Weird Integer boxing in Java

I just saw开发者_StackOverflow中文版 code similar to this:

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

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

When ran, this block of code will print out:

false
true

I understand why the first is false: because the two objects are separate objects, so the == compares the references. But I can't figure out, why is the second statement returning true? Is there some strange autoboxing rule that kicks in when an Integer's value is in a certain range? What's going on here?

The true line is actually guaranteed by the language specification. From section 5.1.7:

If the value p being boxed is true, false, a byte, a char in the range \u0000 to \u007f, or an int or short number between -128 and 127, then let r1 and r2 be the results of any two boxing conversions of p. It is always the case that r1 == r2.

The discussion goes on, suggesting that although your second line of output is guaranteed, the first isn't (see the last paragraph quoted below):

Ideally, boxing a given primitive value p, would always yield an identical reference. In practice, this may not be feasible using existing implementation techniques. The rules above are a pragmatic compromise. The final clause above requires that certain common values always be boxed into indistinguishable objects. The implementation may cache these, lazily or eagerly.

For other values, this formulation disallows any assumptions about the identity of the boxed values on the programmer's part. This would allow (but not require) sharing of some or all of these references.

This ensures that in most common cases, the behavior will be the desired one, without imposing an undue performance penalty, especially on small devices. Less memory-limited implementations might, for example, cache all characters and shorts, as well as integers and longs in the range of -32K - +32K.

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

Integer objects in some range (I think maybe -128 through 127) get cached and re-used. Integers outside that range get a new object each time.

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.

Furthermore, == operators in Java is used to 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.

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.

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.

That is an interesting point. In the book Effective Java suggests always to override equals for your own classes. Also that, to check equality for two object instances of a java class always use the equals method.

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

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

returns:

true
true

Direct assignment of an int literal to an Integer reference is an example of auto-boxing, where the literal value to object conversion code is handled by the compiler.

So during compilation phase compiler converts Integer a = 1000, b = 1000; to Integer a = Integer.valueOf(1000), b = Integer.valueOf(1000);.

So it is Integer.valueOf() method which actually gives us the integer objects, and if we look at the source code of Integer.valueOf() method we can clearly see the method caches integer objects in the range -128 to 127 (inclusive).

/**
 * Returns an {@code Integer} instance representing the specified
 * {@code int} value.  If a new {@code Integer} instance is not
 * required, this method should generally be used in preference to
 * the constructor {@link #Integer(int)}, as this method is likely
 * to yield significantly better space and time performance by
 * caching frequently requested values.
 *
 * This method will always cache values in the range -128 to 127,
 * inclusive, and may cache other values outside of this range.
 *
 * @param  i an {@code int} value.
 * @return an {@code Integer} instance representing {@code i}.
 * @since  1.5
 */
public static Integer valueOf(int i) {
    if (i >= IntegerCache.low && i <= IntegerCache.high)
        return IntegerCache.cache[i + (-IntegerCache.low)];
    return new Integer(i);
}

So instead of creating and returning new integer objects, Integer.valueOf() the method returns Integer objects from the internal IntegerCache if the passed int literal is greater than -128 and less than 127.

Java caches these integer objects because this range of integers gets used a lot in day to day programming which indirectly saves some memory.

The cache is initialized on the first usage when the class gets loaded into memory because of the static block. The max range of the cache can be controlled by the -XX:AutoBoxCacheMax JVM option.

This caching behaviour is not applicable for Integer objects only, similar to Integer.IntegerCache we also have ByteCache, ShortCache, LongCache, CharacterCache for Byte, Short, Long, Character respectively.

You can read more on my article Java Integer Cache - Why Integer.valueOf(127) == Integer.valueOf(127) Is True.

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.

If we check the source code of the Integer class, we can find the source of the valueOf method just like this:

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

This explains why Integer objects, which are in the range from -128 (Integer.low) to 127 (Integer.high), are the same referenced objects during the autoboxing. And we can see there is a class IntegerCache that takes care of the Integer cache array, which is a private static inner class of the Integer class.

There is another interesting example that may help to understand this weird situation:

public static void main(String[] args) throws ReflectiveOperationException {
    Class cache = Integer.class.getDeclaredClasses()[0];
    Field myCache = cache.getDeclaredField("cache");
    myCache.setAccessible(true);

    Integer[] newCache = (Integer[]) myCache.get(cache);
    newCache[132] = newCache[133];

    Integer a = 2;
    Integer b = a + a;
    System.out.printf("%d + %d = %d", a, a, b); // The output is: 2 + 2 = 5
}

In Java 5, a new feature was introduced to save the memory and improve performance for Integer type objects handlings. Integer objects are cached internally and reused via the same referenced objects.

1. This is applicable for Integer values in range between –127 to +127 (Max Integer value).

2. This Integer caching works only on autoboxing. Integer objects will not be cached when they are built using the constructor.

For more detail pls go through below Link:

Integer Cache in Detail

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() {}
}