What is String Pool in Java

String Pool in Java is a memory area where string literals are stored in order to optimize memory usage and improve performance. The idea is to avoid creating multiple identical string objects by storing just one instance of each string literal.

When you create a string literal in Java using double quotes, the compiler checks whether an identical string already exists in the String Pool. If it exists, the reference to the existing instance is returned; otherwise, a new instance is created and added to the String Pool.

For example:

String str1 = "Hello"; // create a literal string in the String Pool
String str2 = "Hello"; // gets the same instance reference in the String Pool
String str3 = new String("Hello"); // create a new instance out of the String Pool

In the example above, "str1" and "str2" point to the same instance in the String Pool, as the string literal "Hello" is shared. On the other hand, "str3" creates a new instance outside of the String Pool, as it uses the "new String()" constructor explicitly.

Using String Pool helps save memory as multiple references to the same string literal share the same instance, avoiding unnecessary duplication of objects.

It is important to note that using the "+" operator to concatenate strings creates new instances and does not necessarily use the String Pool. However, starting in Java 9, strings concatenated using the "+" operator are automatically added to the String Pool when the result is a compile-time constant expression. This is known as "String Concatenation Optimization". Example:

String str4 = "Hello";
String str5 = "World";
String result = str4 + str5; // starting in Java 9, result is added to the String Pool

String Pool is an important optimization in Java, especially when working with many string literals, and contributes to more efficient memory usage.