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Remove duplicate code blocks of overridden methods in Java

My project must comply by contract with all the sonarqube static analyzer rules (to my immense delight).
As shown in the picture I have a base class which is derived n time, and each derived class is derived a second time.
All my DoubleDerived classes override the methods foo1(), ..., fooM() generating n duplicate code blocks of size O(M) lines of code
class DoubleDerivedX extends DerivedX {
#Override
public void foo1() {
...
}
...
#Override
public void fooM() {
...
}
}
Currently my code violates the common-java:DuplicatedBlocks rule for this reason, as far as I can tell (I can't find official documentation about this) the analyzer consider a code block duplicated if it consists of three or more methods and if it is present in multiple file.
It is actually possible to remove the duplicated blocks of code, if yes how?
A trivial solution would be to use multiple inheritance, with a BaseDoubleDerived class derived from Base and from which the n DoubleDerived class derive, but it cannot be implemented in Java.
Edit: The Base and Derived classes are domain classes while the DoubleDerived classes are lazy loading Proxies that fetch data from repositories. The fact is that I can pretty much feel that this is a XY problem but I'm so close to the deadline that I know I don't have probably the time to implement the "right" solution.

Edit: They may say that I don’t believe in ‘if’ anymore is the object-oriented battle song (well, actually they don’t, I just made this up). if statements still have their place.
public class Base {
private boolean needsFooImplementationsForDoubleDerivedClasses;
public void foo() {
if (needsFooImplementationsForDoubleDerivedClasses) {
// DoubleDerivedX foo() implementation here
} else {
// Base foo() implementation here
}
}
}
Now control the value of needsFooImplementationsForDoubleDerivedClasses through constructor arguments. Like this:
class DoubleDerivedX extends DerivedX {
public DoubleDerivedX() {
super(true);
}
}
Original answer
One option is: put the common implementation for double derived classes in a separate method in the base class. Base and single derived classes will not need it and just ignore it.
public class Base {
public void foo() {
// Base foo() implementation here
}
protected void fooImplementationForDoubleDerivedClasses() {
// DoubleDerivedX foo() implementation here
}
}
So your double derived classes only need to do
class DoubleDerivedX extends DerivedX {
#Override
public void foo() {
fooImplementationForDoubleDerivedClasses();
}
}
Depending on your situation putting the common implementation in the base class may feel natural or unmotivated. In the latter case you may also look for a different shared place for it, but assuming that the method needs access to protected parts of Base, moving it out of Base may not be trivial.

Seems like this a candidate for composition over inheritance. If you can enclose the duplicated method in its own class, and invoke it via instances held in the DoubleDerivedX classes, that would eliminate the code duplication. But for that to work, a lot depends on how enmeshed the method is with the rest of the class (which could be an indication there are deeper issues that need addressing).

How to inherits/uses methods from multiple classes

Current I have a base class that contains project init and teardown method, and some large number of common methods, and these methods are inherited/used by its subclasses.
Because the based class is getting huge, so I am trying to move these common methods into newly created classes to improve the modularity of the project. And the original base class inherits the methods from those newly created classes. (multiple inheritances?)
Is there any suggestion to refactor this? also need to minimize the impact to the subclasses.. minimal code changes will be perfect..
Base class
-method1()
-method2()
-method3()
-method4()
-method5()
-method6()--------
| |
| |
subclasse1 subclass2

