String Pool in Java: How It Works and Why It Matters

The String pool in Java is a special memory area where string literals are stored to save memory by reusing identical strings. When you create a string literal, Java checks the pool first and returns the existing string if found, avoiding duplicate objects. This helps improve performance and memory efficiency.

Syntax

The String pool works automatically with string literals and the intern() method.

• String s = "hello"; creates or reuses a string in the pool.
• String s2 = new String("hello"); creates a new object outside the pool.
• s2 = s2.intern(); adds the string to the pool or returns the existing one.

java

String s1 = "hello";
String s2 = new String("hello");
String s3 = s2.intern();

Example

This example shows how string literals share the same object in the pool, while strings created with new do not, unless intern() is used.

java

public class StringPoolExample {
    public static void main(String[] args) {
        String s1 = "java";
        String s2 = "java";
        String s3 = new String("java");
        String s4 = s3.intern();

        System.out.println(s1 == s2); // true, same pool object
        System.out.println(s1 == s3); // false, different objects
        System.out.println(s1 == s4); // true, s4 refers to pool object
    }
}

Output

true
false
true

Common Pitfalls

Many beginners expect == to compare string content, but it compares references (memory addresses). Strings created with new are not in the pool by default, so == returns false even if contents match. Always use equals() to compare string content.

Also, forgetting to use intern() when needed can cause unexpected memory use or comparison results.

java

String a = new String("test");
String b = "test";

// Wrong: compares references
System.out.println(a == b); // false

// Right: compares content
System.out.println(a.equals(b)); // true

// Using intern to get pool reference
String c = a.intern();
System.out.println(b == c); // true

Output

false
true
true

Quick Reference

Concept	Description
String literal	Stored in the String pool automatically
new String()	Creates a new object outside the pool
intern()	Adds string to pool or returns existing pool string
== operator	Compares references, not content
equals() method	Compares string content

Key Takeaways

Java String pool stores string literals to save memory by reusing objects.

Use string literals or intern() to ensure strings are in the pool.

Never use == to compare string content; use equals() instead.

new String() creates a new object outside the pool unless intern() is called.

String pool improves performance and reduces memory usage.