Question

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String comparison and String interning in Java

I understand how String equals() method works but was surprised by some results I had with the String == operator.

I would have expected == to compare references as it does for other objects.

However distinct String objects (with the same content) == returns true and furthermore even for a Static String object (with the same content) which is obviously not the same memory address.

I guess == has been defined the same as equals to prevent its misuse

Answer 1

No, == does just compare references. However, I suspect you’ve been fooled by compile-time constants being interned – so two literals end up refererring to the same string object. For example:

String x = "xyz";
String y = "xyz";
System.out.println(x == y); // Guaranteed to print true

StringBuilder builder = new StringBuilder();
String z = builder.append("x").append("yz").toString();

System.out.printn(x == z); // Will print false

From section 3.10.5 of the Java language specification:

String literals-or, more generally, strings that are the values of constant expressions (§15.28)-are “interned” so as to share unique instances, using the method String.intern.