Object methods

Figure 214. Object methods

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

Figure 215. Get a rectangle's area

‹›

Figure 216. Get a rectangle area method coding

public class Rectangle {

    int width, height;

    int getArea() {
        return width * height;       
    }
}

public static void main(String[] args) {       
   Rectangle  r =  new Rectangle();
   r.width = 20;
   r.height = 30;

   int area = r.getArea();
        
  System.out.println(
    "Area of rectangle is: " + area);
}

Area of rectangle is: 600

Figure 217. Scaling a rectangle

‹›

Figure 218. Scaling method implementation

public class Rectangle {
    int width, height;
 
    int getArea() {...}

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

Rectangle  r =  new Rectangle();
r.width = 20;
r.height = 30;

System.out.println("Area before scaling: "     
      + r.getArea());
r.scale(2);
System.out.println("Area after scaling: "
      + r.getArea());

Area before scaling: 600
Area after scaling: 2400

Figure 219. Scaling method signature

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.

Figure 220. Using the scale(...) method

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

Figure 221. Method definition syntax

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
}

❶	Optional access control modifier either of `public`, `protected` or `private`.
❷	The method's return type either of: `void` The method will not return a value on completion. A data type e.g. `int`, `double`, ... The method will return a value of the given type to its caller.
❸	The method's name.
❹	Arguments being required for execution.
❺	Start of method's body.
❻	The method's implementation.

Figure 222. A rectangle's perimeter

‹›

Figure 223. getPerimeter() method implementation

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

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

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

Figure 224. Using Rectangle.getPerimeter()

Rectangle r = new Rectangle();

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

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

Perimeter=110

exercise No. 89

Compile time error

Try to compile the following snippet:

public int getMinimum(int a, int b) {
  if (a < b) {
    return a;
  } 
}

You'll encounter a “Missing return statement” error. What's wrong here?

On contrary the following code compiles and executes perfectly well:

public class ReturnDemo {

  int minimum;

  public void getMinimum(int a, int b) {
    if (a < b) {
      minimum = a;
      return;
    }
  }
}

The compiler effectively complains about a missing if related path: In case of b <= a no return value has been defined. There are two possible solutions:

public int getMinimum(int a, int b) {
  if (a < b) {
    return a;
  } else {
    return b;
  }
}

Less readable but still producing an identical result:
```
public int getMinimum(int a, int b) {
  if (a < b)
    return a;
  return b;
}
```

On the other hand the return statement in ReturnDemo.getMinimum(...) is unnecessary. The following code produces exactly the same result. The method will terminate anyway when ending the method's body:

public class ReturnDemo {

  int minimum;

  public void getMinimum(int a, int b) {
    if (a < b) {
      minimum = a; // Same as before but no subsequent return statement
    }
  }
}

Albeit executing well the above code is flawed: Calling e.g. getMinimum(4, 3) does not assign any value to our instance variable minimum. Correction requires an else clause:

public class ReturnDemo {

  int minimum;

  public void getMinimum(int a, int b) {
    if (a < b) {
      minimum = a; // Same as before but no subsequent return statement
    } else {
      minimum = b;
    }
  }
}

exercise No. 90

An `Address` class

Consider the following UK postal address sample code and desired execution result:

Code Execution result

final Address address = new Address();

address.name = "Joe";
address.surname = "Simpson";
address.street = "Featherstone Street";
address.number = "49";
address.city = "London";
address.postCode = "EC1Y 8SY";


System.out.println(address.printAddress());

Joe Simpson
49 Featherstone Street
London
EC1Y 8SY

Create an appropriate Address class. Test it using the above code example.

Our Address class requires six attributes of type String and a String printAddress() method:

public class Address {

    String name;
    String surname;
    
    String city;
    String street;
    String number;
    String postCode;

    String printAddress() {
        return name + ' ' + surname + '\n' +
                number + ' ' + street + '\n' +
                city + '\n' +
                postCode;
    }
}

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Object methods

Compile time error

An Address class

An `Address` class