how to protect private field from reflect get and set in java

public class  {
    protected String test = "test string in class ReflectField!";
}

 * subclass of ReflectField
 *
 * 通过子类把父类的test字段隐藏掉，不让反射获取到其值
 * 正确的应该是用java的security manager来管理
 */
import java.lang.reflect.Field;
public class ReflectExtendsField extends  {
    // protected String test = "test string in class ReflectExtendsField!";
    private String test = "test string in class ReflectExtendsField!";
    public static void main(String[] args) throws Exception {
        // Class ref = Class.forName("ReflectExtendsField");
        Class ref = ReflectExtendsField.class;
        Field[] fields = ref.getFields();
        System.out.println(fields.length);
        for (Field field : fields) {
            field.setAccessible(true);
            System.out.println(field.get(ref));
        }
        Field testField = ref.getDeclaredField("test");
        testField.setAccessible(true);
        System.out.println(testField);
        System.out.println(testField.get(ref));
    }
}