First of all, there is no multi inheritance of classes (only multi inheritance of interfaces) in java.
However in java 8 you can write default implementations of interface methods (see "Default Methods" on docs.oracle.com). But there are some restrictions:
You still can't declare fields in a interface
If you have 2 default implementations of the same method in different interfaces, you still need to override them.
You may be better off using the adapter pattern, i.e. refractor your code to something like this:
public class BaseClass {
private final Adaptee1 adaptee1;
private final Adaptee2 adaptee2;
// ...
public BaseClass(Adaptee1 adaptee1, Adaptee2 adaptee2 /* , ...*/) {
this.adaptee1 = adaptee1;
this.adaptee2 = adaptee2;
//...
}
public BaseClass() {
this(new ConcreteAdaptee1(), new ConcreteAdaptee2() /* , ...*/)
}
public void method1() {
adaptee1.method1();
}
public void method2() {
adaptee1.method2();
}
public void method3() {
adaptee2.method3();
}
public void method4() {
adaptee2.method4();
}
// ...
}
public interface Adaptee1 {
void method1();
void method2();
}
public interface Adaptee2 {
void method3();
void method4();
// ...
}
//...
public class ConcreteAdaptee1 implements Adaptee1 {
//...
}
public class ConcreteAdaptee2 implements Adaptee2 {
//...
}
//...
You could even pass the adapter to the adaptees in the method calls, if you need access to methods in other adaptees as well for example.
But you should check first, if the class can be split into multiple classes in a good way (i.e. the adaptees should be independent). Don't split the class into parts at all cost. If you can't make the adaptees independent, you should keep it as a single class and rely on your IDE instead to navigate the code easier (Code folds, bookmarks, ...)

In Java, one class cannot extend multiple other classes, however it can implement as many interfaces as you'd like. Unfortunately, implementing an interface will not provide the methods you need.
As far as I could think this out, you have the following options:
1) If you don't want to change all your references to the Base class, and the Base class has some methods that are really broad and may apply to many classes, you could put those into one class and have your base class extend that one.
example:
public class SuperBaseClass{
method1()
method2()
method3()
}
public class BaseClass extends SuperBaseClass{
method4()
method5()
method6()
}
You could even create a SuperDuperBaseClass which SuperBaseClass extends to continue the chain as far as you'd like. The problem with this technique is it forces a hierarchical structure to your project.
2) You could split the Base class into multiple other base classes (I would recommend this one). Sort out your methods into categories and put them in appropriately named base classes. For example, throw all your methods which crunch numbers into one class named Algorithms. Put all your methods which tie other classes and functionalities from your project into one class called Tools. Do this until your whole project is sorted. It will clean everything up as much as possible and keep things manageable in the future. The problem with this is you will have to change everywhere the Base class used to be referenced.
3) You could just leave the Base class as it is. Since it's only a supporting file, you shouldn't really have to be digging through that code very often, so it won't hold you back too much that this file gets a little messy.

Multiple inheritance in Python; how to do so in Java?

I'm porting some Python code to Java, and I am having trouble dealing with the following problem:
I have some classes which need to have abilities A, B, or C. Class 1 needs ability A, class 2 needs A, B and C, and class 3 needs B and C. Most importantly, I want to easily be able to change what class can have what ability in the future.
I solved this problem pretty easily with multiple inheritance in Python. I'm trying to figure out the best way to do it in Java, but I can't come up with as good of a solution. I know multiple inheritance is frowned-upon, so I'd appreciate being taught a better way.
Thanks!

It depends on your actual use case, but have you already considered decorators?
http://en.wikipedia.org/wiki/Decorator_pattern

Multiple-inheritance ain't frowned upon. What is frowned upon is "implementation inheritance" (also known as "code reuse"), because it leads to the unsolvable "diamond problem". And because, well, code-reuse really hasn't much to do with OO.
What you want to do can be solved using multiple inheritance (and, say, delegation if you need to do "code reuse").
interface A {
void move();
}
interface B {
void eat();
}
interface C {
void think();
}
class One implements A { ... }
class Two implements B { ... }
class Three implements B, C { ... }
Any OOA/OOD using multiple inheritance can be trivially translated to Java. The part where you say that you need to change the "ability" all the time is a bit scary: if, say, a Car can move(), why would it suddenly need to be able to think()?

