Integer comparison using == gets tricky between -128 to 127.

• Integer i=45;                                                                                                                                   Integer k=45;

        System.out.println("i.hashcode= "+i.hashCode());
        System.out.println("k.hashcode= "+k.hashCode());
        System.out.println(i==k);
        System.out.println(i.equals(k));

Out Put:

i.hashcode= 45
k.hashcode= 45
true
true

Great, it seems work fine! now change the value of i=k=201.
        Integer i=201;
        Integer k=201;
        System.out.println("i.hashcode= "+i.hashCode());
        System.out.println("k.hashcode= "+k.hashCode());
        System.out.println(i==k);
        System.out.println(i.equals(k));

OutPut:
i.hashcode= 201
k.hashcode= 201
false // System.out.println(i==k); 
true 

The reason behind this is the IntegerCache, a private static inner class in Integer class. This was feature was introduced in java 1.5 for performance improvement. 

An implementation  of flyweight design pattern . A flyweight is an object that minimises memory use by sharing as much data as possible with other similar objects.

 

so, integer  cache help auto boxing between the range [-128 to 127].

Now how auto boxing  happen?  When we do the following in java 1.5 onwards

Integer K=23;

program flow goes to following method of Integer class

public static Integer valueOf(int i) {

        if (i >= IntegerCache.low && i <= IntegerCache.high)

            return IntegerCache.cache[i + (-IntegerCache.low)];

        return new Integer(i);

    }

so , above code checks if the int i lies between [-128 to 127] then the object from integer cache else it creates a new integer object using new operator.

That’s why checking equality using == operator only holds between the range -128 to 127.

one exception to above rule; if we create object using new operator and even if the integer lies between [-128 to 127] equality using == operator will not hold true when values are same.

Caching also supported by java.lang.Long and java.lang.Character
in Character class cache array hold 128 objects(The ASCII table) unlike Integer and Long.