Core Classes

Figure 412. Openjdk source code repository

Welcome to the JDK!

Figure 413. Java Visualizer

http://prog.mi.hdm-stuttgart.de/java_visualize

Figure 414. Superclass Object

Superclass of all Java™ classes.
Common methods to be redefined by derived classes.

Figure 415. String literals

‹›

Figure 416. OpenJDK String implementation

Implementation of java.lang.String:

public final class String ... {
  private final char value[];
  private int hash;
  private static final long serialVersionUID = -6849794470754667710L;
...
}

Figure 417. String copy constructor

Code	Output
final String s = "Eve"; ❶ final String sCopy = new String(s); ❷ System.out.println("sCopy == s: " + (sCopy == s)); ❸ System.out.println("sCopy.equals(s): " + sCopy.equals(s)); ❹	sCopy == s: false ❸ sCopy.equals(s): true ❹

Code

Output

final String s = "Eve"; ❶
final String sCopy = new String(s); ❷
System.out.println("sCopy == s: " + (sCopy == s)); ❸
System.out.println("sCopy.equals(s): " + sCopy.equals(s)); ❹

sCopy == s: false ❸
sCopy.equals(s): true ❹

❶	The string literal `"Eve"` corresponds to an instance of `String`. Its reference is being assigned to variable `s`.
❷	A clone carrying the same value `"Eve"` is being created by virtue of the `String` class copy constructor.
❸	We indeed have two distinct objects.
❹	Both `String` objects however carry the same value.

Figure 418. Copy constructor and heap

‹›

Figure 419. Operator == and equals()

Primitive type Object

Primitive type	Object
`// equal values int a = 12, b = 12; System.out.println( "==: " + (a == b)); // No equals(...) method // for primitive types`	`String s1 = new String("Kate"), s2 = new String("Kate"); System.out.println( " ==: " + (s1 == s2)); System.out.println( "equals: " + s1.equals(s2));`
==: true	==: false equals: true

// equal values
int a = 12, b = 12;

System.out.println(
    "==: " + (a == b));
// No equals(...) method
// for primitive types

String 
 s1 = new String("Kate"),
 s2 = new String("Kate");

System.out.println(
  "    ==: " + (s1 == s2));
System.out.println(
  "equals: " + s1.equals(s2));

==: true

    ==: false
equals: true

Figure 420. Remarks == vs. equals()

The == operator acting on primitive types compares expression values.
The == operator acting on objects compares for equality of reference values and thus for object identity.
The == operator acting on objects does not check whether two objects carry semantically equal values.
The equals() method defines the equality two objects.

Figure 421. Operator == and equals() implications

Each object is equal by value to itself:

object1 == object2 ⇒ object1.equals(object2)

The converse is not true. Two different objects may be of common value:

Code	Result
`String s = "Hello", copy = new String(s); System.out.println("equals: " + s.equals(copy)); System.out.println(" ==: " + (s == copy));`	equals: true ==: false

Figure 422. equals() is being defined within respective class!

Implementation at https://github.com/openjdk/ .../String.java :

public final class String ... {
public boolean equals(Object anObject) {
  if (this == anObject) {
    return true;
  }
  return (anObject instanceof String aString)
    && (!COMPACT_STRINGS || this.coder == aString.coder)
    && StringLatin1.equals(value, aString.value);
}

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