You can use AspectJ's mixin syntax fairly easily to emulate multiple inheritance (and at compile time too). First, declare an interface for the functionality you want to mixin:
public interface A{
String getSomethingForA();
}
then define an annotation which you can use to signify that you want the mixin applied to a given class:
public #interface WithA {}
then add the annotation to the class you want to use:
#WithA
public class MyClass {}
then, to actually add some functionality:
#Aspect
public class MixinA {
public static class AImpl implements A{
public String getSomethingForA() {
return "it worked!";
}
}
#DeclareMixin("#WithA *")
public static A get() {
return new AImpl();
}
}
You'll need to use the aspectj jars and run the aspects as part of your compile process, but this lets you create truly modularized functionality and then forcibly merge it into your classes later. To access your class with the new functionality, do the following:
MyClass obj = new MyClass();
((A)obj).getSomethingForA();
You can apply the same annotation to another class and cast it as well:
#WithA
#WithB //let's pretend we created this with some other functionality
public class AnotherClass {}
AnotherClass anotherObj = new AnotherClass();
((A)anotherObj).getSomethingForA();
((B)anotherObj).andSetSomethingElseForB("something else");

Multiple inheritance is almost always a bad idea, as its effects can usually be achieved through other mechanisms. Based upon your description of the problem, it sounds like you want to
Use interfaces to define behavior (public interface A) in this scenario, each behavior should probably have its own interface.
If 2 behaviors are tightly coupled (say A & B), define an interface that implements those two atomic interfaces (public interface CombinedAandB extends A, B)
Define an abstract base class that implements the interface to provide default implementations for behaviors
public abstract class BaseAB implements A, B
{
#Override
public void A() { add(0,1); }
#Override
public void B() {add(1,0); }
private void add(int a, int b) //it doesn't return. no soup for you.
{ a + b; //If you know why this is wrong, high five yourself. }
}
Define a concrete class that extends the abstract base class, implements another interface, and provides its own behavior.
public class IDoABAndC extends BaseAB implements C
{
//stuff, etc
}

You can define the abilities in interfaces and implement them in your classes.

In java you don't have multiple inheritance, instead you can implement multiple interfaces.
So your class 1 will implement interface A and B. Class 2 will implement interface A, B and C. Class 3 will implement B and C.

If what you need is interface inheritance, then as mentioned before, you can always implement multiple interfaces.
If you're looking for implementation inheritance, you're somewhat out of luck. The best solution is probably to use delegation — replace the extra superclasses with fields, and implement methods that just delegate to those fields. It does require writing a lot of repetitive delegation methods, but it's rather unavoidable in Java (without resorting to AspectJ or other bytecode-munging tricks; careful, this way madness lies …).

This is a bit tangential, but you can have python code running in Java via Jython (http://www.jython.org/). This addresses the porting to Java part, not the solving multiple inheritance part (I think you need to determine which is relevant)

How do Java Interfaces simulate multiple inheritance?

I am reading "The Java Tutorial" (for the 2nd time). I just got through the section on Interfaces (again), but still do not understand how Java Interfaces simulate multiple inheritance. Is there a clearer explanation than what is in the book?

Suppose you have 2 kinds of things in your domain : Trucks and Kitchens
Trucks have a driveTo() method and Kitchens a cook() method.
Now suppose Pauli decides to sell pizzas from the back of a delivery truck. He wants a thing where he can driveTo() and cook() with.
In C++ he would use multiple inheritance to do this.
In Java that was considered to be too dangerous so you can inherit from a main class, but you can "inherit" behaviors from interfaces, which are for all intents and purposes abstract classes with no fields or method implementations.
So in Java we tend to implement multiple inheritance using delegations :
Pauli subclasses a truck and adds a kitchen to the truck in a member variable called kitchen. He implements the Kitchen interface by calling kitchen.cook().
class PizzaTruck extends Truck implements Kitchen {
Kitchen kitchen;
public void cook(Food foodItem) {
kitchen.cook(foodItem);
}
}
He is a happy man because he can now do things like ;
pizzaTruck.driveTo(beach);
pizzaTruck.cook(pizzaWithExtraAnchovies);
Ok, this silly story was to make the point that it is no simulation of multiple inheritance, it is real multiple inheritance with the proviso that you can only inherit the contract, only inherit from empty abstract base classes which are called interfaces.
(update: with the coming of default methods interfaces now can also provide some behavior to be inherited)

You're probably confused because you view multiple inheritance locally, in terms of one class inheriting implementation details from multiple parents. This is not possible in Java (and often leads to abuse in languages where it's possible).
Interfaces allow multiple inheritance of types, e.g. a class Waterfowl extends Bird implements Swimmer can be used by other classes as if it were a Bird and as if it were a Swimmer. This is the the deeper meaning of multiple inheritance: allowing one object to act like it belongs to several unrelated different classes at once.

