fundamental question about Integer (for example)

[kmansj]

I have a fundamental question about object creation, using an Integer object as an example. This has been bugging me:

Integer x = new Integer(97);

My interpretation of this is, "create an instance x of the class Integer, and call the int constructor to store the value 97."

If I declare again somewhat differently:

Integer y = 97;

I interpret this as, "create an instance y of the class Integer, then store in it the value 97." I'm sure I'm missing some subtlety of the "=," but we'll get to that.

When I put my new instances x and y to the test, they don't quite behave as I expect:

Integer x = new Integer(97);
//x = 97;
Integer y = 97;
if (x == y) {
   System.out.println("x == y");
} else {
   System.out.println("x != y");
}

...
====> x != y

Now if I uncomment the line which assigns 97 to x, I get a different result:
...
====> x == y

So I guess the fundamental question is, why are these 2 things different?

Integer x = new Integer(97);
Integer y = 97;

Does the difference arise from the "=" itself?

Thanks for helping me think this through.

Kman

[Fubarable]

You may be confusing one object being the same object as another vs holding the same value as another. A better test is this one:

    if (x.equals(y)) {
      System.out.println("x.equals(y)");
    } else {
      System.out.println("!x.equals(y)");
    }

[Norm]

Is there any boxing going on here?
Does the compiler generate the new Integer() for you or some such code.

[kmansj]

Fubarable, thanks. Basically you're saying I'm abusing the == operator. I was googling around and ran across an explanation pointing out that == compares references, not values. So my test failed because the references x and y are not the same, rather than their values. I think I get it now.

Thanks again.

Kman

[kmansj]

Norm,

Sorry, not sure what you mean by boxing?

[m00nchile]

Java doesn't like primitive types like ints, doubles and so on. Java likes objects, like Integers, Doubles etc. This is where the process of autoboxing comes in. Note this snippet:

Integer a = 5;

And you think nothing of it. But the compiler looks at this and thinks, oh my, I have an Integer variable, but the value assigned to it is of the type int. What to do, what to do... I know, instead of bothering the programmer about this minutea, I'll simply convert this call to

Integer a = new Integer(5);

Autoboxing is the compiler being lenient towards you by using primitives and their wrapper classes almost interchangably.

[kmansj]

So the compiler is essentially casting the int to Integer when it sees the "=" assignment, right? That's what Norm meant by boxing?

Thanks moon.

-Kman

[curmudgeon]

No, it's not a cast, but rather it's creating a new Integer object even though the code isn't specifically telling the compiler to do this.

[kmansj]

thanks curmudgeon

[Tolls]

Java doesn't like primitive types like ints, doubles and so on. Java likes objects, like Integers, Doubles etc. This is where the process of autoboxing comes in. Note this snippet:

Integer a = 5;

And you think nothing of it. But the compiler looks at this and thinks, oh my, I have an Integer variable, but the value assigned to it is of the type int. What to do, what to do... I know, instead of bothering the programmer about this minutea, I'll simply convert this call to

Integer a = new Integer(5);

Autoboxing is the compiler being lenient towards you by using primitives and their wrapper classes almost interchangably.

It doesn't replace it with a "new" call, but with a call to Integer.valueOf(). This allows it to reuse already defined Integer objects, which is why:

Integer x = 97;
Integer y = 97;
if (x == y) {}

returns true. In fact, this is why you generally shouldn't call "new Integer()", since valueOf() (in cases where you are creating lots of numbers) saves some space.

[JosAH]

Feeling a bit more confident about autoboxing and caching? You won't be after reading this:

import java.util.ArrayList;
import java.util.List;
 
public class Autoboxing {

	public static void main(String[] args) {

		Object foo = new Long(0xcafebabedeadbeefL);
		List<Object> bars = new ArrayList<Object>();
        
		bars.add(true ? (Long)foo : (Long)foo);
		bars.add(true ? (Long)foo : (Number)foo);
		bars.add(true ? (Long)foo : (Double)foo);
		bars.add(true ? (Long)foo : ((Long)foo).longValue());

		System.out.print("==    :");
		for (Object bar : bars)
			System.out.print(" "+(foo == bar));
		System.out.println();

		System.out.print("equals:");
		for (Object bar : bars)
            System.out.print(" "+foo.equals(bar));
		System.out.println();
	}
}

kind regards,

Jos ;-)