Equals method.

[Mubeen Shaik]

Hi All,

I am not understandng thic concept. Why it is giving "False" as result.
This code is from Thinking in Java.

In the same code if i remove static from this code it is giving me error message. Please explain this concept also.

static Test monitor = new Test();

 
 
//: c03:EqualsMethod2.java
import com.bruceeckel.simpletest.*;
 
class Value {
  int i;
}
 
public class EqualsMethod2 {
  static Test monitor = new Test();
  public static void main(String[] args) {
    Value v1 = new Value();
    Value v2 = new Value();
    v1.i = v2.i = 100;
    System.out.println(v1.equals(v2));
    monitor.expect(new String[] {
      "false"
    });
  }
} ///:~
 
 

Thanks in advance,
Mubeen Shaik.

[Mubeen Shaik]

Hi,

I think i have posted this in wrong place. Could you please move this to Java Intemediate forum.


Thanks,
Mubeen Shaik.

[Tim McGuire]

As the book says, the default behavior of the equals() method is to compare references. v1 and v2 are referencing different objects.

equals is inherited from Object and if you want to tell the method to compare values, then you have to override (write your own method of the same name with the same arguments) with something like:

class Value {
   int i;
   public boolean equals(Value v){
        if(v.i == this.i){
           return true;
        } else {
           return false;
        }
   }
}    

[ May 19, 2004: Message edited by: Tim McGuire ]

[John Smith]

public boolean equals(Value v){

A small but important correction: the signature of the equals() method is public boolean equals(Object o). In the form quoted above, the equals() method is overloaded, not overriden. As a result, you will see some strange effects. For example:

        Object v1 = new Value();
        Value v2 = new Value();
 
        System.out.println(v1.equals(v2)); // expected "true", but returns "false"
        System.out.println(v2.equals(v1)); // expected "true", but returns "false"
 

[Tim McGuire]

ooops!
thanks for pointing that out.

[Mubeen Shaik]

Hi Eugene,

I am confused here, then how to Override the method?

Thanks,
Mubeen Shaik.

[Stefan Wagner]

overload:

public boolean equals (Value v)
{
if (v.i == i)
{
return true;
} 
return false;
}

overwrite:

  
public boolean equals (Object o)
{
Value v = null;
try
{
v = (Value) o;
}
catch (ClassCastException cce)
{
return false;
}
 
if (v.i == i)
{
return true;
} 
return false;
}

[John Smith]

Or simply

    public boolean equals(Object o) {
        return (o instanceof Value && ((Value) o).i == i);
    }

[Manish Hatwalne]

And don't forget to override hashcode method if you override equals.

- Manish

[Tim McGuire]

So, getting equals() right and overriding the hashCode() method could look like this?:

class Value{
    int i;
 
    public boolean equals(Object o){
 
        if(o instanceof Value){
            Value v = (Value)o;
            return(v.i == i);
        }
        return false;
    }
 
    public int hashCode(){
        return String.valueOf(i).hashCode();
    }
}
 
public class EqualsMethod2 {
    public static void main(String[] args) {
        Value v1 = new Value();
        Value v2 = new Value();
        v1.i = v2.i = 100;
        System.out.println(v1.equals(v2));
    }
}

[John Smith]

Tim -- yes, that looks better. The hashCode() method could be simplified, though:

    public int hashCode() {
        return i;
    }

Also, a better test would be:

        Object v1 = new Value();
        Value v2 = new Value();
 
        System.out.println(v1.equals(v2));
        System.out.println(v2.equals(v1));

There is much more to equals() and hashCode(). The best reference on this subject (and many others) is Effective Java
[ May 21, 2004: Message edited by: Eugene Kononov ]