So here's the scenario.
I have a class A, and I want some of its methods to behave differently depending on how the class is called.
What I've done now is something like this:
interface Interface{
public void someMethod();
}
class A{
int variable;
Interface getA1(){
return new A1();
}
Interface getA2(){
return new A2();
}
class A1 extends A implements Interface{
public void someMethod(){
variable++;
}
}
class A2 extends A implements Interface{
public void someMethod(){
variable--;
}
}
}
Now the problem with this is that when getA1() or getA2() is called, it will actually return a completely new instance of A1, or A2, where the int variable is totally separate to the parent class A (duh, silly me of course thats how it works).
So the question is. Is it possible in java to return some kind of inner class that allows you to modify the data within the outer class through it's methods?
The key thing is having a single class that can return interfaces that modify that single class in different ways, depending on which method was used to create that interface.
I might be overlooking something simple here, sorry but it's got me stumped! :(
Edit: I think the better way of explaining it is - I want to give direct access to local variables, to a subclass (or another class), and have a method in the original class that can create the other class.
EDIT2: The reason why it created a separate instance when getA1()/getA2(), is likely because class A1 and A2 were declared to extend A. It seems like if the extends clause is removed, then it works as expected.
Yes an anonymous class would do e.g.
interface Interface{
public void someMethod();
}
public class A {
int variable;
Interface getA1() {
return new Interface() {
#Override
public void someMethod() {
variable++;
}
};
}
}
It should be pointed out though that this is equivalent to declaring non-static inner classes albeit more concise.
First of all, I find it very strange that your inner classes are extending the outer class. I can't think of a good reason to do that and I'm not exactly sure what that even does.
Besides that, I don't understand your issue. All inner classes have access to their parent class's fields. For example, the following:
public class Tester {
private int myInt;
public InnerTester getInnerTester() {
return new MyInnerTester();
}
public InnerTester getOtherInnerTester() {
return new OtherInnerTester();
}
public interface InnerTester {
public void doStuff();
}
public class MyInnerTester implements InnerTester{
#Override
public void doStuff() {
myInt++;
}
}
public class MyOtherInnerTester implements InnerTester {
#Override
public void doStuff() {
myInt++;
}
}
public static void main(String[] args) {
Tester tester = new Tester();
System.out.println("Before: " + tester.myInt);
tester.getInnerTester().doStuff();
tester.getOtherInnerTester().doStuff();
System.out.println("After: " + tester.myInt);
}
}
outputs:
Before: 0
After: 2
However, this all seems pretty shady. I'm not exactly sure why you would want to take advantage of this behavior. I've done similar things with action listeners before, but I wouldn't exactly call their parent object an "ActionListenerFactory". Why don't you just have a method to increment the field on the object itself?
Related
Which access modifier, in an abstract class, do I have to use for a method,
so the subclasses can decide whether it should be public or not? Is it possible to "override" a modifier in Java or not?
public abstract class A {
??? void method();
}
public class B extends A {
#Override
public void method(){
// TODO
}
}
public class C extends B {
#Override
private void method(){
// TODO
}
}
I know that there will be a problem with static binding, if
someone calls:
// Will work
A foo = new B()
foo.method();
// Compiler ?
A foo = new C();
foo.method();
But maybe there is another way. How I can achieve that?
It is possible to relax the restriction, but not to make it more restrictive:
public abstract class A {
protected void method();
}
public class B extends A {
#Override
public void method(){ // OK
}
}
public class C extends A {
#Override
private void method(){ // not allowed
}
}
Making the original method private won't work either, since such method isn't visible in subclasses and therefore cannot be overriden.
I would recommend using interfaces to selectively expose or hide the method:
public interface WithMethod {
// other methods
void method();
}
public interface WithoutMethod {
// other methods
// no 'method()'
}
public abstract class A {
protected void method();
}
public class B extends A implements WithMethod {
#Override
public void method(){
//TODO
}
}
public class C extends B implements WithoutMethod {
// no 'method()'
}
... then only work with the instances through the interfaces.
When overriding methods, you can only change the modifier to a wider one, not vice versa. For example this code would be valid:
public abstract class A {
protected void method();
}
public class B extends A {
#Override
public void method() { }
}
However, if you try to narrow down the visibility, you'd get a compile-time error:
public abstract class A {
protected void method();
}
public class B extends A {
#Override
private void method() {}
}
For your case, I'd suggest to make C not implementing A, as A's abstraction implies that there's a non-private method():
public class C {
private void method(){
//TODO
}
}
Another option is to make the method() implementation in C throwing a RuntimeException:
public class C extends A {
#Override
public void method(){
throw new UnsupportedOperationException("C doesn't support callbacks to method()");
}
}
What you are asking for is not possible for very good reasons.