Here is a way to achieve multiple inheritance through interfaces in java.
What to achieve?
class A extends B, C // this is not possible in java directly but can be achieved indirectly.
class B{
public void getValueB(){}
}
class C{
public void getValueC(){}
}
interface cInterface{
public getValueC();
}
class cChild extends C implemets cInterface{
public getValueC(){
// implementation goes here, call the super class's getValueC();
}
}
// Below code is **like** class A extends B, C
class A extends B implements cInterface{
cInterface child = new cChild();
child.getValueC();
}

given the two interfaces below...
interface I1 {
abstract void test(int i);
}
interface I2 {
abstract void test(String s);
}
We can implement both of these using the code below...
public class MultInterfaces implements I1, I2 {
public void test(int i) {
System.out.println("In MultInterfaces.I1.test");
}
public void test(String s) {
System.out.println("In MultInterfaces.I2.test");
}
public static void main(String[] a) {
MultInterfaces t = new MultInterfaces();
t.test(42);
t.test("Hello");
}
}
We CANNOT extend two objects, but we can implement two interfaces.

Interfaces don't simulate multiple inheritance. Java creators considered multiple inheritance wrong, so there is no such thing in Java.
If you want to combine the functionality of two classes into one - use object composition. I.e.
public class Main {
private Component1 component1 = new Component1();
private Component2 component2 = new Component2();
}
And if you want to expose certain methods, define them and let them delegate the call to the corresponding controller.
Here interfaces may come handy - if Component1 implements interface Interface1 and Component2 implements Interface2, you can define
class Main implements Interface1, Interface2
So that you can use objects interchangeably where the context allows it.

It's pretty simple. You can implement more than one interface in a type. So for example, you could have an implementation of List that is also an instance of Deque (and Java does...LinkedList).
You just can't inherit implementations from multiple parents (i.e. extend multiple classes). Declarations (method signatures) are no problem.

You know what, coming from the perspective of a JavaScript dev trying to understand what the heck is going on with this stuff, I'd like to point out a couple things and somebody please tell me what I'm missing here if I'm way off the mark.
Interfaces are really simple. Stupidly, insanely simple. They're as stupidly, insanely simple as people initially think, which is why there are so many duplicate questions on this exact subject because the one reason to use them has been made unclear by people trying to make more of them than they are and there is widespread misuse in every Java server-side code-base I've ever been exposed to.
So, why would you want to use them? Most of the time you wouldn't. You certainly wouldn't want to use them ALL the time as many seem to think. But before I get to when you would, let's talk about what they're NOT.
Interfaces are NOT:
in any way a workaround for any sort of inheritance mechanism that Java lacks. They have nothing to do with inheritance, they never did, and in no way simulate anything inheritance-like.
necessarily something that helps you with stuff you wrote, so much as it helps the other guy write something meant to be interfaced by your stuff.
They really are as simple as you think they are on first glance. People misuse stupidly all the time so it's hard to understand what the point is. It's just validation/testing. Once you've written something conforms to an interface and works, removing that "implements" code won't break anything.
But if you're using interfaces correctly, you wouldn't want to remove it because having it there gives the next developer a tool for writing an access layer for another set of databases or web services that they want the rest of your app to continue using because they know their class will fail until they get the 100% complete-as-expected-interface in place. All interfaces do is validate your class and establish that you have in fact implemented an interface as you promised you would. Nothing more.
They're also portable. By exposing your interface definitions you can give people wanting to use your unexposed code a set of methods to conform to in order for their objects to use it correctly. They don't have to implement the interfaces. They could just jot them down on a piece of notepad paper and double-check that. But with the interface you have more of a guarantee nothing is going to try to work until it has a proper version of the interface in question.
So, any interface not likely to ever be implemented more than once? Completely useless. Multiple-inheritance? Stop reaching for that rainbow. Java avoids them for a reason in the first place and composited/aggregate objects are more flexible in a lot of ways anyway. That's not to say interfaces can't help you model in ways that multiple-inheritance allows but it's really not inheritance in any way shape or form and shouldn't be seen as such. It's just guaranteeing that your code won't work until you've implemented all of the methods you established that you would.

