equals() problem

[Nitesh Nandwana]

class InstEqual 
{	
	private int value;
	InstEqual(int value){
		 this.value=value;
	}
	public static void main(String[] args) 
	{
		InstEqual obj1=new InstEqual(1);
		InstEqual obj2=new InstEqual(1);
		
		if(obj1.equals(obj2))
		{
			System.out.println("objects are equal");
		}
		else
		{
			System.out.println("objects are not equal");
		}
	}
}

I'm not getting,why objects are not equal here

[Zandis Murāns]

You mums override equals method and do your comparing stuff there.

[Nitesh Nandwana]

ok

[Matthew Brown]

To give a bit more detail: since you don't override the equals method, InstEqual inherits the equals method from the Object class. That just checks to see if they are the same object (obj1 == obj2). If you want equality to be defined in a different way (for instance, according to obj.value in this case), then you override the method to define it.

And if you override equals(), you really ought to override hashCode() as well to be consistent.

[Nitesh Nandwana]

Matthew Brown wrote: if you override equals(), you really ought to override hashCode() as well to be consistent.

class InstEqual 
{
	private int value;
	InstEqual(int value){
		 this.value=value;
	}
	public boolean equals(Object obj){
		InstEqual m=(InstEqual)obj;
		if(this.value==m.value)
		{
			return true;
		}
		else
			return false;
	}

	
	public static void main(String[] args) 
	{
		InstEqual obj1=new InstEqual(12);
		InstEqual obj2=new InstEqual(2);
		System.out.println(obj1.equals(obj2));
	}
}

But i am not getting why to override hashCode() and what is need here because my program is working well.

[Matthew Brown]

Without overriding hashCode() you'll fine, until you do something that relies on it. Most likely, that will involve putting your objects in a HashMap or HashSet. Then it won't work quite as you expected. If you look at the documentation for Object.equals() and Object.hashCode() it explains how they are supposed to work.

I've got a couple of comments about your equals method:

1. You should check obj instanceof InstEqual before casting it, to avoid the possible exception. The should return false if the cast would fail.

2. Instead of:

if(this.value==m.value) {  
    return true;  
}  
else {  
    return false;
}

it's easier to read if you just use:

return (this.value == m.value);

[Nitesh Nandwana]

Matthew Brown wrote:Without overriding hashCode() you'll fine, until you do something that relies on it. Most likely, that will involve putting your objects in a HashMap or HashSet. Then it won't work quite as you expected. If you look at the documentation for Object.equals() and Object.hashCode() it explains how they are supposed to work.

I've got a couple of comments about your equals method:

1. You should check obj instanceof InstEqual before casting it, to avoid the possible exception. The should return false if the cast would fail.

2. Instead of:

if(this.value==m.value) {  
    return true;  
}  
else {  
    return false;
}

it's easier to read if you just use:

return (this.value == m.value);

thank you matthew for detailed explanation and i really like that shortcut

[Javin Paul]

1. always override hashcode if you override equals.
2. follow all contracts of equals specified in javadoc.
3. if you are implementing comparable then make sure equals() and compareTo() must be compatible to each other . ie. compareTo() must return zero if two object are equal with equals method.

you can also check my post How to correctly override equals in java

[Nitesh Nandwana]

Javin Paul wrote:
thank you Javin for help