The string constant pool

String a = "java";
String b = "java";
String c = new String("java");
String d = c.intern();

a == b;   // true  — same pooled literal
a == c;   // false — new String() always allocates
a == d;   // true  — intern() returns the pooled instance

Constant expressions are folded at compile time, so "ja" + "va" is also pooled and == "java" is true. Concatenation involving a non-final variable happens at runtime and is not.

Why it matters

Identity comparison on strings is a bug that passes its own unit tests, because the test data is usually literals. Always compare with equals.