Objects and Classes

Using Helper.factorial(...) computing $(\begin{matrix} n \\ k \end{matrix}) = \frac{n!}{k! (n - k)!}$

Objects and Classes

The concept of classes: Blueprints for objects.

Example: Rectangles

UML diagram visualization.

In object oriented languages classes:

are blueprints for objects.
contain attributes and methods.
allow for implementation hiding.
allow for tailored access to methods and attributes.

public class Rectangle {
  int width;
  int height;

  // solid or dashed:
  boolean hasSolidBorder;
}

Objects and Classes
- ➟ Working with objects
  
  Classes and creation of related instances.
  
  The «new» operator.
  
  Assignment of attribute values. Working with «null».

Rectangle dashedRectangle = new Rectangle();

Rectangle solidRectangle = new Rectangle();

...

new class-name ([argument 1[, argument 2] ...] )

Wording examples:

“Create an instance of class Rectangle”.
“Create a Rectangle object.”
“Create a Rectangle”.

Rectangle dashedRectangle = new Rectangle();

dashedRectangle.width = 28;
dashedRectangle.height = 10;
dashedRectangle.hasSolidBorder = false;

Syntax accessing object attributes:

variable.attributeName = value;

Rectangle r = new Rectangle();// Creating an object

r.width = 28; // o.K.

r = null;// removing reference to object

r.width = 28; // Runtime error: NullPointerException (NPE)

Exception in thread "main" java.lang.NullPointerException
  at de.hdm_stuttgart.mi.classdemo.App.main(App.java:17)

Rectangle r;

... // possible object assignment to variable r.

if (null == r) {
  System.out.println("No rectangle on offer");
} else {
  System.out.println("Width:" + r.width);
}

Objects and Classes
- ➟ Packages
  
  Significance for larger projects.
  
  Fully qualified vs. «import».
  
  java.lang and other exceptions.

Grouping of related classes (e.g. subsystems).
Structuring big systems.
Provide access restrictions using:

public, private and protected modifier
Resolving class name clashes. Example:

java.lang.String vs. my.personal.String

Package names below java. are reserved.
Package names should not start with javax. either.
Package names must not contain operators:

mi.hdm-stuttgart.de --> de.hdm_stuttgart.mi.
Packages should start with reversed DNS avoiding clashes.

Fully qualified class name:

java.util.Scanner ❶ scanner =          // Clumsy and
   new java.util.Scanner ❷(System.in); // redundant

Using import:

import java.util.Scanner; ❶

public class Q {

  public static void main(String[] args) {
     Scanner ❷ scanner = new Scanner ❷(System.in);
       ...
  }
}

Bad

Good

import java.util.*;

public class Q {
  public static void
        main(String[] args) {
    Scanner s = 
      new Scanner(System.in);
    Date today = new Date();
  }
}

import java.util.Scanner;
import java.util.Date;

public class Q {
  public static void
         main(String[] args) {
    Scanner s =
       new Scanner(System.in);
    Date today = new Date();
  }
}

import java.lang.String; ❶  // Optional
import java.util.Scanner;❷  // Required
public class Q {

  public static void main(String[] args) {
    String message = "Hello!";
    Scanner ❸ s = new Scanner(System.in);
  }
}

package my.first.javapackage;
        ❷ ❸     ❹
public class Print { 
              ❺
  ...


}

Objects and Classes
- ➟ Object methods
  
  Object behaviour, state transitions, derived values.
  
  Selected examples both of state-changing and state-conserving methods.
  
  Rectangle sample methods.

Change an object's state.: Example: Scale a rectangle.
Get dependent values: Example: Calculate a rectangle's perimeter.
Combined: Scale a rectangle and calculate its new perimeter.

public class Rectangle {
    int width, height;
    boolean hasSolidBorder;

    public void scale (int factor) {
        width *= factor;
        height *= factor;
    }
}

void ❶ scale (int factor ❷) {
...}

❶	No value is being returned to caller.
❷	A single value of type `int` is being provided as method argument.

Rectangle r = new Rectangle();
r.width = 33;
r.height = 22;

r.scale(2);

System.out.println("width=" + r.width);
System.out.println("height=" + r.height);

width=66
height=44

public ❶ void ❷ scale❸ (int factor ❹) { ❺
   width *= factor; ❻
   height *= factor;
}

[access modifier] ❶ return_type ❷ methodName ❸ ([arguments] ❹) {❺
   [statement(s)] ❻
}

public class Rectangle {
    int width, height;
    boolean hasSolidBorder;

    public void scale (int factor) { ... }

    public int getPerimeter() {
        return 2 * (width + height);
    }
}

Rectangle r = new Rectangle();

r.width = 33;
r.height = 22;

System.out.println("Perimeter=" + r.getPerimeter());

Perimeter=110

Objects and Classes
- ➟ Object methods
  - ➟ Encapsulation and access control
    
    Justifying access restrictions.
    
    public, protected «package local» and private.
    
    Best practices.

Fine-grained control on attributes and methods.
Support encapsulation / Information hiding.

Purpose: Hide implementation details within class or package

public class Time {
  private long ❶ secondsSince_2000_01_01;

  public int getSecond() { ❷
    return
    (int) secondsSince_2000_01_01 % (60);
  }
  public int getMinute() {❷
    return (int)
      (secondsSince_2000_01_01 / 60) % 60;
  }
    ...
}

public class Time {
  private int second, minute,❶
         hour,day, month, year;

  public int getSecond() { ❷
        return second;
  }
  public int getMinute() { ❷
        return minute;
  }
    ...
}

No access to private field of alien class Time:

public class Q {
  public static void main(String[] args) {

      Time t = new Time();

      // Error: 'secondsSince_2000_01_01' has private access in 'Time'
      t.secondsSince_2000_01_01 = 33445354L;
  }
}

Access Level	Other package	Child class	Same package	Same class
`public`	yes	yes	yes	yes
`protected`	no	yes	yes	yes
Default	no	no	yes	yes
`private`	no	no	no	yes

Use the most restrictive access level that makes sense for a particular member.
Use private unless you have a good reason not to.
Avoid public fields except for constants. Public fields tend linking to a particular implementation and limit your flexibility in changing your code.

