What is the String Pool?

Direct Answer

The String pool is the JVM's canonical set of interned String values. String literals and string-valued constant expressions are interned, and String.intern() returns the pooled representative equal to a String. In HotSpot, pooled String objects are heap objects, but the specification does not promise one physical table layout.

Deep Dive & Architecture

Automatic string interning

A literal such as "java" is interned. The compiler can also fold the constant expression "ja" + "va", so it denotes the same interned value. If one operand is a runtime variable, concatenation happens during execution and the resulting String is not guaranteed to be the pooled representative even when its characters match the literal.

new String("java") deliberately asks for a distinct String object initialized from the literal's value. Its equals result matches the literal because their characters match, while == observes different references. Calling intern() returns the canonical pooled reference. This is a teaching example for identity, not a reason to compare application text with ==.

When string interning is appropriate

Interning can reduce duplicate identities for a small, repeated vocabulary, but enums or domain value objects often express fixed choices more clearly. Interning user IDs, URLs, or other unbounded external values turns input cardinality into pool lookup and memory pressure. Measure a real benefit before adding it as a cache.

API guarantees and HotSpot implementation details

The Java API specifies canonical equality behaviour, not a public hash-table design or permanent address. Current HotSpot keeps pooled String objects in the managed heap; older PermGen explanations are historical. Pooling also does not cause immutability—String is immutable whether pooled, newly constructed, or returned by a runtime operation.

Code Example

class StringPool {
    public static void main(String[] args) {
        String firstLiteral = "java";
        String secondLiteral = "ja" + "va";
        String created = new String("java");

        System.out.println(firstLiteral == secondLiteral);
        System.out.println(firstLiteral == created);
        System.out.println(firstLiteral.equals(created));
        System.out.println(firstLiteral == created.intern());
    }
}