Boxing, unboxing and the Integer cache
Autoboxing silently converts between primitives and wrappers. Values from -128 to 127 come from a shared cache, so == appears to work on small numbers and fails on large ones.
Integer a = 127, b = 127;
Integer c = 128, d = 128;
a == b; // true — both from the cache
c == d; // false — two separate objectsInteger.valueOf caches −128…127 by specification. Boolean, Byte,
Character up to 127, Short and Long cache too; Float and Double
never do.
The three real hazards
Unboxing a null. map.get(missingKey) returns null, and assigning it to
an int throws NullPointerException at a line that contains no visible
method call.
Map<String, Integer> counts = new HashMap<>();
int n = counts.get("absent"); // NPEMixed comparison. Integer == int unboxes and compares numerically;
Integer == Integer compares references. One character changes the meaning.
Silent allocation in loops. A Long accumulator in a tight loop boxes on
every iteration. Declare accumulators as primitives.
Rule
Use primitives unless you need null, a collection element, or a generic type
argument. Compare wrappers with equals, or call intValue() first.