Objects and Classes
- ➟ Object methods
  - ➟ Getter and setter methods

package hour_second.direct;

public class Time {
    public int hour, minute;
}

package hour_second;
import hour_second.direct.Time;

public class DirectAccess {
    Time time = new Time();
    void init() {
        time.hour = 17;
        time.minute = 45;
    }
}

package hour_second.setter;

public class Time {
    private int hour, minute;
    public void setTime(int h, int m) {
        minute = m;
        hour = h;
    }
}

package hour_second;
import hour_second.setter.Time;
public class SetterAccess {
    Time time = new Time();
    void init() { time.setTime(17, 45);}
}

Allow for change of implementation.
Allow for non-business logic concerns. Examples:
- Logging
- Adding persistence

package direct.access;
public class Time {
  // Minutes since 00:00
  public int minute;
}

import direct.access.Time;
public class DirectAccess {
  final Time time = new Time();
  void init() {
    time.minute = 720;// 12:00
  }
}

package access.by.setter;
public class Time {
  private int minute;   // Minutes since 00:00
  public void set(int h, int m) {
    minute = m + 60 * h;
  }
  public int getMinute(){...} // 0 to 59
  public int getHour(){...}   // 0 to 23
}

import access.by.setter.Time;
public class SetterAccess {
    final Time time = new Time();
    void init() { time.setTime(12, 0);}
}

Implementing getter methods

Objects and Classes
- ➟ Object methods
  - ➟ Signatures
    
    Type signatures.
    
    Method signatures: Prerequisite understanding method overloading.

boolean ❶ startsWith(String prefix, int toffset) ❷

❶	Return type boolean
❷	Arguments among with their respective types and order: Type `String` Type `int`

Method	Return type	Argument type list
`void print()`	void	(void)
`int add (int a, int b)`	int	(int, int)
`int max (int a, int b)`	int	(int, int)
`void print (int a, float b)`	void	(int, float)
`void display (float a, int b)`	void	(float, int)

boolean startsWith❶(String prefix, int toffset) ❷

❶	Method name `startsWith`.
❷	Number of arguments among with their respective types and order.

Method	Method name	Method signature
`void print()`	print	(void)
`int add (int a, int b)`	add	(int, int)
`int max (int a, int b)`	max	(int, int)
`void print (int a, float b)`	print	(int, float)
`void display(float a, int b)`	display	(float, int)

Method signature variants

Objects and Classes
- ➟ Object methods
  - ➟ Method overloading
    
    Same name, different argument types.
    
    «C» programming language comparison.
    
    Common examples.

public class Print {
  public void print() {             // (void)
    System.out.println("No argument");
  }
  public void print(int i) {        // (int)
    System.out.println("int value " + i);
  }
  public void print(double d) {     // (double)
    System.out.println("double value " + d);
  }
  public void print(int i, int j) { // (int, int)
      System.out.println("Two int values "+
         i + " and " + j);
  }
}

Print p = new Print();
p.print();
p.print(33);
p.print(4.333);
p.print(-1, 7);

No argument
int value 33
double value 4.333
Two int values -1 and 7

Static polymorphism.
Compile time binding.
Early binding.

#include <stdio.h>

void print() {
  printf("No argument\n");
}

void print(int i) {              /* Error: redefinition of ‘print’ */
  printf("int value %d\n", i);
}

void main(void) {
  print();
  print(33);
}

Code in file print.c Compile / execute

#include <stdio.h>

void print() {
  printf("No argument\n");
}

/* Different function name */
void printIntValue(int i) {   
  printf("int value %d\n", i);
}

void main(void) {
  print();
  printIntValue(33);
}

Compiling print.c to executable file print

> cc -o print print.c

Executing file print

> ./print
No argument
int value 33

public class Person {
  String getDetails() { return "dummy";}
  int getDetails() { return 1;} // Error: 'getDetails()' is already
}                                // defined in 'Person'

Return type	Method signature
Return type	Method name	Argument type list
String	getDetails	(void)
int	getDetails	(void)

Only method signature support in Java™ ignoring return type.

In Java™ method signatures allow for uniquely addressing a method within a given class e.g.:

The method named print having an int argument followed by a double:

print(int, double)

Will a match be found?

`print(boolean b)`	`print(char c)`
`print(char[] s)`	`print(double d)`
`print(float f)`	`print(int i)`
`print(long l)`	`print(Object obj)`
`print(String s)`

Objects and Classes
- ➟ Object methods
  - ➟ Constructors
    
    Combined instance creation and initialization.
    
    Constructors are methods, but ...
    
    Constructor overloading and a class' default constructor.

Combining statements desired:
`int a; a = 33;`	`Rectangle r = new Rectangle(); r.width = 28; r.height = 10; r.hasSolidBorder = false;`
`int a = 33; // works!`	Rectangle r = new Rectangle(28, 10, false);//how ???

public class Rectangle {
    int width, height;
    boolean hasSolidBorder;
         ...
    public ❶ Rectangle ❷ (int width, int height, boolean hasSolidBorder){
        this.width = width;
        this.height = height;
        this.hasSolidBorder = hasSolidBorder;
    }
}

[access modifier] constructorName (listOfArguments) {
    [constructor body]
}

Empty argument list

Default constructor e.g. obj = new MyClass().

Non-empty argument list

Non-default constructor e.g. :

obj = new String("xyz");

Can only be executed on object creation.
Are being called prior to any non-constructor method.
Only one of potentially multiple constructors will be executed exactly one time.

However nesting is possible.

public class Rectangle {
    int width, height;

    public Rectangle() {
        width = height = 1;
    }
    public Rectangle(int width, int height){
        this.width = width;
        this.height = height;
    }
    public Rectangle(int widthAndHeight) {
        width = height = widthAndHeight;
    }
}

public class Rectangle {
  int width, height;

  public Rectangle(int width,
                int height){
    this.width = width;
    this.height = height;
  }
  public Rectangle() {
    width = height = 1;
  }
  public Rectangle(
     int widthAndHeight) {
    width = height =
          widthAndHeight;
  }
}

   public class Rectangle {
     int width, height;

 ┏━▶ public Rectangle(int width,
 ┃                    int height){
 ┃     this.width = width;
 ┃     this.height = height;
 ┃   } 
 ┃   public Rectangle() { 
 ┣━◀━━ this(1, 1);           ❶
 ┃   }
 ┃   public Rectangle(
 ┃     int widthAndHeight) {
 ┗━◀━━ this(widthAndHeight,   ❷
            widthAndHeight); 
     }
   }

Rectangle standard = new Rectangle(); // 1 x 1
Rectangle square = new Rectangle(2); // 2 x 2
Rectangle individual = new Rectangle(2, 7); // 2 x 7

Equivalent:

Rectangle r =
                              new Rectangle();

public class Rectangle {
    int width, height;
    boolean hasSolidBorder;

    // Default constructor, empty body.
    public Rectangle ( ){}
}

public class Rectangle {
    int width, height;
    boolean hasSolidBorder;

  // No constructor at all
}

public class Rectangle {
  int width, height;
  boolean hasSolidBorder;

  // Non-default constructor
  public Rectangle(int width,
                  int height,
    boolean hasSolidBorder){
      this.width = width;
      this.height = height;
      this.hasSolidBorder =
                hasSolidBorder;
  }

 // No defined default constructor.
}

// o.K.: Using non-default
// constructor.

Rectangle r = 
   new Rectangle(3, 6, false);

// Wrong: Default constructor
// undefined, but non-default
// constructor present.

Rectangle r = new Rectangle();

Objects and Classes
- ➟ Object methods
  - ➟ Scopes
    
    Class, method and block scopes.
    
