String Constant Pool vs String pool

Question

I am confused about these two things. I need a help. Please clear my doubt, whether String Constant Pool and String pool both are same concept. I faced this question on interview. I have already read lot of sites and blogs but, my doubt is not cleared.Please clear my doubts.

Thanks in Advance.

Answer 1

Both are the same thing. String Constant Pool contains constant string objects. Constant can be defined as String object holds the value at compile time. For more refer JLS.

    String s="abc";
    String s1="def";
    String s2=s+"def";
    String s3="abc"+"def";
    System.out.println(s2==s3); // print false

But if you make s as final then

    final String s="abc";
    String s1="def";
    String s2=s+"def";
    String s3="abc"+"def";
    System.out.println(s2==s3); // print true

In above case s3 is a compile time constant as s is final .

Answer 2

The String pool (= "String constant pool"):

• This is an informal nickname given to String's class-level/static intern storage. Note: javadoc mentions "pool of strings" http://docs.oracle.com/javase/6/docs/api/java/lang/String.html#intern%28%29.

• It's a set containing every unique String value interned during app execution. Interning happens automatically for all compile-time String constants (literals and fixed expressions) and also for all runtime String values where String.intern() is called. The JLS mandates that compile-time constant expressions of type String are always "interned" so as to share unique instances, using the method String.intern. http://docs.oracle.com/javase/specs/jls/se7/html/jls-15.html#jls-15.28

• The JVM spec does not mandate any particular internal structure for objects. http://docs.oracle.com/javase/specs/jvms/se7/html/jvms-2.html#jvms-2.7

• The Java Language Spec does mandate that a String object has a constant (unchanging) value. http://docs.oracle.com/javase/specs/jls/se7/html/jls-4.html#jls-4.3.3
  That means that a String variable can only change value by referencing a new String object with a new value - of course, this is internally managed by the compiler & JVM . That also means that all items in the pool are String constants.

The Constant Pool (not focused on Strings, but does include Strings):

• Exists within each class file (and in memory, for each class loaded). Is a per-class record of the constants (i.e. final variables) in use. http://docs.oracle.com/javase/specs/jvms/se7/html/jvms-4.html#jvms-4.4 http://docs.oracle.com/javase/specs/jvms/se7/html/jvms-2.html#jvms-2.5.5