Java String intern() method example

The Java String intern() return the reference of a equal string literal present in string pool. If there is an existing string literal present in string pool then it’s reference is returned. Otherwise a new string with same content is created and the reference of new string is returned.

String equality is checked with String.equals() method.

1. String pool

String pool is a reserved memory area in heap memory which Java uses for storing string constants. Note that Java strings are immutable by default.

Java stores only one copy of every distinct String value in the string pool. It helps to reuse String objects to save memory during program execution. There may be many references to a string in running program, but there will be only copy of string inside string pool.

1.1. Two ways to create string

In Java we can create strings in two ways.

String str1 = new String("hello world");

String str2 = "hello world";

In above example, both ways are used to create strings, but later is recommended which uses string literals. String literals always go to string pool.

When we create string with new keyword, two objects will be created i.e. one in the Heap Area and another in the String constant pool. The created string object reference always points to heap area object.

To get the reference of same object created in string pool, use intern() method.

2. Java String intern() method

The String.intern() returns a reference to equal string literal present in string pool.

As we know that all string literals are automatically created in String pool, so intern() method is applicable to String objects created via 'new' keyword.

String intern() is native method. By the help of intern() method, we can get the reference of corresponding String constant pool object of an original string object.

3. Java String intern() example

Java program intern a string with String.intern() method.

public class StringExample 
{
    public static void main(String[] args) 
    {
        //String object in heap
        String str1 = new String("hello world");
        
        //String literal in pool
        String str2 = "hello world";
        
        //String literal in pool
        String str3 = "hello world";
        
        //String object interned to literal
        //It will refer to existing string literal
        String str4 = str1.intern();
        
        
        System.out.println(str1 == str2);       //false
        System.out.println(str2 == str3);       //true
        System.out.println(str2 == str4);       //true
    }
}

Program output.

false
true
true

In this example, we learned to intern a string in Java. This is native method and provides very high performance.

References:

A Guide to Java String
String Java Doc