    Documentation, «good» variable names and the this keyword.

/** Representing circles.
 */
public class Circle {
  private double radius;

  /** Creating a circle.
   *  @param r representing the circle's radius
   */
  public Circle(double r) {
    radius = r;
  }
  public double getDiameter() {
    return 2 * radius;
  }
}

/** Representing circles.
 */
public class Circle {
  private double radius;

  /** Creating a circle.
   *  @param radius Circle's size
   */
  public Circle(double radius) {
    radius = radius; // Warning: Variable 'radius' is assigned to itself.
  }
  public double getDiameter() {
    return 2 * radius;
  }
}

Constructors variable names and “this”.

Objects and Classes
- ➟ Class members and methods
  
  Motivating class attributes and methods.
  
  Example: A club and its affiliated members.
  
  Syntax of static and employment considerations

public class Helper {
  // Find the larger of two values
  public int maximum(int a, int b) {
    if (a < b) {
      return b;
    } else {
      return a;
    }
  }
}

final Helper instance = new Helper();

// Why do we need an instance just for
// computing the maximum of two values?
System.out.println("Maximum: " +
            instance.maximum(-3, 5));

Maximum: 5

Observation: The instance's state is irrelevant for finding the maximum of two values.

public class Helper { ❶

  static ❷ public int maximum(int a, int b)
   {
    if (a < b) {
      return b; 
    } else {
      return a;
    }
  }
}

// No instance required any longer
System.out.println("Maximum: " +
     Helper.maximum(-3, 5)); ❶

Maximum: 5

Each club member has got a name.
Each member has got an ascending unique membership number.
The overall club's member count needs to be accounted for.

Solution:

Class level: Advance club's member count by each new member.
Instance level: New members receive name and current member count plus 1.

public class ClubMember {

  final private String name;

  public ClubMember(final String name) {
    this.name = name;
  }
  public String toString() {
    return "Member " + name;
  }
}

final ClubMember
   john = new ClubMember("John"),
  karen = new ClubMember("Karen");

System.out.println(john.toString());
System.out.println(karen.toString());

Member John
Member Karen

public class ClubMember {

  static ❶ private int memberCount = 0;

  final private int memberNumber; ❷
  final private String name;

  public ClubMember(final String name) {
    this.name = name;
    memberNumber = ++memberCount; ❸
  }
  public String toString() {
    return "Member " + name + ", member number " +  memberNumber ❹;
  }
}

final ClubMember
   john = new ClubMember("John"),
  karen = new ClubMember("Karen");

System.out.println(john); ❶ // toString() is being
System.out.println(karen);   // called implicitly

Member John, member number 1
Member Karen, member number 2

public class ClubMember {

  static private int memberCount = 0;
  ...
  public ClubMember(final String name) {
    this.name = name;
    memberNumber = ++memberCount;
  }
  static public int getMemberCount() ❶ {
    return memberCount; ❷
  }
  ...
}

final ClubMember
   john = new ClubMember("John"),
  karen = new ClubMember("Karen"),
  petra = new ClubMember("Petra");

System.out.println(karen.toString());

System.out.println("Club's member count:"
   + ClubMember.getMemberCount());
// Good: Prevent tampering memberCount
// variable.

Member Karen, member number 2
Club's member count:3

Class Variable: {class name}.{variableName}
Class Method: {class name}.{methodName}([parameter])

public class X {
         int a; ❶
  static int b; ❷

❶	Variable `a` defined once per instance of class `X`.
❷	Variable `b` defined once per class `X`.

   ❶   ❷    ❸
System.out.print(...)

❶	Class `System` in package java.lang.
❷	Class variable (static) out of type `PrintStream` in class `System`
❸	One of 9 overloaded methods in class `PrintStream`.

Objects and Classes
- ➟ Project dependencies and Maven
  
  Cross project dependencies.
  
  Repositories and project artifacts.

Automated third-party class import and dependency management
Executing automated tests
Complete project lifecycle:

compile, test, execute, package, deploy
Extensible plugin architecture

Source: CreatePdf.java PDF output

import com.itextpdf.kernel.pdf.PdfDocument;
...
final PdfWriter writer = new PdfWriter("helloWorld.pdf");

final PdfDocument pdf = new PdfDocument(writer);

final Document document = 
   new Document(pdf, PageSize.A8.rotate());

document.add(new Paragraph("Hello World!").
     setFontColor(ColorConstants.MAGENTA).
     setFontSize(20));

document.close();

<project ...>
  ...
  <dependencies>
    <dependency>
      <groupId>com.itextpdf</groupId>
      <artifactId>itext-core</artifactId>
      <version>8.0.3</version>
      <type>pom</type>
    </dependency>
  </dependencies>
...

</project>

CDN: Maven Central
Company local: Sonatype Nexus, e.g. MI Maven repository

Dealing with IBAN numbers

Blueprints for projects.
Based on the pom.xml file:

Project Object Model
Initial project creation.
CLI and IDE support.
Extensibility by archetype catalogs.

mvn archetype:generate
[INFO] Scanning for projects...
...
1: remote -> am.ik.archetype:elm-spring-boot-blank-archetype ...
2: remote -> am.ik.archetype:graalvm-blank-archetype ...
3: remote -> am.ik.archetype:graalvm-springmvc-blank-archetype ...
...
2083: remote -> org.apache.maven.archetypes:maven-archetype-quickstart...
...
3330: remote -> za.co.absa.hyperdrive:component-archetype_2.12 (-)
Choose a number or apply filter ...

Helper.java pom.xml

package mi.calc.common;
public class Helper {
  static public long
    factorial(int n) {
    long result = 1;
    for (int i = 2;
        i <= n; i++) {
      result *= i;
    }
    return result;
  }
}

<project xmlns="http://maven.apache.org...>
     ...
 <groupId>mi.calc</groupId>
 <artifactId>common</artifactId>
 <version>1.0</version>
     ...
</project>

goik@goiki Helper> mvn install
[INFO] Scanning for projects...
[INFO] -------------------------------------------
[INFO] Building helper 0.9
...
[INFO] Installing .../Helper/target/helper-0.9.jar to
  /ma/goik/.m2/repository/mi/calc/common/1.0/common-1.0.jar

goik@goiki tmp> unzip  ...hdm_stuttgart/de/mi/sd1/helper/0.9/helper-0.9.jar
Archive:  .../.m2/repository/.../sd1/helper/0.9/helper-0.9.jar
   creating: META-INF/
  inflating: META-INF/MANIFEST.MF
   creating: de/
   creating: de/hdm_stuttgart/
   creating: de/hdm_stuttgart/mi/
   creating: de/hdm_stuttgart/mi/sd1/
  inflating: de/hdm_stuttgart/mi/sd1/Helper.class
   creating: META-INF/maven/
   creating: META-INF/maven/de.hdm_stuttgart.mi.sd1/
   creating: META-INF/maven/de.hdm_stuttgart.mi.sd1/helper/
  inflating: META-INF/maven/de.hdm_stuttgart.mi.sd1/helper/pom.xml
  inflating: META-INF/maven/de.hdm_stuttgart.mi.sd1/helper/pom.properties

