compare objects

[Vicken Karaoghlanian]

class Test {
public static void main(String [] args) {
Test t1 = new Test();
Test t2 = new Test();

String s1 = new String();
String s2 = new String();

System.out.println("t1.equals(t2)? " + t1.equals(t2));
System.out.println("s1.equals(s2)? " + s1.equals(s2));
}
}

output: false
true
isn't supposed that the first output be 'true' since they are both the same object? however the second output is correct as expected.

[Andres Gonzalez]

isn't supposed that the first output be 'true' since they are both the same object?
They might be the same object type, but the reference variables points to different objects.
try this:

class B {
  public static void main(String [] args) 
  {
    B t1 = new B();
    B t2 = t1;  // I changed this!!!  :wink: 
    String s1 = new String();
    String s2 = new String();
    System.out.println("t1.equals(t2)? " + t1.equals(t2));
    System.out.println("s1.equals(s2)? " + s1.equals(s2));
  }
}

What's the result?
[ July 29, 2003: Message edited by: Andres Gonzalez ]

[Vicken Karaoghlanian]

They might be the same object type, but the reference variables points to different objects.

if this is the case then System.out.println("s1.equals(s2)? " + s1.equals(s2)) should output false (which isn't the case)
please also note that the purpose of my question is to understand the different behaviour of the test and String objects
[ July 29, 2003: Message edited by: Vicken Karaoghlanian ]

[Andres Gonzalez]

Ohh.. i see.. sorry
The problem here is with the equals() method. The String class overrides the equals() method from Object, so when you do this:

Test t1 = new Test();
Test t2 = new Test();
t1.equals(t2));

Test is not overriding equals, so

The equals method for class Object implements the most discriminating possible equivalence relation on objects; that is, for any reference values x and y, this method returns true if and only if x and y refer to the same object (x==y has the value true).

http://java.sun.com/j2se/1.4.1/docs/api/java/lang/Object.html#equals
This is the main reason it returns false.
Now, String class overrides equals, so when you do equals() it returns true.

String and wrapper classes override equals to check for values.

From KnB book.
I hope this helps. let me know
[ July 29, 2003: Message edited by: Andres Gonzalez ]

[Anupam Sinha]

Hi Vicken
As Andres pointed out the output is so because the Object's class equals() method compares for object equality by checking if the references to both the objects point to the same thing or not. But in the String class the String class overrides equals() method to provide the equals method with functionality that two strings would be considered equal if their contents are equal.
Code for the Object's equals() method :

public boolean equals(Object obj) {
return (this == obj);
    }

Code for String's equals() method :

    public boolean equals(Object anObject) {
if (this == anObject) {
    return true;
}
if (anObject instanceof String) {
    String anotherString = (String)anObject;
    int n = count;
    if (n == anotherString.count) {
char v1[] = value;
char v2[] = anotherString.value;
int i = offset;
int j = anotherString.offset;
while (n-- != 0) {
    if (v1[i++] != v2[j++])
return false;
}
return true;
    }
}
return false;
    }

[Anupam Sinha]

One more thing the StringBuffer class doesn't overrides the equals() method.