String interning : intern()
Java by default doesn't put all String object into String pool, instead we have flexibility to store any arbitrary String object in String pool explicitly.
java.lang.String.intern() method used to put String object to String pool.
Double quoted literal is known as String literal and the cache which stored these String instances are known as String pool.
Points to remember:
String pool was located in permgen area of heap up-to Java 1.6, but in Java 1.7 updates it’s moved to main heap area.
Earlier since it was in PermGen space, it was always a risk to create too many String object (java.lang.OutOfMemory: permgen space), because it’s a very limited space, default size 64 MB and used to store class metadata e.g. .class files.
Now because String pool is moved to a much larger memory space, it's much safer.
By the way, don't misuse memory here, always try to minimize temporary String object e.g. "a", "b" and then "ab". So don't forget to use StringBuffer and StringBuilder for string concatenation; it will reduce number of temporary Object in String pool.
Compare two String object using equals() method and never compare them using == operator, because we never know which one is coming from pool and which one is created using new() operator.
Perm space is used to keep information for loaded classes and few other advanced features like String Pool, which usually get created by String.intern() methods.
PermGen Space speeds up our String equality searching.
As your application (number of classes) will grow this space shall get filled quickly, since the garbage collection on this Space is not much effective to clean up as required, you quickly get Out of Memory: perm gen space error. After then, no application shall run on that machine effectively even after having a huge empty JVM.