Literal Strings - same value, same object?

Question

I've read that if two literal Strings contain the same value, they reference the same object to save memory. Is this guaranteed? And is the same thing true of primitive wrappers with literal values like Integer(42)?
The reason I ask is because I'm thinking of using a Map as a sparse array indexed by Integers... or, if not that, then Strings representing integer values.

Answer 1

Ok so you put a string into the map using one of the Integer references as a key and took it out using the other one.
The problem here is how a Map works when you get() something. (Including HashMap).
"Returns the value to which the specified key is mapped, or null if this map contains no mapping for the key.
"More formally, if this map contains a mapping from a key k to a value v such that (key==null ? k==null : key.equals(k)), then this method returns v; otherwise it returns null. (There can be at most one such mapping.)"
http://java.sun.com/javase/6/docs/api/java/util/Map.html#get(java.lang.Object)
The important bit is the second paragraph. Do you see where for non-null keys it checks key.equals(k)? In your case the two 666 Integers were checked for equality of value using equals(), not for identity of reference (using ==). They are different objects, but they represent the same integer value so they compare true with equals(). Hence the map finds a match even when you use a different key.
A more straightforward test is to simply print:
{code}
Integer i1 = new Integer(666);
Integer i2 = new Integer(666);
if(i1 == i2) {
System.out.println("Same");
} else {
System.out.println("Different");
{code}
You'll find the same behaviour - +different+ key objects returning the same value from a Map - even when you use small Integer objects like new Integer(66).