<project ...>
  ...
  <groupId>de.hdm_stuttgart.mi.sd1</groupId>
  <artifactId>lottery</artifactId>
  <version>0.9</version>

  <packaging>jar</packaging>
  <name>lottery</name>

  <dependencies>
    <dependency>
      <groupId>de.hdm_stuttgart.mi.sd1</groupId>
      <artifactId>helper</artifactId>
      <version>0.9</version>
    </dependency>
...
</project>

static public long binomial(int n, int k) {
  return (Helper.factorial(n) / Helper.factorial(k)
                              / Helper.factorial(n - k));
}

public static void main(String[] args) {
  System.out.println("There are " + binomial(5, 2) +
      " ways to draw 2 out of 5 numbers");

  System.out.println("There are " + binomial(49, 6) +
      " ways to draw 6 out of 49 numbers");
}

Objects and Classes
- ➟ Project dependencies and Maven
  - ➟ Maven command line usage

mvn --batch-mode -e archetype:generate \
-DgroupId=de.hdm_stuttgart.mi.sd1 -DartifactId=second -Dversion=0.9 \
-DarchetypeGroupId=org.apache.maven.archetypes -DarchetypeArtifactId=maven-archetype-quickstart -DarchetypeVersion=1.4

[INFO] Scanning for projects...
      ...
[INFO] BUILD SUCCESS ...

See artifact reference.

MI nexus repository server

Configuration:

mkdir -p ~/.m2 ❶
nano ~/.m2/settings.xml ❷

mvn --batch-mode -e archetype:generate \
  -DgroupId=de.hdm_stuttgart.mi.sd1 -DartifactId=second -Dversion=0.9 \
  -DarchetypeGroupId=de.hdm_stuttgart.mi -DarchetypeArtifactId=mi-maven-archetype-quickstart -DarchetypeVersion=2.3
[INFO] Error stacktraces are turned on.
[INFO] Scanning for projects...
      ...
[INFO] BUILD SUCCESS ...

mvn --batch-mode ❶ -e archetype:generate ❷ \
                                              \
 -DarchetypeGroupId=de.hdm_stuttgart.mi \ ❸
 -DarchetypeArtifactId=mi-maven-archetype-quickstart \
 -DarchetypeVersion=2.3 \
                        \
 -DgroupId=de.hdm_stuttgart.mi.sd1 ❹ \
 -DartifactId=first  \
 -Dversion=0.9

DNS inflicted groupId / package names clashes

> cd first        # Enter project directory
> find . -type f  # Search recursively for files
./.gitignore ❶
./src/main/java/de/hdm_stuttgart/mi/sd1/App.java ❷
./pom.xml ❸

> mvn compile
[INFO] Scanning for projects...
       ...
[INFO] Building first 0.9
   ...
[INFO] Changes detected - recompiling the module!
[INFO] Compiling 1 source file to /ma/goik/first/target/classes
[INFO] --------------------------------------------------------
[INFO] BUILD SUCCESS

> find target/classes -type f
./target/classes/de/hdm_stuttgart/mi/sd1/App.class
...

> cd target/classes ❶
> java de.hdm_stuttgart.mi.sd1.App ❷
Hi there, let's have
fun learning Java! ❸

❶

Change to base directory containing compiled Java™ classes.

❷

Application execution. Note:

Our App class is being prefixed by the package name de.hdm_stuttgart.mi.sd1 defined by the groupId parameter in Figure 277, “CLI archetype details”.

❸

The expected output result.

Note

Executing this particular class requires a configuration in our project's pom.xml file:

...
<archive>
  <manifest>
    <mainClass>de.hdm_stuttgart.mi.sd1.test.ShowReachedPoints</mainClass>
  </manifest>
</archive> ...

> mvn package
...
 T E S T S
...
Tests run: 1, Failures: 0, Errors: 0, Skipped: 0
   ...
[INFO] Building jar: /ma/goik/first/target/first-0.9.jar
...
[INFO] Replacing /ma/goik/first/target/first-0.9.jar with 
         /ma/goik/first/target/first-0.9-shaded.jar
...
[INFO] BUILD SUCCESS

java -jar target/first-0.9.jar
Hi there, let's have
fun learning Java!

Remark: This will execute HelloWorld.class being contained in first-0.9.jar.

Details on execution

> mvn javadoc:javadoc
[INFO] Scanning for projects...
...
Generating /ma/goik/First/target/site/apidocs/allclasses-noframe.html...
Generating /ma/goik/First/target/site/apidocs/index.html...
Generating /ma/goik/First/target/site/apidocs/overview-summary.html...
Generating /ma/goik/First/target/site/apidocs/help-doc.html...

See e.g. class String documentation.

> mvn clean
...
[INFO] 
[INFO] --- maven-clean-plugin:2.5:clean (default-clean) @ first ---
[INFO] Deleting /ma/goik/first/target
[INFO] ------------------------------------------------------------------------
[

Objects and Classes
- ➟ Project dependencies and Maven
  - ➟ Intellij IDEA on top of Maven

Built in Maven support in Intellij IDEA.
MI supplementary archetypes require MI archetype configuration in ~/.m2/settings.xml.

Your Maven project depends on its pom.xml file.
Reloading changed pom.xml file.

Objects and Classes
- ➟ Unit testing
  
  Test categories.
  
  Why unit testing? Idea of test «driven development».
  
  Required steps.

Unit test: Test individual methods, classes and packages in isolation.
Integration Test: Test a group of associated components/classes.
Acceptance / Functional Test: Operate on a fully integrated system, testing against the user interface.
Regression Test: Ensure system integrity after (implementation) change.
Load test: Responsiveness vs. system load.

Informal problem specification:

A prime number is a whole number greater than 1 whose only factors are 1 and itself.

Sample values: 2, 3, 5, 7, 11, 13, 17, 23, ...

First write tests, then implement.

Specify but not yet implement classes / methods.
Write skeleton (dummy) implementations.
Write corresponding unit tests.
Implement skeleton.
Test your implementation.

Objects and Classes
- ➟ Unit testing
  - ➟ Tests and implementation
    
    Providing both an implementation and related tests possibly revealing coding flaws.
    
    Providing prime number related tests.

/**
 * Dealing with prime numbers.
 */
public class Prime {
  /**
   * Check whether a given value is prime or not ❶
   * @param value A positive value
   * @return true if and only if value is a prime number.
   */
  public static boolean isPrime(int value) {
    return true ❷; //TODO: Dummy value to be implemented correctly 
  }
}

for (int i = 1; i < 20;i++) {
  System.out.println(i + " is " + (Prime.isPrime(i) ? " a " : " not a ")
    + " prime number");
}

1 is a prime number
2 is a prime number
3 is a prime number
4 is a prime number
5 is a prime number
...

