A question about "equals"

[Vince Cheung]

Why does the answer is "false"? How do I change it to "true"?

public class A {
    private int num;
    private String name;
 
    public A (int num, String name) {
this.num = num;
this.name = name;
    }
}
 
public class B {
    public B (){
injudge();
    }
 
    public void injudge(){
A aa = new A(25, "zhangsan");
A bb = new A(25, "zhangsan");
if (aa.equals(bb)) {
    System.out.println("true");
} else {
            System.out.println("false");
}
    }
 
    public static void main(String[] args) {
B abc = new B();
    }
}  

[fred rosenberger]

you need to override the equals method your class A inherits from Object.

By default, the inherited method compares (i think) the memory address of the two objects. since you have two distinct objects, they have a different memory space, so they are not equal.

in your class A, create an equals method that does what you want, like

if ((this.num == input.num) && (this.string.equals(input.string))){
   return true;
   }
else
   {
   return false;
   }

[ March 10, 2005: Message edited by: fred rosenberger ]

[Mike Gershman]

To override Object.equals(Object) in accordance with its contract, you need to do a little more.

public boolean equals(Object obj) {
    if ( ! obj instanceof A || name == null )
       return false;
    else
        return num == obj.num && name.equals( obj.name );
}

Note that passing equals(Object) an argument that is null or is a different type is legal and returns false. This is different from compareTo(Object), which throws a ClassCastException if the argument is not the same type and a NullPointerException if the argument is null.

[fred rosenberger]

sorry my answer was incomplete. i was trying to get something helpful up quickly, and... well... i just plead being tired.

[James Carman]

Also, remember to override hashCode() when you override equals().

[Chengwei Lee]

Hi Vince,

The equals method in your class needs to be implemented in order for it to make sense. By default, it doesn't contains any implementation, since you could have so many different kinds of objects & to test for equality, many require different criterias.

Hence, it is up to the developer to implement their own equals method if they wish to use it.

To make sense & see how to do it, read the Effective Java by Joshua Bloch. Sun had provided a few sample chapters & one of them shows to how to override a few important methods such as equals, hashCode, toString & clone.

Or if you just want to get things done, try using Jakarta Commons Lang.

HTH.