SCJP 1.5 "In order to save memory ??" QUOTE

[Harry Henriques]

Hi,

In Chapter 3, p236 of K & B SCJP 1.5 Certification Text has the following excerpt. Can you tell me how the JVM saves memory with this feature implementation? Thanks.

Integer i1 = 1000;
Integer i2 = 1000;
if(i1 != i2) System.out.println("different objects");
if(i1.equals(i2)) System.out.println("meaningfully equal");

Produces the output:
different objects
meaningfully equal

Integer i3 = 10;
Integer i4 = 10;
if(i3 == i4) System.out.println("same object");
if(i3.equals(i4)) System.out.println("meaningfully equal");

This example produces the output:
same object
meaningfully equal

Yikes! The equals() method seems to be working, but what happened with ==
and != ? Why is != telling us that i1 and i2 are different objects, when == is saying
that i3 and i4 are the same object? In order to save memory, two instances of the
following wrapper objects will always be == when their primitive values are the same:
-- Boolean
-- Byte
-- Character from \u0000 to \u007f (7f is 127 in decimal)
-- Short and Integer from -128 to 127

From K & B SCJP Certification Text Errata, the following is abstracted:

236....clarify...Sentence before bullet points s/b: In order to save memory, two instances of the following wrapper objects, created through autoboxing, will always be equal...

[Brandon Bernie]

It looks like K & B have answered your question in your quoted text. What are you unclear about?

[David Newton]

Because they're the same objects there's only one object instead of two.

[Harry Henriques]

How can i3 and i4 be the same object when I instantiated them separately? i3 is a reference to one object, and i4 is a reference to a different object. They are meaningfully equal. I think I see the reason now, but it isn't that obvious. After incrementing i3, it points to a different wrapper object.

class NotEqualObjects{
public static void main(String [] args)
{
Integer i3 = 10;
Integer i4 = 10;
System.out.println("i3 = " + i3);
System.out.println("i4 = " + i4);
i3++;
System.out.println("i3 = " + i3);
System.out.println("i4 = " + i4);
}
}

[Henry Wong]

How can i3 and i4 be the same object when I instantiated them separately? i3 is a reference to one object, and i4 is a reference to a different object. They are meaningfully equal.

But you did *not* instantiate i3 and i4. You declared i3 and i4. And you used autoboxing to "create" the object. Autoboxing uses the valueOf() method, which may instantiate a new object, or may get you a copy from an internal cache.

Henry

[Srinivasa Chebrolu]

-- Short and Integer from -128 to 127

i3 & i4 assigned values fall with in -128 - 127 and their primitive values same hence i3 == i4.

[Harry Henriques]

Srinivasa,

i3 is a reference to an Integer object, not an int primitive.
i4 is also a reference to an Integer object, not an int primitive.

Integer is a wrapper class.

i3 and i4 are references to the same object, an Integer wrapper object that wraps a primitive with a value of 10. Because i3 and i4 are referencing the same object, the JVM saves memory. Because i3 and i4 are referencing the same object, i3==i4.

Harry

[Jesper de Jong]

Autoboxing works like this: If you write a line such as:

Integer i3 = 10;

The compiler will automatically convert this to:

Integer i3 = Integer.valueOf(10);

Now, the implementation of class Integer contains something smart to avoid making a lot of Integer objects for often used values. Class Integer has a cache of Integer objects, which represent all the values between -128 and 127. When you call the valueOf() method, it first looks if the value that you pass in is between -128 and 127. If it is, then it returns an Integer object from the cache; otherwise, it creates a new Integer object with the value you specified.

If you do something like this:

Integer i3 = 10;
Integer i4 = 10;

This will be converted to:

Integer i3 = Integer.valueOf(10);
Integer i4 = Integer.valueOf(10);

In both these lines, valueOf() will return the same Integer object from its cache. If you compare i3 with i4 by using ==, you will get 'true', because they refer to the same Integer object.

If you would use a value outside the range -128 to 127 instead of 10, the result would be 'false'.

[Jesper de Jong]

Also, note that we have a separate SCJP forum. I will move this topic there for you.