Input	Expected output	Input	Expected output
`1`	`false`	`7`	`true`
`2`	`true`	`8`	`false`
`3`	`true`	`9`	`false`
`4`	`false`	`10`	`false`
`5`	`true`	`11`	`true`
`6`	`false`	`12`	`false`

public class PrimeTest {

  @Test ❶ public void test_1_isNotPrime() {
    Assert.assertFalse(Prime.isPrime(1));
  }
  @Test ❶ public void test_2_isPrime() {
    Assert.assertTrue(Prime.isPrime(2));
  }

  void someOrdinaryMethod() ❷ {...}
...

goik@goiki Prime_v01> mvn test
...
Running de.hdm_stuttgart.mi.sd1.PrimeTest
Tests run: 2, Failures: 1, Errors: 0, Skipped: 0, 
         Time elapsed: 0.065 sec <<< FAILURE!
...
test_1_isNotPrime(de.hdm_stuttgart.mi.sd1.PrimeTest)
         Time elapsed: 0.001 sec  <<< FAILURE!
java.lang.AssertionError
	at org.junit.Assert.fail(Assert.java:86)
	at org.junit.Assert.assertTrue(Assert.java:41)
	at org.junit.Assert.assertFalse(Assert.java:64)
	at org.junit.Assert.assertFalse(Assert.java:74)
...

@Test public void test_Primes() {
  Assert.assertTrue(Prime.isPrime(3));
  Assert.assertTrue(Prime.isPrime(5));
  Assert.assertTrue(Prime.isPrime(7));
  Assert.assertTrue(Prime.isPrime(11));
    ...  }
@Test public void testOddNonPrimes() {
  Assert.assertFalse(Prime.isPrime(9));
  Assert.assertFalse(Prime.isPrime(15));
  Assert.assertFalse(Prime.isPrime(21));...}

@Test public void testEvenNonPrimes() {
  for (int i = 2; i < 100; i++) {
    Assert.assertFalse(Prime.isPrime(2 * i));
  }
}

public static boolean isPrime(int value) {
  for (int i = 2; i < value; i++) {
    if (0 == value % i) { // i divides value
      return false;
    }
  }
  return value != 1;
}

goik@goiki Prime_v01> mvn test
...
-------------------------------------------------------
 T E S T S
-------------------------------------------------------
Running de.hdm_stuttgart.mi.sd1.PrimeTest
Tests run: 5, Failures: 0, Errors: 0, Skipped: 0, Time elapsed: 0.055 sec

Results :

Tests run: 5, Failures: 0, Errors: 0, Skipped: 0

Objects and Classes
- ➟ Unit testing
  - ➟ Improving the algorithm
    
    Try improving the algorithm.
    
    Introducing an error.

101 / 2  = 50 remainder 1
101 / 3  = 33 remainder 2
101 / 4  = 25 remainder 1
101 / 5  = 20 remainder 1
101 / 6  = 16 remainder 5
101 / 7  = 14 remainder 3
101 / 8  = 12 remainder 5
101 / 9  = 11 remainder 2
101 / 10  = 10 remainder 1
101 / 11  = 9 remainder 2
101 / 12  = 8 remainder 5
101 / 13  = 7 remainder 10
...

Big performance gain:

public static boolean isPrime(int value) {
  for (int i = 2; i * i < value; i++) {
    if (0 == value % i) {
      return false;
    }
  }
  return value != 1;
}

public static boolean isPrime(int value) {

//for (int i = 2; i * i < value; i++) {
  for (int i = 2; i * i <= value; i++) {
    if (0 == value % i) {
      return false;
    }
  }
  return value != 1;
}

Objects and Classes
- ➟ Unit testing
  - ➟ Helpful Junit methods
    
    Selected assertion methods.
    
    Considerations when comparing float and double values.

java.lang.AssertionError: Use assertEquals(expected, actual, delta)
  to compare floating-point numbers

at org.junit.Assert.assertEquals(Assert.java:656)
at qq.doubleCompareTest.test1_3(doubleCompareTest.java:15)
...

System.out.println("3.6 - 3 * 1.2 == " + (3.6 - 3 * 1.2));

Result:

3.6 - 3 * 1.2 == 4.440892098500626E-16

public class doubleCompareTest {
  static final double delta = 1E-15;
  /**
   * Comparing 3.6 and 3 * 1.2 within delta's limit
   */
  @Test
  public void test1_3() {
    Assert.assertEquals(3.6, 3 * 1.2 , delta);
  }
}

Objects and Classes
- ➟ Unit testing
  - ➟ Technical details
    
    Junit Building Blocks: Classes and interfaces
    
    Integration using Maven.

public
@interface Test {
...
}

@interface defining an annotation.
Purpose: Adding meta information for automated detection of test methods.

public class Assert {

  public static void assertTrue(
    String message, boolean condition) { ...}

  public static void assertEquals(
    long expected, long actual) { ...}
...
}

<project ...>
...
  <dependencies>
    <dependency>
      <groupId>junit</groupId>
      <artifactId>junit</artifactId>
      <version>4.13</version>

      <scope>test</scope>
    </dependency>
    ...
  </dependencies>
  ...
</project>

Local: /home/goik/.m2/repository/junit/junit/4.13/junit-4.13.jar

Remote: https://mvnrepository.com/artifact/junit/junit/4.13

> jar -tf junit-4.13.jar
META-INF/
META-INF/MANIFEST.MF
org/
org/junit/
...
org/junit/Assert.class
...

Objects and Classes
- ➟ Value types and reference types
  
  Primitive and value types revisited.

Value type: Holding data in associated memory location

byte
short

int
long

float
double

char
boolean

Reference type: Holding a reference to an object

Array or class instances i.e. String, java.util.Scanner etc.

Value type

int a = 1;
int b = a;
a = 5;
System.out.println("a=" + a);
System.out.println("b=" + b);

a=5
b=1

Reference type

StringBuffer r = new StringBuffer("Joe");
StringBuffer s = r;
r.append(" Simpson");
System.out.println("r=" + r);
System.out.println("s=" + s);

r=Joe Simpson
s=Joe Simpson

public static void main(String[] args) {
  int value = 3;
  System.out.println(
    "Before printDuplicateValue: "
    + value);
  printDuplicateValue(value);
  System.out.println(
    "After printDuplicateValue: "
     + value);
}
static void printDuplicateValue(int n) {
  n = 2 * n;
  System.out.println(
  "printDuplicateValue: " + n);
}

Before printDuplicateValue: 3
printDuplicateValue: 6
After printDuplicateValue: 3

public static void main(String[] args) {
  StringBuffer buffer = new StringBuffer("My");
  System.out.println("Before duplicateString: "
    + buffer);
  duplicateString(buffer);
  System.out.println("After duplicateString: "
    + buffer);
}
static void duplicateString(StringBuffer b) {
    b.append(b); // Append self
}

Before duplicateString: My
After duplicateString: MyMy

public static void main(String[] args) {
  StringBuffer buffer = new StringBuffer("My");
  System.out.println("Before duplicateString: "
    + buffer);
  replaceString(buffer);
  System.out.println("After duplicateString: "
    + buffer);
}
static void replaceString(StringBuffer b) {
  b = new StringBuffer("Replacement");
}

Before duplicateString: My
After duplicateString: My

// Passing a reference
// to variable n
void printDuplicateValue(int& n) {
  n = 2 * n;
  cout << "duplicateValue: " << n 
       << endl;
}
int main() {
  int value = 3;
  cout << "Before call: "
    << value << endl;
  printDuplicateValue(value);
  cout << "After call: "
    << value << endl;
}

Before call: 3
duplicateValue: 6
After call: 6

Objects and Classes
- ➟ Method calls, the details
  
  Method calls, behind the scenes.
  