It's not a simulation of multiple inheritance. In java you can't inherit from two classes, but if you implements two interfaces "it seems like you inherited from two different classes" because you can use your class as any of your two intefaces.
For example
interface MyFirstInteface{
void method1();
}
interface MySecondInteface{
void method2();
}
class MyClass implements MyFirstInteface, MySecondInteface{
public void method1(){
//Method 1
}
public void method2(){
//Method 2
}
public static void main(String... args){
MyFirstInterface mfi = new MyClass();
MySecondInterface msi = new MyClass();
}
}
This will work and you can use mfi and msi, it seems like a multi inheritance, but it's not because you don't inherit anything, you just rewrite public methods provided by the interfaces.

You need to be precise:
Java allows multiple inheritance of interface, but only single inheritance of implementation.
You do multiple inheritance of interface in Java like this:
public interface Foo
{
String getX();
}
public interface Bar
{
String getY();
}
public class MultipleInterfaces implements Foo, Bar
{
private Foo foo;
private Bar bar;
public MultipleInterfaces(Foo foo, Bar bar)
{
this.foo = foo;
this.bar = bar;
}
public String getX() { return this.foo.getX(); }
public String getY() { return this.bar.getY(); }
}

Just by the way, the reason why Java does not implement full multiple inheritance is because it creates ambiguities. Suppose you could say "A extends B, C", and then both B and C have a function "void f(int)". Which implementation does A inherit? With Java's approach, you can implement any number of interfaces, but interfaces only declare a signature. So if two interfaces include functions with the same signature, fine, your class must implement a function with that signature. If interfaces you inherit have functions with different signatures, then the functions have nothing to do with each other, so there is no question of a conflict.
I'm not saying this is the only way. C++ implements true multiple inheritance by establishing precedence rules of which implementation wins. But the authors of Java decided to eliminate the ambiguity. Whether because of a philosophical belief that this made for cleaner code, or because they didn't want to do all the extra work, I don't know.

It's not fair to say that interfaces 'simulate' multiple inheritance.
Sure, your type can implement multiple interfaces and act as many different types polymorphically. However, you obviously won't inherit behaviour or implementations under this arrangement.
Generally look at composition where you think you may need multiple inheritance.
OR A potential solution to achieving something multiple inheritance like is the Mixin interface - http://csis.pace.edu/~bergin/patterns/multipleinheritance.html. Use with care!

They don't.
I think that the confusion comes from people believing that implementing an interface constitutes some form of inheritance. It doesn't; the implementation can simply be blank, no behavior is forced by the act or guaranteed through any contract. A typical example is the Clonable-interface, which while alluding to lots of great functionality, which defines so little that's it's essentially useless and potentially dangerous.
What do you inherit by implementing an interface? Bubkes! So in my opinion, stop using the words interface and inheritance in the same sentence. As Michael Borgwardt said, an interface is not a definition but an aspect.

You can actually "inherit" from multiple concrete classes if they implement interfaces themselves. innerclasses help you achieve that:
interface IBird {
public void layEgg();
}
interface IMammal {
public void giveMilk();
}
class Bird implements IBird{
public void layEgg() {
System.out.println("Laying eggs...");
}
}
class Mammal implements IMammal {
public void giveMilk() {
System.out.println("Giving milk...");
}
}
class Platypus implements IMammal, IBird {
private class LayingEggAnimal extends Bird {}
private class GivingMilkAnimal extends Mammal {}
private LayingEggAnimal layingEggAnimal = new LayingEggAnimal();
private GivingMilkAnimal givingMilkAnimal = new GivingMilkAnimal();
#Override
public void layEgg() {
layingEggAnimal.layEgg();
}
#Override
public void giveMilk() {
givingMilkAnimal.giveMilk();
}
}

