NullPointerException Gah

All Java developers dislike NullPointerException(NPE). To avoid NPE we should use Optional type or annotations like @Nullable/@Notnull. But they are optional and sometimes it's wrong(null was assigned to @Notnull field). These days null-safe language like Scala/Kotlin is a good solution for this problem. However, we need to write Java for some reasons.

NPE Gah is a fuzz test tool to avoid NPE. You can find null-unsafe codes during unit testing.

public class Main {
    public static class Foo {
        String field1;
        String field2;

        public Foo() {
        }

        @Override
        public String toString() {
            return "Foo{" +
                "field1='" + field1 + '\'' +
                ", field2='" + field2 + '\'' +
                '}';
        }

    }

    void targetMethod(Foo v) {
        v.field1.length();
        v.field2.length();
    }

    @Test
    void nullPointerDetector() {
        FuzzyObjectGenerator generator = new FuzzyObjectGenerator();
        for (Foo v : generator.generate(Foo.class)) {
            try {
                targetMethod(v);
                System.out.println(v.toString() + " OK");
            } catch (NullPointerException e) {
                System.out.println(v.toString() + " NG L:" + e.getStackTrace()[0].getLineNumber());
            }
        }
    }
}

Output is below. You can see some arguments which cause NPE.

Foo{field1='null', field2='null'} NG L:24
Foo{field1='string-value', field2='string-value'} OK
Foo{field1='null', field2='string-value'} NG L:24
Foo{field1='string-value', field2='null'} NG L:25