  Stack and heap memory.
  
  Stack frames and value propagation. Using the debugger.

public class Circle {
  static final double
    PI = 3.141592653589793;
  double r;
  /** Change a circle's area
   *  @param area The desired new area
   *  @return The circle's new radius */
  double setArea(final double area ❶) {
    double val ❷ = area  / PI ❸;
    return ❹ r ❺ = Math.sqrt(val);
  }
}

❶	Passing arguments.
❷	Defining method local variables.
❸	Accessing class variable.

❹	returning values.

❺	Accessing instance variable.

public class Circle {

  static final double
    PI ❶ = 3.141592653589793;

  double r ❷ ;

  double setArea(final double area ❸) {
    double val ❸ ...
    ...
  }
}

❶	Class scope.
❷	Instance scope
❸	Method scope

Class scope (`static`)	Application process
Instance scope	Object lifetime: `new (...)` until being garbage collected.
Method scope	Method invocation until `return`.

Heap memory

Allocation of class or array instances:

new String()

new float[200]

De-allocation subject to garbage collection.

Execution stack

One instance per process thread.
Hosting variables (values or references)

Method	Description	Precondition	Access type
`push(...)`	Add object on top	-	Modify
`pop()`	Read + remove topmost object	Stack not empty	Modify
`top()`	Read topmost object	Stack not empty	Read only
`empty()`	`true` if and only if stack is empty	-	Read only

Objects and Classes
- ➟ Enumerations (enum)

Modeling finite sets of discrete states.
Examples:

A room's door: {OPEN, CLOSED}

State of matter: {SOLID, LIQUID, GASEOUS}
No dynamic change of state set.

Objects and Classes
- ➟ Enumerations (enum)
  - ➟ Enumeration by integer representation
    
    Example: Weekdays Monday till Sunday represented by constant int values.
    
    Common pitfall examples.
    
    Insufficient means of language protection against self - inflicted problems.

public class Lecture {

    public final int dayHeld; /* e.g. to be held on Tuesdays */

    public final String title; /* e.g. «PHP introduction» */

    public Lecture(final int dayHeld, final String title) {
        this.dayHeld = dayHeld;
        this.title = title;
    }
}

Quick and dirty:

Class Driver:

final Lecture
  phpIntro     = new Lecture(1 /* Monday */, "PHP introduction"),
  advancedJava = new Lecture(5 /* Friday */, "Advanced Java");

Error prone:

Weeks start on Mondays?
Index starts with 0 or 1?

public class Day {

  static public final int
    MONDAY    = 1,
    TUESDAY   = 2,
    WEDNESDAY = 3,
    THURSDAY  = 4,
    FRIDAY    = 5,
    SATURDAY  = 6,
    SUNDAY    = 7;
}

Class Driver:

final Lecture
  phpIntro     = new Lecture(Day.MONDAY, "PHP introduction"),
  advancedJava = new Lecture(Day.FRIDAY, "Advanced Java");

public class Day {
...
  public static String getDaysName(final int day) {
    switch (day) {
      case MONDAY:    return "Monday";
      case TUESDAY:   return "Tuesday";
      case WEDNESDAY: return "Wednesday";
      case THURSDAY:  return "Thursday";
      case FRIDAY:    return "Friday";
      case SATURDAY:  return "Saturday";
      case SUNDAY:    return "Sunday";

      default:        return "Illegal day's code: " + day;
    }
  }
}

public class Lecture {
  public final int dayHeld;
    ...

  public String toString() {
    return "Lecture «" + title + "» being held each " +
           Day.getDaysName(dayHeld);
  }
}

// Class Driver
final Lecture 

  phpIntro = new Lecture(
    Day.MONDAY, "PHP introduction"),

  advancedJava = new Lecture(
    Day.FRIDAY, "Advanced Java");

System.out.println(phpIntro);
System.out.println(advancedJava);

Lecture «PHP introduction» 
  being held each Monday
Lecture «Advanced Java» 
  being held each Friday

// Class Screwed

final Lecture phpIntro =
    new Lecture(88, "PHP introduction");

System.out.println(phpIntro);

Lecture «PHP introduction» being
held each Illegal day's code: 88

/**
* Charge double prices on weekends
* @param day Day of week
* @param amount
* @return the effective amount for
*         given day of week.
*/
static public int getPrice(
   final int day, final int amount) {
  switch (day) {
    case Day.SATURDAY:
    case Day.SUNDAY: return 2 * amount;

    default: return amount;
  }
}

// Correct
System.out.println(
   getPrice(Day.SUNDAY,  2));

// Argument mismatch
System.out.println(
   getPrice(2, Day.SUNDAY));

4
7

Bad: No warning message whatsoever!

Objects and Classes
- ➟ Enumerations (enum)
  - ➟ Enumeration by dedicated class
    
    Representing enumeration values by instances gaining type safety.
    
    Partial problem solution.

Roadmap:

Define a dedicated enumeration representing class.
Create exactly one class instance per enumeration value.
Enumeration value equality comparison by virtue of the == operator.

public class Day {

  static public final Day
    MONDAY    = new Day(),
    TUESDAY   = new Day(),
    WEDNESDAY = new Day(),
    THURSDAY  = new Day(),
    FRIDAY    = new Day(),
    SATURDAY  = new Day(),
    SUNDAY    = new Day();
}

Note: Class without instance attributes.

