Why Does Java's Integer Constant Pool Behave Differently Above 127?

Question: Divergence in Java's Integer Constant Pool Behavior at 127

Introduction:

The Integer constant pool is a mechanism in Java that optimizes the caching of common integer values to improve performance. However, the behavior of this pool exhibits a change at 127, giving rise to confusion among developers.

Understanding the Behavior:

For integers ranging from -128 to 127, Java guarantees identical references for variables referencing the same constant.

Integer i1 = 127;
Integer i2 = 127;
System.out.println(i1 == i2); // True

This is because these values are cached in the constant pool.

Divergence at 128:

However, when the integer value exceeds 127, the behavior changes.

Integer i1 = 128;
Integer i2 = 128;
System.out.println(i1 == i2); // False

This is due to an implementation optimization. While the JLS guarantees caching for values up to 127, beyond that range, implementations may choose to cache values differently or even not at all.

Additional Considerations:

Even within the guaranteed range (-128 to 127), the caching behavior is not always reliable. For example, using new Integer() can bypass the constant pool and create new references.

Integer i1 = new Integer(127);
Integer i2 = new Integer(127);
System.out.println(i1 == i2); // False

Conclusion:

The behavior of the Integer constant pool at 127 stems from implementation optimizations and the guaranteed range defined by the JLS. Developers should be aware of this change in behavior to avoid unexpected results.

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