I'd like to point out something that bit me in the behind, coming from C++ where you can easily inherit many implementations too.
Having a "wide" interface with many methods means that you'll have to implement a lot of methods in your concrete classes and you can't share these easily across implementations.
For instance:
interface Herbivore {
void munch(Vegetable v);
};
interface Carnivore {
void devour(Prey p);
}
interface AllEater : public Herbivore, Carnivore { };
class Fox implements AllEater {
...
};
class Bear implements AllEater {
...
};
In this example, Fox and Bear cannot share a common base implementation for both it's interface methods munch and devour.
If the base implementations look like this, we'd maybe want to use them for Fox and Bear:
class ForestHerbivore implements Herbivore
void munch(Vegetable v) { ... }
};
class ForestCarnivore implements Carnivore
void devour(Prey p) { ... }
};
But we can't inherit both of these. The base implementations need to be member variables in the class and methods defined can forward to that. I.e:
class Fox implements AllEater {
private ForestHerbivore m_herbivore;
private ForestCarnivore m_carnivore;
void munch(Vegetable v) { m_herbivore.munch(v); }
void devour(Prey p) { m_carnivore.devour(p); }
}
This gets unwieldy if interfaces grow (i.e. more than 5-10 methods...)
A better approach is to define an interface as an aggregation of interfaces:
interface AllEater {
Herbivore asHerbivore();
Carnivore asCarnivore();
}
This means that Fox and Bear only has to implement these two methods, and the interfaces and base classes can grow independetly of the aggregate AllEater interface that concerns the implementing classes.
Less coupling this way, if it works for your app.

I don't think they do.
Inheritance is specifically an implementation-oriented relationship between implementations. Interfaces do not provide any implementation information at all, but instead define a type. To have inheritance, you need to specifically inherit some behaviors or attributes from a parent class.
I believe there is a question here somewhere specifically about the role of interfaces and multiple inheritance, but I can't find it now...

There's really no simulation of multiple inheritance in Java.
People will sometimes say that you can simulate multiple inheritance using Interfaces because you can implement more than one interface per class, and then use composition (rather than inheritance) in your class to achieve the behaviors of the multiple classes that you were trying to inherit from to begin with.

If it makes sense in your object model, you can of course inherit from one class and implement 1 or more interfaces as well.

There are cases where multiple-inheritance turns to be very handy and difficult to replace with interfaces without writing more code. For example, there are Android apps that use classes derived from Activity and others from FragmentActivity in the same app. If you have a particular feature you want to share in a common class, in Java you will have to duplicate code instead of let child classes of Activity and FragmentsActivity derive from the same SharedFeature class. And the poor implementation of generics in Java doesn't help either because writing the following is illegal:
public class SharedFeature<T> extends <T extends Activity>
...
...

There is no support for multiple inheritance in java.
This story of supporting multiple inheritance using interface is what we developers cooked up. Interface gives flexibility than concrete classes and we have option to implement multiple interface using single class. This is by agreement we are adhering to two blueprints to create a class.
This is trying to get closer to multiple inheritance. What we do is implement multiple interface, here we are not extending (inheriting) anything. The implementing class is the one that is going to add the properties and behavior. It is not getting the implementation free from the parent classes. I would simply say, there is no support for multiple inheritance in java.

No, Java does not support multiple inheritance.
Neither using class nor using interface. Refer to this link for more info
https://devsuyed.wordpress.com/2016/07/21/does-java-support-multiple-inheritance

I also have to say that Java doesn't support multiple inheritance.
You have to differentiate the meaning between extends and implements keywords in Java. If we use extends, we are actually inheriting the class after that keyword. But, in order to make everything simple, we can't use extends more than once. But you can implement as many Interfaces as you wish.
If you implement an interface, there's a zero chance that you will miss the implementation of all the methods in each interface (Exception: default implementations of interface methods introduced in Java 8) So, you are now fully aware of what is happening with the things that you have embedded to your fresh class.
Why Java doesn't allow multiple inheritance is actually, multiple inheritance makes the code somewhat complex. Sometimes, two methods of parent classes might conflict due to having the same signatures. But if you are forced to implement all the methods manually, you will get the full understanding about what's going on, as I mentioned above. It makes your code more understandable to you.
If you need more info on Java interfaces, check out this article, http://www.geek-programmer.com/introduction-to-java-interfaces/