Reverting to if .. else if ...

public static String getDaysName(final Day day) {
  if (MONDAY == day) {  // Switch no longer possible, sigh!
    return "Monday";
  } else if (TUESDAY == day) {
      ...
  } else if (SUNDAY == day) {
    return "Sunday";
  } else {
    return "Illegal day instance: " + day;
  }
}

/**
 * Charge double prices on weekends
 * @param day Day of week
 * @param amount
 * @return the effective amount depending on day of week.
 */
static public int getPrice(final Day day, final int amount) {
  if (Day.SATURDAY == day || Day.SUNDAY == day) {
    return 2 * amount;
  } else {
    return amount;
  }
}

Preventing method argument order mismatch:

// Class Driver

// o.K.
System.out.println(Screwed2.getPrice(Day.SUNDAY,  2));

// Argument mismatch causing compile time type violation error
System.out.println(Screwed2.getPrice(2, Day.SUNDAY));

Class Screwed:

final Day PAST_SUNDAY = new Day();

final Lecture phpIntro = new Lecture(
     PAST_SUNDAY, "PHP introduction");

System.out.println(phpIntro.toString());

Lecture «PHP introduction» being
held each Illegal day instance: 
de.hdm_stuttgart.mi.sd1.
class_wrapper.Day@63961c42

Objects and Classes
- ➟ Enumerations (enum)
  - ➟ Defining a private constructor
    
    Clearing the mess.
    
    Sad: switch still not working.

public class Day {

  // Disallow object creation outside class

  private Day() {} 

  static public final Day
    MONDAY    = new Day(),
    TUESDAY   = new Day(),
  ...
    SUNDAY    = new Day();
}

Class Screwed:

Day PAST_SUNDAY = new Day();

Lecture phpIntro = new Lecture(
     PAST_SUNDAY, "PHP introduction");

System.out.println(phpIntro.toString());

Compile time error:

'Day()' has private access in
'de.hdm_stuttgart.mi.sd1.
class_wrapper_private.Day'

public class Day {
  public final String name;

  private Day(final String name) {
    this.name = name;
  }
  static public final Day
    MONDAY    = new Day("Monday"),
  ...
    SUNDAY    = new Day("Sunday");

    public String toString() { return name; }
}

Objects and Classes
- ➟ Enumerations (enum)
  - ➟ enum replacing class
    
    Introducing enum as kind of restricted class.
    
    Makes switch working again!

public enum Day {

  MONDAY("Monday"),
  TUESDAY("Tuesday"),
  ...
  SUNDAY("Sunday");

  final String name;
  Day(final String name) { this.name = name;}

  public String toString() { return name;}
}

public enum Day {
...
  public static String getItalianDayName(final Day day) {
    switch (day) {
      case MONDAY:    return "Lunedì";
      case TUESDAY:   return "Martedì";
   ...
      case SUNDAY:    return "Domenica";
    }
    return null; // Actually unreachable, but static
                 // compiler code analysis is limited
  }
}

public enum Day {
  ...
  private Day(final String name)
    { ...

Compile time warning:

Modifier 'private' is redundant for enum constructors

public enum Day {
  ...
  public Day(final String name)
    { ...

Compile time error:

Modifier 'public' not allowed here

Prohibits enum external instance creation.

Objects and Classes
- ➟ Using git

Git:

A completely ignorant, childish person with no manners.
A person who feels justified in their callow behaviour.
A pubescent kid who thinks it's totally cool to act like a moron on the internet, only because no one can actually reach through the screen and punch their lights out.

Objects and Classes
- ➟ Using git
  - ➟ Working with git locally.
    
    Creating a git project
    
    Important commands: git add, git commit, git log, ...

git init
Result: Sub folder .git containing project's metadata and versioning history

git config --global user.email "foo@company.com"
git config --global user.name "Helen Foo"

public class Math {

  static public int add(
   final int a, final int b) {
     return a + b;
  }
}

git status: Math.java yet unversioned.
git add Math.java
git status: Math.java became versioned.

git commit --message "New math class containing single method"
Result: Commit change set to repository adding given message to project log.

git status
On branch master ❶

No commits yet ❷

Untracked files: ❸
  (use "git add <file>..." to include in what will be committed)

    Math.java

nothing added to commit but untracked files present (use "git add" to track) ❹

public class Math {

  static public int add(
   final int a, final int b) {
     return a + b;
  }
}

public class Math {
 /**
  * Summing two int values.
  * @param a first value.
  * @param b second value.
  * @return The sum of both.
  */
  static public int add(
   final int a, final int b) {
     return a + b;
  }
}

>git checkout -- ❶ Math.java ❷

>git diff Math.java ❸
>

❶	Double dashes '--' disambiguating branch names from file names.

❷	Replace working tree file `Math.java` by repository `master`.

❸	No difference working tree to `master` branch.

> javac Print.java Math.java  # Compilation creating Math.class and Print.class
 
> git status
On branch master
Your branch is up to date with 'origin/master'.

Untracked files:
  (use "git add <file>..." to include in what will be committed)

        Math.class
        Print.class

nothing added to commit but untracked files present (use "git add" to track)

Math.class and Print.class are being generated from Math.java and Print.java.
Rule of thumb: Do not version dependent objects.

Solution: Add a .gitignore file to versioning to contain:

# ignore generated .class files
*.class

git log
commit 0137ccd857a242f4751e36bdbce365c6130c3a32 ❶(HEAD -> master) 
Author: Martin Goik <goik@hdm-stuttgart.de>
Date:   Sat May 25 11:56:00 2019 +0200

    Removing duplicate headline ❷

commit 7f119fac36e02e4c5a7f04f022217b6f744d6e1d ❸
Author: Martin Goik <goik@hdm-stuttgart.de>
Date:   Sat May 25 11:49:52 2019 +0200

    Project Readme.md ❹ ...

git checkout 7f119fac36e02e4c5a7f04f022217b6f744d6e1d
Note: checking out '7f119fac36e02e4c5a7f04f022217b6f744d6e1d'.

You are in 'detached HEAD' state. You can look around, make experimental
changes and commit them, and you can discard any commits you make in this
state without impacting any branches by performing another checkout.

If you want to create a new branch to retain commits you create, you may
do so (now or later) by using -b with the checkout command again. Example:

  git checkout -b <new-branch-name>

HEAD is now at 7f119fa Project Readme.md

git checkout master
Previous HEAD position was 7f119fa Project Readme.md
Switched to branch 'master'

git local, DIY

Objects and Classes
- ➟ Using git
  - ➟ Shared development with centralized remote.
    
    Connecting local to remote repository or clone
    
    push local to remote
    
    pull from remote to local

See multi+remote/shared git:

Create empty remote repository on gitlab.mi.hdm-stuttgart.de.
Connect local repository to remote
push or pull content

Step 2: Retrieve remote repository address

git remote add origin ❶ https://gitlab.mi.hdm-stuttgart.de/goik/gitintro.git

❶	origin is an alias for our remote repository `https://gitlab.mi.hdm-stuttgart.de/goik/gitintro.git`.

git push --set-upstream origin ❶ master
Username for 'https://gitlab.mi.hdm-stuttgart.de': goik ❷
Password for 'https://goik@gitlab.mi.hdm-stuttgart.de': 
Counting objects: 5, done.
Delta compression using up to 6 threads.
Compressing objects: 100% (4/4), done.
Writing objects: 100% (5/5), 507 bytes | 507.00 KiB/s, done.
Total 5 (delta 0), reused 0 (delta 0)
To https://gitlab.mi.hdm-stuttgart.de/goik/gitintro.git
 * [new branch]      master -> master ❸
Branch 'master' set up to track remote branch 'master' from 'origin'.