The Liskov substitution principle basically says: a class S is a subclass of another class T only then, when you can replace any occurrence of some "T object" with some "S object" - without noticing.
If you would allow that S is reducing a public method to private, then you can't do that any more. Because all of a sudden, that method that could be called while using some T ... isn't available any more to be called on S.
Long story short: inheritance is not something that simply falls out of the sky. It is a property of classes that you as the programmer are responsible for. In other words: inheritance means more than just writing down "class S extends T" in your source code!
This is impossible because of the polymorphism. Consider the following. You have the method in class A with some access modifier which is not private. Why not private? Because if it was private, then no other class could even know of its existence. So it has to be something else, and that something else must be accessible from somewhere.
Now let's suppose that you pass an instance of class C to somewhere. But you upcast it to A beforehand, and so you end up having this code somewhere:
void somewhereMethod(A instance) {
instance.method(); // Ouch! Calling a private method on class C.
}
One nice example how this got broken is QSaveFile in Qt. Unlike Java, C++ actually allows to lower access privileges. So they did just that, forbidding the close() method. What they ended up with is a QIODevice subclass that is not really a QIODevice any more. If you pass a pointer to QSaveFile to some method accepting QIODevice*, they can still call close() because it's public in QIODevice. They “fixed” this by making QSaveFile::close() (which is private) call abort(), so if you do something like that, your program immediately crashes. Not a very nice “solution”, but there is no better one. And it's just an example of bad OO design. That's why Java doesn't allow it.
Edit
Not that I missed that your class is abstract, but I also missed the fact that B extends C, not A. This way what you want to do is completely impossible. If the method is public in B, it will be public in all subclasses too. The only thing you can do is document that it shouldn't be called and maybe override it to throw UnsupportedOperationException. But that would lead to the same problems as with QSaveFile. Remember that users of your class may not even know that it's an instance of C so they won't even have a chance to read its documentation.
Overall it's just a very bad idea OO-wise. Perhaps you should ask another question about the exact problem you're trying to solve with this hierarchy, and maybe you'll get some decent advises on how to do it properly.
Here is a part of the #Override contract.
The answer is : there isn't any possibility to achieve what you have.
The access level cannot be more restrictive than the overridden
method's access level. For example: if the superclass method is
declared public then the overridding method in the sub class cannot be
either private or protected.
This is not a problem concerning abstract classes only but all classes and methods.
THEORY:
You have the determined modifiers order:
public <- protected <- default-access X<- private
When you override the method, you can increase, but not decrease the modifier level. For example,
public -> []
protected -> [public]
default-access -> [public, default-access]
private -> []
PRACTICE:
In your case, you cannot turn ??? into some modifier, because private is the lowest modifier and private class members are not inherited.
I would like to prevent a class from calling its own method. The method shall only be callable by its super class.
Right now, I cannot think of any way to achieve this (cleanly). But maybe someone knows a solution?
In code:
public abstract class A {
protected abstract void foo();
private void barA() {
//do smth
foo();
}
}
public class B extends A {
#Override
protected void foo() {
//do smth
}
private void barB() {
//must not be able to call foo() here
}
}
Edit: the explanation why I would like to do this:
A is lets say a vehicle. B can be a car or an airplane. The method foo() would be startEngines(). -> I want to make sure that the engines can only be started by calling the method barA().... does that make any sense?
There is a way to do it, but you need to use Google Error Prone. This is an extension of the Java compiler that aims to provide more and more helpful warnings and errors (similar to FindBugs and PMD, but with less false alarms). I can only recommend it, it has already helped us to find some bugs.
Specifically, it contains an annotation #ForOverride and an according compile-time check. This annotation is meant to be used for protected methods that the sub-class and any other class should not call, but only the defining class.
So using
public abstract class A {
#ForOverride
protected abstract void foo();
private void barA() {
//do smth
foo();
}
}
would exactly achieve what you want.
You can integrate Error Prone into most build systems like Maven and Ant. Of course, it won't help if somebody compiles your source without Error Prone (for example in Eclipse), but using it in a continous-integration system would still allow you to find such issues. The source code still stays compatible with regular Java compilers (provided you have error_prone_annotations.jar on the class path), other compilers will simply not do the additional checks.
this answer has a good hint.
add below method in your class (class B):
public static String getMethodName(final int depth)
{
final StackTraceElement[] ste = Thread.currentThread().getStackTrace();
return ste[ste.length - 1 - depth].getMethodName();
}
and change the foo method in class B to this:
#Override
protected void foo() {
//....
if (getMethodName(0)=="barB"){
// tell you are not able to call barB
}
}
Considering your vehicle and engine scenario, I think you need to reconsider your design a bit.
