Sometimes you need dirty hacks. This is one that can be useful in testing scenarios — how to change fields that are not meant to be changed.

If we have class Knowledge with the answer for everything:

public class Knowledge {

    private static final Integer ANSWER = 42;

    public String askQuestion(String question) {
        return "The answer to '" + question + "' is: " + ANSWER;
    }
}

We now want to change the answer for testing purposes:

public class KnowledgeTest {

    @Test
    public void testAskQuestion() throws Exception {
        Knowledge knowledge = new Knowledge();

        String answer = knowledge.askQuestion("question?");
        assertThat(answer, is("The answer to 'question?' is: 42"));

        setFinalStaticField(Knowledge.class, "ANSWER", 41);

        answer = knowledge.askQuestion("question?");
        assertThat(answer, is("The answer to 'question?' is: 41"));
    }

    private static void setFinalStaticField(Class<?> clazz, String fieldName, Object value)
            throws ReflectiveOperationException {

        Field field = clazz.getDeclaredField(fieldName);
        field.setAccessible(true);

        Field modifiers = Field.class.getDeclaredField("modifiers");
        modifiers.setAccessible(true);
        modifiers.setInt(field, field.getModifiers() & ~Modifier.FINAL);

        field.set(null, value);
    }
}

We make this possible by setting the field to accessible and removing the final modifier. Then the field can be set using reflection.

Please use this with care and only in test scopes. Also, for primitive types the compiler makes use of constant folding — then such a solution has no effect.

Happy (dirty) hacking!