InternMethod.java

import java.util.HashMap;
import java.util.Map;
import org.checkerframework.checker.interning.qual.Interned;

public class InternMethod {

    private static Map<Foo, @Interned Foo> pool = new HashMap<Foo, @Interned Foo>();

    class Foo {

        @SuppressWarnings("interning")
        public @Interned Foo intern() {
            if (!pool.containsKey(this)) {
                pool.put(this, (@Interned Foo) this);
            }
            return pool.get(this);
        }
    }

    void test() {
        Foo f = new Foo();
        @Interned Foo g = f.intern();
    }

    public static @Interned String intern(String a) {
        return (a == null) ? null : a.intern();
    }
}