checker/tests/nullness/TestPolyNull.java

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

public class TestPolyNull {
  @PolyNull String identity(@PolyNull String str) {
    return str;
  }

  void test1() {
    identity(null);
  }

  void test2() {
    identity((@Nullable String) null);
  }

  public static @PolyNull String[] typeArray(@PolyNull Object[] seq, @Nullable String nullable) {
    @SuppressWarnings("nullness") // ignore array initialization here.
    @PolyNull String[] retval = new @Nullable String[seq.length];
    for (int i = 0; i < seq.length; i++) {
      if (seq[i] == null) {
        // null can be assigned into the PolyNull array, because we
        // performed a test on seq and know that it is nullable.
        retval[i] = null;
        // and so can something that is nullable
        retval[i] = nullable;
        // One can always add a dummy value: nonnull is the bottom
        // type and legal for any instantiation of PolyNull.
        retval[i] = "dummy";
      } else {
        retval[i] = seq[i].getClass().toString();
        // :: error: (assignment)
        retval[i] = null;
        // :: error: (assignment)
        retval[i] = nullable;
      }
    }
    return retval;
  }

  public static @PolyNull String identity2(@PolyNull String a) {
    return (a == null) ? null : a;
  }

  public static @PolyNull String identity3(@PolyNull String a) {
    if (a == null) {
      return null;
    }
    return a;
  }
}