Strings in Java : equals vs == [duplicate]

Answer 1

You should be using .equals and not == when comparing java strings.

== Compares references and therefore s2.ToString() returns s2.

From the java glossary about heap / stack:

In Sun’s JVM, the interned Strings (which includes String literals) are

stored in a special pool of RAM called the perm gen, where the JVM also loads classes and stores natively compiled code. However, the intered Strings behave no differently than had they been stored in the ordinary object heap.

Answer 2

Given that == compares references, you can see from s2 == s3 being true that s2.toString() returns s2.

Answer 3

The method String.intern() can be used to ensure that equal strings have equal references. String constants are interned, so s1 and s2 will reference the same string. String.toString() simply returns itself, that is, a.toString() returns a, when a is a String. So, s2 also == s3.

In general, strings should not be compared by reference equality, but by value equality, using equals(). The reason is that it's easy to get two strings that are equivalent but different references. For example, when creating substrings. An exception to this rule is that if you know both strings have been interned beforehand (or you intern them as part of the comparison.)

To answer your implied question about heap or stack, Strings are allocated on the heap. Even if they were allocated on the stack, such as with the upcoming escape analysis and stack allocation, the semantics of the program will not change, and you will get the same result for both heap and stack allocation.