在Java中：可以对方法链进行代码重用以调用继承层次结构吗？

Question

I have some class inheritance SubClass < MidClass < SuperClass and want to perform some TASK upward for all these classes. TASK is quite complex with only minor changes in the 3 classes, which I moved into the private methods m2().

My current solution is very boiler plate:

class SuperClass {
  protected void m1() {
    //TASK (calls m2())
  }

  private void m2() {
    //...
  }
}

class MidClass extends SuperClass {
  protected void m1() {
    //same TASK (calls m2())
    super.m1();
  }

  private void m2() {
    //...
  }
}

class SubClass extends MidClass {
  protected void m1() {
    //same TASK (calls m2())
    super.m1();
  }

  private void m2() {
    //...
  }
}

Can I exploit some code reuse mechanism instead of copying TASK?

Something like the following, with m1() only in SuperClass, does not work:

class SuperClass {
  protected final void m1() {
    //TASK (calls m2())
    if (!(this.getClass().equals(SuperClass.class))) {
      super.m1();
  }
}

because super.m1() does not refer to execution of the same inherited method in the context of a super class, but to the overridden method implementation. Since m1() does not exist in Object, I additionally get a compiler error...

Putting TASK in a protected final helper() method in SuperClass and calling helper() instead of copying TASK won't work, since then always SuperClass.m2() gets called.

The only alternative I can think of is slow, complicated and unsafe: using a type token as parameter, ie protected final void m1(Class<? extends SuperClass> clazz) in SuperClass, and fulfilling TASK via reflection (requires to make m2() public static or use setAccessible(true) on m2()).

Do you know some better solution? AOP? Maybe some framework where you can inject a method into classes (as in C#)? Or am I missing something???

Answer 1

How about this?

class SuperClass {
  protected void m1() {
    //TASK (calls m2())
  }

  protected void m2() {
    //...
  }
}

class MidClass extends SuperClass {

  protected void m2() {
    //...
  }
}

class SubClass extends MidClass {
  protected void m2() {
    //...
  }
}

The m1 method is inherited, and will always call the m2 method. Since m2 is protected, it's called polymorphically. So, if invoked on a SubClass instance, SubClass.m2() will be called.

Answer 2

Expanding on my comment to JB's answer:

class SuperClass {
 protected void m1() {
   m2();
 }

 protected void m2() {
   System.out.println("start super task");
   System.out.println("end super task");
 }
}

class MidClass extends SuperClass {
  protected void m2() {
   super.m2();
   System.out.println("start mid task");
   System.out.println("end mid task");
  }
}

class SubClass extends MidClass {
 protected void m2() {
  System.out.println("start sub task");
  super.m2();
  System.out.println("end sub task");
 }
}

new SubClass().m1() yields this result:

start sub task
start super task
end super task
start mid task
end mid task
end sub task

Note that all 3 versions of m2() are executed in the defined order: sub is started, then execution continues with super and mid and finished with sub again.

Answer 3

Solution for my concrete example of mixed-type equals() with default value constraints instead of ignoring the subclass value fields. Instead of Angelika Langer's solution (see http://www.angelikalanger.com/Articles/JavaSolutions/SecretsOfEquals/Equals-2.html ) with private methods _compareFields() and a protected method _navigateClassHierarchy() that has to be copied into each subclass, only a protected method compareOwnFields() is used, which has to be overridden correctly in each subclass.

class SuperClass {
    // ...

    @Override
    public final boolean equals(final Object other) {
        if (other == this) { return true; }
        if (!(other instanceof SuperClass)) {
            return false;
        }
        final SuperClass otherSuperClass = (SuperClass) other;

        return compareOwnFields(otherSuperClass, false)  
        && otherSuperClass.compareOwnFields(this, true);
    }

    protected boolean compareOwnFields(final SuperClass other, 
        final boolean firstTraversal) {
        if (!firstTraversal) {
            return true;
        }
        if (field1 != other.getField1()) {
           return false;
        } 
        // compare other fields similarly ...
        return true;
    }

}    

class SubClass {
    // ...

    @Override
    protected boolean compareOwnFields(final SuperClass other, 
        final boolean firstTraversal) {
        if (other instanceof SubClass && !firstTraversal) {
            return true;
        if (other instanceof SubClass) {
            if (field1 != ((SubClass) other).getField1()) {
                return false;
            }
            // compare other fields similarly ...
            return super.compareOwnFields(other, firstTraversal);
        } else {
            if (field1 != DEFAULT_FIELD1) {
                return false;
            }
            // check other fields for default values similarly ..
            return super.compareOwnFields(other, firstTraversal);
        }
    }
}

But this does not answer my question in general, it's rather a redesign that avoids the problem. So further answers on how to solve the problem with the Java language features are very welcome!