Whizlabs question - Generics/Wrapper classes

[Vyas Sanzgiri]

In chapter 3 (K&B Java 5) Page 236 it says that in order to save memory 2 instances of following wrapper objects will always be == when their primitive values are the same: Boolean, Byte, Character from \u000 to \u007f, Short and Integer from -128 to 127. So why does the first comparison return false and second returns true.

I checked the classes and both return java.lang.Integer. Both return values 18 (within range).

   
import java.util.*;

class Gen<T>
{
    T obj;
    Gen(T o) {
        obj=o; }
    T get() {
        return obj;
    }
}

class Generics20 {
    public static void main(String[] args) {
        Gen<Integer> i1=new Gen<Integer>(new Integer(18)); // line1
        Gen<Integer> i2=new Gen<Integer>(18); //line2
        System.out.println((i1.get()).getClass());
        System.out.println((i2.get()).getClass());
        System.out.println(i1.get()==i2.get()); //line 3
        System.out.println(i1.get().intValue()==i2.get().intValue()); //line 4
        Integer i3 = 18;
        Integer i4 = 18;
        if (i3 == i4) System.out.println("they are equal");
    }
}
 

Output:

class java.lang.Integer
class java.lang.Integer
false
true
they are equal

[Henry Wong]

In chapter 3 (K&B Java 5) Page 236 it says that in order to save memory 2 instances of following wrapper objects will always be == when their primitive values are the same: Boolean, Byte, Character from \u000 to \u007f, Short and Integer from -128 to 127. So why does the first comparison return false and second returns true.

I think you may have taken the sentence out of context... This is only true if autoboxing (or the valueOf(int) method) is used. In your example, at line #1, you are *not* using autoboxing, so the instance is not the same instance that is used at line 2 (where autoboxing is used).

If both line #1 and line #2 used autoboxing, then the statement would be true.

Henry

[Vyas Sanzgiri]

ok!! Now I understand it is the pool concept as in String