Java - Pattern Matching for instanceof

Enhance the Java programming language with pattern matching for the instanceof operator. Pattern matching allows common logic in a program, namely the conditional extraction of components from objects, to be expressed more concisely and safely.

For example, all Java programmers with the instanceof-and-cast idiom:

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

There are three things going on here: a test (is obj a String?), a conversion (casting obj to String), and the declaration of a new local variable (s) so we can use the string value.

The use of pattern matching in instanceof should dramatically reduce the overall number of explicit casts in Java programs. Moreover, type test patterns are particularly useful when writing equality methods. Consider the following equality method taken from Item 10 of the Effective Java book:

@Override public boolean equals(Object o) {
    return (o instanceof CaseInsensitiveString) &&
        ((CaseInsensitiveString) o).s.equalsIgnoreCase(s);
}
Using a type test pattern means it can be rewritten to the clearer:

@Override public boolean equals(Object o) {
    return (o instanceof CaseInsensitiveString cis) &&
        cis.s.equalsIgnoreCase(s);
}
The instanceof grammar is extended accordingly:

RelationalExpression:
     ...
     RelationalExpression instanceof ReferenceType
     RelationalExpression instanceof Pattern

Pattern:
     ReferenceType Identifier