Main.java

package com.demonwav.stupid;

import java.lang.reflect.Field;
import java.lang.reflect.Method;
import java.lang.reflect.Modifier;

public class Main {
    public static final Object helloWorldHolder = new Object() {
        private final Object words = "Goodbye World!";
    };

    public static final Object stringPrinter = new Object() {
        private void printString(Object object) {
            System.out.println(object);
        }
    };

    public static void main(String[] args) throws Throwable {
        setField(helloWorldHolder, "words", "Hello World!");
        forMethod(stringPrinter, "printString", Object.class).call(getField(helloWorldHolder, "words"));
    }

    public static class MethodCaller {
        private final Class<?> aClass;
        private final Object object;
        private final String method;
        private final Class<?>[] types;

        public MethodCaller(Class<?> aClass, Object object, String method, Class<?>[] types) throws Throwable {
            this.aClass = aClass;
            this.object = object;
            this.method = method;
            this.types = types;
        }

        public Object call(Object... params) throws Throwable {
            final Method declaredMethod = aClass.getDeclaredMethod(method, types);
            declaredMethod.setAccessible(true);
            return declaredMethod.invoke(object, params);
        }
    }

    public static MethodCaller forMethod(Object object, String method, Class<?>... types) throws Throwable {
        final Class<?> aClass = object.getClass();
        return new MethodCaller(aClass, object, method, types);
    }

    public static Object getField(Object object, String field) throws Throwable {
        final Class<?> aClass = object.getClass();
        final Field declaredField = aClass.getDeclaredField(field);
        declaredField.setAccessible(true);
        return declaredField.get(object);
    }

    public static Object setField(Object object, String field, Object value) throws Throwable {
        final Class<?> aClass = object.getClass();
        final Field declaredField = aClass.getDeclaredField(field);
        declaredField.setAccessible(true);

        if (Modifier.isFinal(declaredField.getModifiers())) {
            final Field modifiers = declaredField.getClass().getDeclaredField("modifiers");
            modifiers.setAccessible(true);
            modifiers.setInt(declaredField, declaredField.getModifiers() & ~Modifier.FINAL);
        }

        final Object oldVal = getField(object, field);
        declaredField.set(object, value);
        return oldVal;
    }
}