Between two Java class multiple Inheritance directly is not possible. In this case java recommend Use to interface and declare method inside interface and implement method with Child class.
interface ParentOne{
public String parentOneFunction();
}
interface ParentTwo{
public String parentTwoFunction();
}
class Child implements ParentOne,ParentTwo{
#Override
public String parentOneFunction() {
return "Parent One Finction";
}
#Override
public String parentTwoFunction() {
return "Parent Two Function";
}
public String childFunction(){
return "Child Function";
}
}
public class MultipleInheritanceClass {
public static void main(String[] args) {
Child ch = new Child();
System.out.println(ch.parentOneFunction());
System.out.println(ch.parentTwoFunction());
System.out.println(ch.childFunction());
}
}

Is it possible to hide or lower access to Inherited Methods in Java?

I have a class structure where I would like some methods in a base class to be accessible from classes derived directly from the base class, but not classes derived from derived classes. According to the Java Language specification it is possible to override access specifications on inherited methods to make them more public, but not more private. For example, this is the gist of what I need to do, but is illegal:
// Defines myMethod
public class Base {
protected void myMethod() {}
}
// Uses myMethod and then hides it.
public class DerivedOne extends Base {
#Override
private void myMethod();
}
// can't access myMethod.
public class DerivedTwo extends DerivedOne {
}
Is there any way to accomplish this?
Edited to explain why I would like to do this:
In this case the class structure is a data handling and import structure. It reads in and parses text files full of tabular data and then stores them in a database.
The base class is the base table class managing the database handling part of it. There is a fair amount of functionality contained in it that is common to all table types - as once they are in the database they become uniform.
The middle class is specific to the kind of table in the file being parsed, and has the table parsing and import logic. It needs access to some of the base class's database access functions.
The top level class is specific to the table and does nothing more than initialize the table's layout in a way the parent classes can understand. Also users of the base class do not need to see or access the database specific functions which the middle class do. In essence, I want to reveal these functions only to one level above the base class and no one else.
I ask because, although the code I posted as an example is illegal, there may be some other means to accomplish the same end. I'm asking if there is.
Perhaps hiding is the wrong way to phrase this - what I really need to do is expose some functionality that should be private to the base class to the class one level up in the hierarchy. Hiding would accomplish this - but I can see how hiding would be a problem. Is there another way to do this?

I think the very nature of the problem as you've posed it exposes conceptual problems with your object model. You are trying to describe various separate responsibilities as "is a" relationships when actually what you should be doing is describing "has a" or "uses a" relationships. The very fact that you want to hide base class functionality from a child class tells me this problem doesn't actually map onto a three-tiered inheritance tree.
It sounds like you're describing a classic ORM problem. Let's look at this again and see if we can re-map it onto other concepts than strict "is a" inheritance, because I really think your problem isn't technical, it's conceptual:
You said:
The base class is the base table class
managing the database handling part of
it. There is a fair amount of
functionality contained in it that is
common to all table types - as once
they are in the database they become
uniform.
This could be more clear, but it sounds like we have one class that needs to manage the DB connection and common db operations. Following Single Responsibility, I think we're done here. You don't need to extend this class, you need to hand it to a class that needs to use its functionality.
The middle class is specific to the
kind of table in the file being
parsed, and has the table parsing and
import logic. It needs access to some
of the base class's database access
functions.
The "middle class" here sounds a bit like a Data Mapper. This class doesn't need to extend the previous class, it needs to own a reference to it, perhaps injected on the constructor or a setter as an interface.
The top level class is specific to the
table and does nothing more than
initialize the table's layout in a way
the parent classes can understand.
Also users of the base class do not
need to see or access the database
specific functions which the middle
class do. In essence, I want to reveal
these functions only to one level
above the base class and no one else.
I'm not clear why a high-level class seems to have knowledge of the db schema (at least that's what the phrase "initialize the table's layout" suggests to me), but again, if the relationship between the first two classes were encapsulation ("has a"/"uses a") instead of inheritance ("is a"), I don't think this would be a problem.

