Reference equality uses Java’s == operator to ask whether two references denote the same object. It is not the usual choice for comparing object contents: equals() may report equivalent objects even when == is false. section 7.0 section 7.1
The boxed-integer trap
Integer i1 = 100; Integer i2 = 100; System.out.println(i1 == i2); // often true Integer i3 = 1000; Integer i4 = 1000; System.out.println(i3 == i4); // may be false Autoboxing commonly reuses cached Integer objects for values from -128 through 127. Larger values may create separate objects, so code that accidentally relies on == can change behavior when its data changes. section 7.1
Avoid accidental identity comparisons with these rules:
- Use equals() when comparing object values or content. section 7.0 section 7.1
- Do not infer correctness from a true result with small boxed values; caching can conceal the mistake. section 7.1
- Remember that Boolean and Byte values are cached during autoboxing, while explicitly constructed wrapper objects can still be distinct. section 7.1
Integer i1 = 100; Integer i2 = 100; System.out.println(i1 == i2); #1
Inspect the minimal boxed-integer example that demonstrates why reference equality can appear to work. section 7.1
