Why Does `==` Sometimes Fail to Compare `Integer` Objects Correctly in Java?

The Mystery of Integer Equality in Java

As Java introduced autoboxing in version 1.5, the use of Integer objects became interchangeable with primitive ints. However, it has been observed that using == for comparison of Integer objects may lead to unexpected results.

In a perplexing situation, code utilizing == for comparing Integer variables was found to incorrectly set a boolean flag as true when the values were in fact equal. However, switching the comparison to !cdiCt.equals(cdsCt) resolved the issue.

This unusual behavior arises from the JVM's caching mechanism for Integer values. For values within the range of -128 to 127, the JVM reuses cached instances, ensuring efficient memory utilization. Therefore, integers in this range appear as identical regardless of their actual references.

When using == to compare Integer objects, the reference equality is checked rather than the content. Therefore, if the cached instances are used, the comparison returns true even when the object references are distinct.

This issue does not occur when using the equals() method because it compares the values of the objects, disregarding the references. As a result, it is recommended to use equals() for comparison of Integer objects, especially when the range of values may extend beyond the cached range or when precise equality determination is required.

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