No. I'm not sure why you'd quote the spec and then ask if there's any way to do the opposite of what the spec says...
Perhaps if you explain why you want to do this, you could get some suggestions on how.

When overriding a method you can only make it more public, not more private. I don't know why you use the word "general"
Remember that, ordering from least to most restrictive:
public<protected<default<private
Yes, "protected" is a less restrictive access modifier than default (when no modifier is used), so you can override a default method marking the overriding method as protected, but not do the opposite.
Can:
You can override a protected method with a public one.
Can't:
You can't override a public method with a protected one.

If you did this then DerivedOne would not be a Base, from the DerivedTwo's point of view. Instead what you want is a wrapper class
//Uses myMethod but keeps it hidden
public class HiddenBase {
private final Base base = new Base();
private void myMethod();
public void otherMethod() {base.otherMethod();}
}
You can't access protected methods of the base though this way...

What you describe comes close to what the protected access class is for, derived classes can access, all others cannot.
If you inherit from base classes you have no control over this might pose a problem, you can make the method inaccesible to others by throwing an exception while making the inherited code available to your classes by calling super directly, something like:
// Uses myMethod and then hides it.
public class DerivedOne extends Base {
#Override
public void myMethod() {
throw new IllegalStateException("Illegal access to myMethod");
}
private void myPrivateMethod() {
super.myMethod();
}
}
Edit: to answer your elaboration, if I understand you correctly you need to specify behaviour in the context of the base class which is defined in the middle class. Abstract protected methods won't be invisible to the classes deriving from the middle class.
One possible approach is to define an interface with the methods you would need to be abstract in the base class, keeping a private final reference in the base class and providing a reference to the implementation when constructing the middle class objects.
The interface would be implemented in a (static?) nested inside the middle class. What I mean looks like:
public interface Specific {
public void doSomething();
}
public class Base {
private final Specific specificImpl;
protected Base(Specific s) {
specificImpl = s;
}
public void doAlot() {
// ...
specificImpl.doSomething();
// ...
}
}
public class Middle extends Base {
public Middle() {
super(new Impl());
}
private Impl implements Specific {
public void doSomething() {
System.out.println("something done");
}
}
}
public class Derived extends Middle {
// Access to doAlot()
// No access to doSomething()
}

Inheritance works because everywhere you can use the base class, you can also use one of it's subclasses. The behavior may be different, but the API is not. The concept is known as the Liskov substitution principle.
If you were able to restrict access to methods, the resulting class would not have the same API and you would not be able to use substitute an instance of the base class for one of the derived classes, negating the advantage of inheritance.
What you actually want to accomplish can be done with interfaces:
interface IBase1 {
}
class Derived1 implements IBase1 {
public void myMethod() {
}
}
class Derived2 implements IBase1 {
}
class UseMe {
public void foo(IBase1 something) {
// Can take both Derived1 and Derived2
// Can not call something.myMethod()
}
public void foo(Derived1 something) {
something.myMethod();
}
public void foo(Derived2 something) {
// not something.myMethod()
}
}

It is possible, but requires a bit of package manipulation and may lead to a structure that is a bit more complex than you would like to work with over the long haul.
consider the following:
package a;
public class Base {
void myMethod() {
System.out.println("a");
}
}
package a;
public class DerivedOne extends Base {
#Override
void myMethod() {
System.out.println("b");
}
}
package b;
public class DerivedTwo extends a.DerivedOne {
public static void main(String... args) {
myMethod(); // this does not compile...
}
}
I would recommend being nice to yourself, your co-workers and any other person that ends up having to maintain your code; rethink your classes and interfaces to avoid this.

you have to make method final when override it
public class Base {
protected void myMethod() {}
}
// Uses myMethod and then hides it.
public class DerivedOne extends Base {
#Override
final protected void myMethod(); //make the method final
}
public class DerivedTwo extends DerivedOne {
// can't access myMethod here.
}