Equal below one hundred, unequal above
A medium Java puzzle in the equality and identity family. One line below contains a real bug. Try to spot it first, then read why it fails and how to fix it.
Highlighted: line 3. Play this one as a puzzle without the answer.
What is wrong
Both parameters are Integer objects, so == compares references. Autoboxing reuses cached objects for values from -128 to 127, which makes small numbers appear equal, while larger values create separate objects and compare false.
The fix
How to avoid it next time
Never compare wrapper objects with ==. Use equals, or unbox to int when both sides are known to be non-null.
Want the background? Read Autoboxing and wrapper types in the Java concepts library.
About equality and identity
In Java == compares primitive values but only compares references for objects. Strings, boxed numbers and enums behave differently, and a class that overrides equals without hashCode quietly breaks every hash-based collection.
In code review: Whenever you see == between two objects, stop and ask whether identity was really intended. Use equals or Objects.equals, and look for equals and hashCode written as a pair.
More puzzles in this family
Frequently asked questions
What is the bug in "Equal below one hundred, unequal above"?
Both parameters are Integer objects, so == compares references. Autoboxing reuses cached objects for values from -128 to 127, which makes small numbers appear equal, while larger values create separate objects and compare false. The faulty line is line 3: return ordered == shipped;
How do I fix it?
Replace it with: return ordered.equals(shipped);. Never compare wrapper objects with ==. Use equals, or unbox to int when both sides are known to be non-null.
How do I catch equality and identity in code review?
Whenever you see == between two objects, stop and ask whether identity was really intended. Use equals or Objects.equals, and look for equals and hashCode written as a pair.
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