What Is the String Pool in Java?

Direct Answer

The string pool is the JVM's canonical set of interned strings. String literals and compile-time constant string expressions are interned, and String.intern() returns the canonical object for equal content. Pooled strings are ordinary String objects managed in the heap in modern HotSpot; do not use == as a content test even though equal interned references are identical.

Code Example

public class StringPoolDemo {
    public static void main(String[] args) {

        // Literals — both go through the pool
        String s1 = "hello";
        String s2 = "hello";
        System.out.println(s1 == s2);     // true — same pooled object
        System.out.println(s1.equals(s2)); // true — same content

        // new String() is not itself automatically interned — always creates a new heap object
        String s3 = new String("hello");
        System.out.println(s1 == s3);     // false — different objects
        System.out.println(s1.equals(s3)); // true — same content

        // Always use .equals() to compare content — never == for Strings
        // == checks address (are they the same object?)
        // .equals() checks content (do they have the same characters?)
    }
}

Common Mistakes & Pitfalls

• Thinking every String lives in the pool — literals, compile-time constant string expressions, and strings represented canonically through intern() are pooled; new String("hello"), concatenation results, and strings from input or files land on the regular heap.
• Comparing content with == — == checks whether two references point to the same object, not whether the characters match; it returns true for two literals by luck of pooling and false for a new String. Use .equals().
• Assuming new String("hello") holds different text — the content is identical to the literal; only the object identity differs, which is exactly why == and .equals() disagree.
• Believing the pool still lives in PermGen — since Java 7 it sits in the normal heap, so pooled strings are subject to garbage collection like any other object.