Why does Java's Constant Pool treat Integer values differently for numbers greater than 127?

Constant Pool Behavior for Integers: Understanding the Numerical Boundaries

Traditionally, Java's Constant Pool is known to treat String literals similarly for identical values. However, the behavior of the Constant Pool for Integers introduces a subtle distinction. While it holds true for values up to 127, it differs when the value exceeds this threshold.

To understand this disparity, it is crucial to recognize the difference between Integer wrapper objects and primitive int values. When you declare an integer literal within the range of -128 to 127, it is automatically interned in the Constant Pool. Consequently, two variables assigned to the same value, such as "Integer i1 = 127; Integer i2 = 127;", will point to the same object reference, leading to the expected result of "i1 == i2" evaluating to true.

However, boxing an int value into an Integer object changes this dynamic entirely. Unlike Strings, the Constant Pool for Integers only caches literal values, meaning that the boxing operation itself initiates an allocation in the Java heap. Consider the following snippet: "Integer i1 = new Integer(127); Integer i2 = new Integer(127);". In this case, although both variables represent the same value, they are not interned in the Constant Pool and are thus distinct objects. This distinction is reflected in "i1 == i2" evaluating to false.

Now, the intrigue arises when integer literals cross the 127 threshold. At this point, the Constant Pool no longer interns the values, regardless of whether they are boxed or not. This is evident in the example: "Integer i1 = 128; Integer i2 = 128;", where both variables refer to different object instances and "i1 == i2" evaluates to false.

The justification for this change in behavior lies in the implementation pragmatics outlined in the Java Language Specification (JLS). While the JLS guarantees memory optimization for a small range of pooled values, it allows for a wider range at the discretion of the implementation. Most Java Virtual Machines (JVMs) opt to utilize a wider range, leveraging techniques like lazy or eager caching, resulting in performance enhancements without compromising behavioral expectations for typical use cases.

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