Test.java

package examples;

import org.checkerframework.checker.initialization.qual.UnknownInitialization;
import org.checkerframework.checker.nullness.qual.NonNull;

public class Test {

    void a() {
        @NonNull String f = "abc";

        //:: error: (assignment.type.incompatible)
        f = null;
    }

    void b() {
        @UnknownInitialization @NonNull Test f = new Test();
    }
}