equals() vs ==

[Faraah Dabhoiwala]

Hello,

Actually i wanted to know the exact difference between equals and ==

as far as i have understood from scjp 5 K & B,,

i have concluded that equals() compares values whereas == compares values as well as object type..

but when i run code it gives unexpected answers everytime....

can anyone please explain me what is the exact meaning of these two??

[John Bengler]

Hi Shadab,

if I want to formulate it in a short statement like your's I would say:

equals() compares values and object type whereas == compares the object instance (the adress in memory).


You can control the result of equals() by overriding your classes hashCode() and equals() methods.


You can find more on this if you search the forum for "equals".



John

[Faraah Dabhoiwala]

class Equals
{
	public static void main(String args[])
	{
		Integer I=4;
		int i=4;
		Double D=4.0;
		double d=4.0;
		
		System.out.println(i==4);
		System.out.println(I==4.0);
		System.out.println(d==4.0);
		System.out.println(D==4.0);
		//System.out.println(i.equals(4));
		System.out.println(I.equals(4));
		//System.out.println(d.equals(4));
		System.out.println(D.equals(4);
		System.out.println(i==I);
		System.out.println(i==d);
		System.out.println(i==D);
		System.out.println(I==d);	
		//System.out.println(I==D);
		System.out.println(D==d);
		
	}

}

   

output

  
true
true
true
true
true
false
true
true
true
true
true


  

Here it gives true for i==D ,I==d,i==I,d==D and so on....


and the lines in comment shows compile time error....if i remove the comments

[Akanksha Mittal]

  //System.out.println(i.equals(4));   
        System.out.println(I.equals(4));   
        //System.out.println(d.equals(4));   
        System.out.println(D.equals(4);    

Since Integer and Double override equals method, thats why the uncommented lines work and commented do not.

[Jesper de Jong]

Because variable d is a double (primitive type), and not a Double (instance of wrapper class).
You can't call methods on primitive types, therefore d.equals(...) does not work.

The same holds ofcourse for i, which is an int (primitive type) and not an Integer (object).

[Faraah Dabhoiwala]

Thanks for solving my second doubt but my 1st doubt was difference between equals () and ==

[Jesper de Jong]

When you use the == operator on two values, it will check if the two values refer to the exact same object. Note that if you have to different objects that have the same value, == will return false. For example:

Integer a = new Integer(200);
Integer b = new Integer(200);

// Prints 'false' because a and b refer to two distinct Integer objects
System.out.println(a == b);

When used on variables of a primitive type, == will compare values. For example:

int a = 200;
int b = 200;

// Prints 'true'
System.out.println(a == b);

The equals() method is just a normal method, that you can override in your own classes. It's supposed to compare two objects by value - it should return true when the value of the two objects is the same (and false otherwise).

[Faraah Dabhoiwala]

Thanks...

ya but


when i did System.out.println(i==I);

it returned true why so??

Here i is primitive type and I is an Object of type Integer

so as far as i understood we are not comparing objects hence here it should return false !!

[Deepak Borania]

Shadab Wadiwala wrote:Thanks...

ya but


when i did System.out.println(i==I);

it returned true why so??

I think the value in 'I' is unboxed and is then compared with the primitive 'i' . Hence true.
I can't think of any other reason, why it should return true.

[Tanmoy Dhara]

Why the rules of auto-boxing and auto-unboxing didn't apply to equals method??(code is given in the third post of this topic thread by the starter). When he was saying -
int i=4;
System.out.println(i.equals(4));

why i is not automatically converted to an object of Integer class ?

[Deepak Borania]

Because 'i' is primitive with no methods whatsoever.
However you can box it yourself to get the desired result like this :

 
int i =4;
Integer I= new Integer(4);
System.out.println (((Integer)i).equals(4));   //prints true
//However following is an campile time error
//System.out.println (i.equals(4));
   

JLS conversions

[Tanmoy Dhara]

System.out.println (((Integer)i).equals(4)); //prints true

in the JLS link you provided,only boxing conversions are clarified(no mention of autoboxing). So, i still don't understand that why (Integer)i part of the above mentioned statement you wrote, can not be done automatically as far as J2SE 5.0 or later versions are concerned.