Identity vs Equality

Identity vs equality

== on objects asks: is this the same object in memory? (identity) .equals() asks: do these two objects represent the same value? (equality)

String a = new String("hi");
String b = new String("hi");
a == b        // false — two distinct objects
a.equals(b)   // true  — same value

The JVM caches small strings and small Integer boxes, so == can accidentally return true for equal values — until it doesn't. Never use == for value comparison on objects. Primitives (int, long, ...) are the exception: == is correct there because there is no object identity.

Integer x = 127, y = 127;
Integer p = 128, q = 128;
x == y   // true — cached box (do not rely on this!)
p == q   // false — new boxes above 127

This is the classic intermediate trap: code works in tests, breaks in production with bigger numbers.