> git pull ❶
Username for 'https://gitlab.mi.hdm-stuttgart.de': goik ❷
Password for 'https://goik@gitlab.mi.hdm-stuttgart.de': 
remote: Enumerating objects: 8, done.
remote: Counting objects: 100% (8/8), done.
remote: Compressing objects: 100% (6/6), done.
remote: Total 6 (delta 1), reused 0 (delta 0)
Unpacking objects: 100% (6/6), done.
From https://gitlab.mi.hdm-stuttgart.de/goik/gitintro ❸
   733e541..ffff092  master     -> origin/master
Updating 733e541..ffff092 ❹
Fast-forward
 Math.java | 10 ++++++++-- ❺
 1 file changed, 8 insertions(+), 2 deletions(-)

Create new possibly non-empty project at https://gitlab.mi.hdm-stuttgart.de.

> git clone https://gitlab.mi.hdm-stuttgart.de/goik/gitintro.git
Cloning into 'gitintro'...
Username for 'https://gitlab.mi.hdm-stuttgart.de': goik
Password for 'https://goik@gitlab.mi.hdm-stuttgart.de': 
remote: Enumerating objects: 5, done.
remote: Counting objects: 100% (5/5), done.
remote: Compressing objects: 100% (4/4), done.
remote: Total 5 (delta 0), reused 0 (delta 0)
Unpacking objects: 100% (5/5), done.

Objects and Classes
- ➟ Using git
  - ➟ Conflicts
    
    Line level conflicts
    
    Manual merge

User A: Print.java

User B: Print.java

// Driver class
public class Print {
  public static void
    main(String[] args) {
      System.out.println
        (Math.add(2, 3));
  }
}

/**
 * Application entry point
 */
public class Print {
  public static void
    main(String[] args) {
      System.out.println
        (Math.add(2, 3));
  }
}

User A	User B
Edit: ... Driver class ...	-
git commit, git push	-
-	edit: ... Application entry point ...
-	git commit
-	git push: Fail!

>git push ...
To https://gitlab.mi.hdm-stuttgart.de/goik/gitintro.git
 ! [rejected]        master -> master (fetch first)
error: failed to push some refs to
        'https://gitlab.mi.hdm-stuttgart.de/goik/gitintro.git'
hint: Updates were rejected because the remote contains work that you do
hint: not have locally. This is usually caused by another repository pushing
hint: to the same ref. You may want to first integrate the remote changes
hint: (e.g., 'git pull ...') before pushing again.

>git pull 
...
From https://gitlab.mi.hdm-stuttgart.de/goik/gitintro
   b003a82..dbbedb0  master     -> origin/master
Auto-merging Print.java
CONFLICT (content): Merge conflict in Print.java
Automatic merge failed; fix conflicts and then commit the result.

>git diff Print.java
diff --cc Print.java
index fc36ae6,7b27edf..0000000
--- a/Print.java
+++ b/Print.java
@@@ -1,6 -1,4 +1,10 @@@
++<<<<<<< HEAD
 +/**                        ❶
 + * Application entry point
 + */
++=======
+ // Driver class ❷
++>>>>>>> dbbedb0fc29d77beeaaada37f2538d78f82bac93
public class Print {
  public static void
    main(String[] args) {
      System.out.println
        (Math.add(2, 3));
  }
}

<<<<<<< HEAD
/**
 * Application entry point 
 */
=======
// Driver class 
>>>>>>> 10cf21c ... 759462c
public class Print {
  public static void
    main(String[] args) {
      System.out.println
        (Math.add(2, 3));
  }
}

➱

/**
 * Driver class, application entry point 
 */
public class Print {
  public static void
    main(String[] args) {
      System.out.println
        (Math.add(2, 3));
  }
}

>git add Print.java 

>git commit --message "Merging changes" 
[master 173487a] Merging changes

>git push
...
To https://gitlab.mi.hdm-stuttgart.de/goik/gitintro.git
   10cf21c..173487a  master -> master

git distributed, DIY

Overview

Instances of a Class

General class structure

What's a class anyway?

Rectangle objects

A class describing rectangles

Rectangle class and instances

Rectangle class and instances

Rectangle class and instances

Rectangle class and instances

Rectangle class and instances

Generated diagrams

Overview

The new operator: Creating rectangle instances

Syntax creating instances

Assigning attribute values to class instances

Instance memory representation

References and null

Checking for object presence

Overview

Why packages ?

Rules and conventions

Fully qualified class name vs. import

Don't be too lazy!

Special: Classes in package java.lang

Class, package and file system

Source hierarchy view

Overview

Object methods

Scaling a rectangle

Scaling a rectangle

Scaling a rectangle

Scaling method implementation

Scaling method signature

Using the scale(...) method

Method definition syntax

A rectangle's perimeter

A rectangle's perimeter

A rectangle's perimeter

getPerimeter() method implementation

Using Rectangle.getPerimeter()

Followup exercises

Overview

Access control: Overall objectives

Example: Implementing time and date

Access violation

Access rules

“Tips on Choosing an Access Level”

Followup exercises

Overview

Direct access vs. setter method

Why adding setter methods?

Implementation by minutes only

Followup exercise

Overview

Defining type signatures

Type signature examples

Defining method signatures

Method signature examples

Followup exercise

Overview

Method overloading: Same name, different signature

Overloading, alternate names

No overloading in »C«

»C« requires unique function names

No distinction on return type

Method signatures rationale

Method signatures rationale

Followup exercise

Example: System.out.print(...)

Overview

Creating and initializing rectangles

Defining a constructor

Constructor syntax

Constructors

Multiple overloaded constructors

Constructor calls within constructor

Instances by overloaded constructors

No constructor vs. default constructor

Non - default constructor, but no default constructor

The `new` operator: Creating rectangle instances

References and `null`

Fully qualified class name vs. `import`

Using the `scale(...)` method

`getPerimeter()` method implementation

Using `Rectangle`.`getPerimeter()`

`Circle` and variable scopes

`Circle` and variable scopes

`Circle` and variable scopes

`Documenting classes and methods`

`Generated Javadoc`

`Generated Javadoc`

`Generated Javadoc`

`Generated Javadoc`

`Generated Javadoc`

`Refactoring «r» ⇒ «radius»`

`Refactoring «r» ⇒ «radius»`

`Refactoring «r» ⇒ «radius»`

`Scope assignment problem`

`this` overriding method scope

`this` overriding method scope

`this` overriding method scope

`this` overriding method scope

`this` overriding method scope

Finally understanding `System.out.print(ln)`

`helper-0.9.jar` archive content

Using Helper.factorial(...) computing $(\begin{matrix} n \\ k \end{matrix}) = \frac{n!}{k! (n - k)!}$