Lazy.java

import org.checkerframework.checker.initialization.qual.*;
import org.checkerframework.checker.nullness.qual.*;

public class Lazy {

    @NonNull String f;
    @MonotonicNonNull String g;
    @MonotonicNonNull String g2;
    @org.checkerframework.checker.nullness.qual.MonotonicNonNull String _g;
    @org.checkerframework.checker.nullness.qual.MonotonicNonNull String _g2;

    // Initialization with null is allowed for legacy reasons.
    @MonotonicNonNull String init = null;

    public Lazy() {
        f = "";
        // does not have to initialize g
    }

    void test() {
        g = "";
        test2(); // retain non-null property across method calls
        g.toLowerCase();
    }

    void _test() {
        _g = "";
        test2(); // retain non-null property across method calls
        _g.toLowerCase();
    }

    void test2() {}

    void test3() {
        //:: error: (dereference.of.nullable)
        g.toLowerCase();
    }

    void test4() {
        //:: error: (assignment.type.incompatible)
        g = null;
        //:: error: (monotonic.type.incompatible)
        g = g2;
    }

    void _test3() {
        //:: error: (dereference.of.nullable)
        _g.toLowerCase();
    }

    void _test4() {
        //:: error: (assignment.type.incompatible)
        _g = null;
        //:: error: (monotonic.type.incompatible)
        _g = _g2;
    }
}