Why create a String object no need to use constructor?

Question

If we create a Java String object, we will do:
String s = "Hello";
And we won't do:
String s = new String("Hello");
In API doc, String() constructor description says "Initializes a newly created String object so that it represents an empty character sequence. Note that use of this constructor is unnecessary since Strings are immutable."
I am not sure how immutable is related to this??
Also, I wonder if the compiler will convert
String s = "Hello" to
String s = new String("Hello");
Thanks!

Answer 1

String s = new String("Hello");
This is a valid statement too..... But the compiler will not convert String s = "Hello" to String s = new String("Hello") as you suggested. The reason is that java has a sort of a String Bag. Everytime you do a String s = "Hello" it will check the bag and see if such a string already exists. If it does, it merely creates a pointer to it, because strings are immutable, it doesn't need to worry about others modifying that string. If the string doesn't exist then it creates the necessary memory for the string object and adds the reference to the bag.
However, once you do a String s = new String("Hello") what you are saying to the compiler is, "Hey, don't check the bag just create the String." This is all fine and dandy, except that it increases the memory size of your program unnecessarily.
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