What is the purpose of the String Pool in Java?

String Pool is a region of memory in Java used for storing string literals. Its main purpose is to save memory and improve performance by preventing the creation of duplicate strings with the same value. When a string literal is created, the JVM first checks the String Pool. If a string with the same value already exists, a reference to the existing object is returned. If not, a new String object is created, placed in the String Pool, and a reference to it is returned.

Example:

// String literals "hello"
String s1 = "hello"; 
String s2 = "hello"; 

// s1 and s2 refer to the same object in the String Pool
System.out.println(s1 == s2); // Outputs true

// Creating a new String object "hello" not through a literal
String s3 = new String("hello");

// s3 refers to a different object in memory, not from the String Pool by default
System.out.println(s1 == s3); // Outputs false

// The intern() method adds the string to the String Pool if it is not already there, and returns a reference from the pool
String s4 = s3.intern(); 

// s4 refers to the same object in the String Pool as s1/s2
System.out.println(s1 == s4); // Outputs true

Using the == operator to compare strings checks whether the variables refer to the same object in memory. The equals() method compares the contents of the strings.

The String Pool only stores string literals declared with double quotes ("..."). String objects created with new String(...) are not placed in the String Pool by default unless the intern() method is explicitly called.

Advantages of String Pool:

• Memory savings: Fewer duplicate strings occupying space in the heap.
• Performance improvement: String comparison (==) becomes faster because it compares references, not contents.

Disadvantages:

• intern() can slow down performance if used on a very large number of unique strings, as it requires searching the pool.