Weird Integer boxing in Java

Question

I just saw code similar to this:

public class Scratch
{
    public static void main(String[] args)
    {
        Integer a = 1000, b = 1000;
        System.o         


        

Answer 1

My guess is that Java keeps a cache of small integers that are already 'boxed' because they are so very common and it saves a heck of a lot of time to re-use an existing object than to create a new one.

Answer 2

Yes, there is a strange autoboxing rule that kicks in when the values are in a certain range. When you assign a constant to an Object variable, nothing in the language definition says a new object must be created. It may reuse an existing object from cache.

In fact, the JVM will usually store a cache of small Integers for this purpose, as well as values such as Boolean.TRUE and Boolean.FALSE.

Answer 3

public class Scratch
{
   public static void main(String[] args)
    {
        Integer a = 1000, b = 1000;  //1
        System.out.println(a == b);

        Integer c = 100, d = 100;  //2
        System.out.println(c == d);
   }
}

Output:

false
true

Yep the first output is produced for comparing reference; 'a' and 'b' - these are two different reference. In point 1, actually two references are created which is similar as -

Integer a = new Integer(1000);
Integer b = new Integer(1000);

The second output is produced because the JVM tries to save memory, when the Integer falls in a range (from -128 to 127). At point 2 no new reference of type Integer is created for 'd'. Instead of creating a new object for the Integer type reference variable 'd', it only assigned with previously created object referenced by 'c'. All of these are done by JVM.

These memory saving rules are not only for Integer. for memory saving purpose, two instances of the following wrapper objects (while created through boxing), will always be == where 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

Answer 4

Class Integer contains cache of values between -128 and 127, as it required by JLS 5.1.7. Boxing Conversion. So when you use the == to check the equality of two Integers in this range, you get the same cached value, and if you compare two Integers out of this range, you get two diferent values.

You can increase the cache upper bound by changing the JVM parameters:

-XX:AutoBoxCacheMax=<cache_max_value>

or

-Djava.lang.Integer.IntegerCache.high=<cache_max_value>

---

See inner IntegerCache class:

/**
 * Cache to support the object identity semantics of autoboxing for values
 * between -128 and 127 (inclusive) as required by JLS.
 *
 * The cache is initialized on first usage.  The size of the cache
 * may be controlled by the {@code -XX:AutoBoxCacheMax=<size>} option.
 * During VM initialization, java.lang.Integer.IntegerCache.high property
 * may be set and saved in the private system properties in the
 * sun.misc.VM class.
 */

private static class IntegerCache {
    static final int low = -128;
    static final int high;
    static final Integer cache[];

    static {
        // high value may be configured by property
        int h = 127;
        String integerCacheHighPropValue =
            sun.misc.VM.getSavedProperty("java.lang.Integer.IntegerCache.high");
        if (integerCacheHighPropValue != null) {
            try {
                int i = parseInt(integerCacheHighPropValue);
                i = Math.max(i, 127);
                // Maximum array size is Integer.MAX_VALUE
                h = Math.min(i, Integer.MAX_VALUE - (-low) -1);
            } catch( NumberFormatException nfe) {
                // If the property cannot be parsed into an int, ignore it.
            }
        }
        high = h;

        cache = new Integer[(high - low) + 1];
        int j = low;
        for(int k = 0; k < cache.length; k++)
            cache[k] = new Integer(j++);

        // range [-128, 127] must be interned (JLS7 5.1.7)
        assert IntegerCache.high >= 127;
    }

    private IntegerCache() {}
}

Answer 5

In Java the boxing works in the range between -128 and 127 for an Integer. When you are using numbers in this range you can compare it with the == operator. For Integer objects outside the range you have to use equals.

Answer 6

Integer Cache is a feature that was introduced in Java Version 5 basically for :

1. Saving of Memory space
2. Improvement in performance.

Integer number1 = 127;
Integer number2 = 127;

System.out.println("number1 == number2" + (number1 == number2); 

OUTPUT: True

---

Integer number1 = 128;
Integer number2 = 128;

System.out.println("number1 == number2" + (number1 == number2);

OUTPUT: False

HOW?

Actually when we assign value to an Integer object, it does auto promotion behind the hood.

Integer object = 100;

is actually calling Integer.valueOf() function

Integer object = Integer.valueOf(100);

Nitty-gritty details of valueOf(int)

    public static Integer valueOf(int i) {
        if (i >= IntegerCache.low && i <= IntegerCache.high)
            return IntegerCache.cache[i + (-IntegerCache.low)];
        return new Integer(i);
    }

Description:

This method will always cache values in the range -128 to 127, inclusive, and may cache other values outside of this range.

When a value within range of -128 to 127 is required it returns a constant memory location every time. However, when we need a value thats greater than 127

return new Integer(i);

returns a new reference every time we initiate an object.

== operators in Java compares two memory references and not values.

Object1 located at say 1000 and contains value 6.
Object2 located at say 1020 and contains value 6.

Object1 == Object2 is False as they have different memory locations though contains same values.