What is pattern matching for instanceof?

The 30 second answer

Since Java 16, instanceof can test the type and bind a variable in one step, so I don't need a separate cast afterwards. For example, if obj is a String s, I can use s directly. The variable only exists where the compiler knows the match succeeded, so it also works after an and condition, or after an early return when the check fails.

// Before
if (obj instanceof String) { String s = (String) obj; use(s); }

// Java 16+
if (obj instanceof String s && !s.isEmpty()) { use(s); }

if (!(obj instanceof Order o)) return;
process(o);                         // o is in scope here (flow scoping)

The binding is in scope only where the match definitely succeeded; obj instanceof String s || s.isEmpty() doesn't compile. It also makes equals shorter:

public boolean equals(Object o) {
    return o instanceof Point p && x == p.x && y == p.y;
}

Previewed in Java 14 and 15, final in Java 16. Java 21 extends patterns to switch and to record deconstruction.

Sources: JEP 394: Pattern Matching for instanceof, JLS 6.3.1 Scope for Pattern Variables

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