Your vehicle could be a car, aeroplane, etc but car, aeroplane, ... each have separate engines and therefore different startEngine method. So declare your class vehicle as abstract like you did and class startEngine as abstract method . Next , subclass Vehicle and implement startEngine in them , now you can invoke startEngine on the subclass instances
abstract class Vehicle{
abstract void startEngine();
}
public class Car extends Vehicle{
public void startEngine(){
//implementation
}
public static void main(String[] arg){
Vehicle v=new Car();
v.startEngine();
}
}
Add Anonymouse inner class to barA method via Interface, so you will need to implement a method for foo() (functional interface). It won't be part of Class B.
you could put an interface as a member in the super class given to it via the constructor. the child class implements the method but can't call it except by making it static.
interface Foo {
void stopEngines();
void startEngines();
}
abstract class Base {
final private Foo foo;
public Base(final Foo foo) {
this.foo = foo;
}
private void barA() {
// do smth
foo.startEngines();
}
}
class Child extends Base {
public Child() {
super(new Foo() {
boolean engineRunning;
#Override
public void stopEngines() {
this.engineRunning = false;
}
#Override
public void startEngines() {
this.engineRunning = true;
}
});
}
private void barB() {
// can't call startEngines() or stopEngines() here
}
}
class Child2 extends Base {
public Child2() {
super(new Foo() {
#Override
public void stopEngines() {
stopEngines();
}
#Override
public void startEngines() {
startEngines();
}
});
}
static void stopEngines() {
// influence some static state?
}
static void startEngines() {
// influence some static state?
}
private void barB() {
// can call stopEngines() and startEngines(), but at least they have to be static
}
}
Of course, this is not really what you asked for, but about as much as you can do about it in Java, I guess.
Seeing the startEngines explanation, this solution might even suffice.
I guess you wouldn't care about the class calling its static methods, since they can only influence a static state, which is used seldom. The methods within the anonymous interface implementation can mutually call each other, but I guess that would be OK, since you only seem to be trying to prevent others to start the engines in some different way.
I guess this is similar to the problem AWT/Swing has with overriding the paint(Graphics g) method on a component (or onCreate(..) in Android Activities). Here you are overriding the paint method but you should never call it.
I think the best thing you can do is add documentation to the method to clarify that it should never be explicitly called by the subclasses OR re-evaluate your design.
I have a basic question about generics in Java.
I have a class X which is instantiated by another class T. In every class T which will be used has a method called as methodOfT(). However, Java gives me compiler time error since it does not know obj and methodOfT().
public class X<T>
{
T obj;
public void methodOfX()
{
obj.methodOfT();
}
}
In order to avoid this problem, what I did is I defined another class XSuper. And every class now which wants to instantiate X will extend this XSuper class. This removes the compile time error and allows me to achieve what I want.
public abstract class XSuper
{
public abstract void methodOfT();
}
public class UserOfX extends XSuper
{
X<UserOfX> objX = new X<UserOfX>();
public void methodOfT()
{
}
}
However, I want to know the cleaner way of doing this thing. Since I want to derive class UserOfX from another Class. Another Problem is that I want to define methodOfT() method as -
public methodOfT(T objOfT)
{
}
In this case, the above solution fails. Could someone help.
public class X<T>
{
T obj;
public void methodOfX()
{
obj.methodOfT();
}
}
The compiler doesn't know what T is so it is evaluated as Object. Object does not have a methodOfT method, so compilation fails. Here's how to solve that:
public interface SomeInterface{
void methodOfT();
}
public class X<T extends SomeInterface>
{
T obj;
public void methodOfX()
{
obj.methodOfT();
}
}
In this case, the compiler knows that the supplied T will implement the interface SomeInterface and hence have the method methodOfT. (You can also use classes instead of interfaces, but that's less elegant)
Given your additional requirements, we're going t have to change this code some more:
public interface SomeInterface<X>{
void methodOfT(X object);
}
public class X<T1, T2 extends SomeInterface<T1>>
{
T1 obj1;
T2 obj2;
public void methodOfX()
{
obj2.methodOfT(obj1);
}
}
public class Testing extends JDialog {
public MyClass myClass;
public Testing() {
}
}
given the above code, is it possible to override a method in myClass in Testing class?
say myClass has a method named computeCode(), will it be possible for me to override it's implementations in Testing? sorry it's been a long time since I've coded.
if you want to override a method from MyClass then your testing class must extend that. for overriding a method one must complete IS-A relationship whereas your code comes under HAS-A relationship.
Yes, it is generally possible (note that as others have correctly mentioned - you'd need to extend it to override the method). Refer to this sample:
public class Animal {
public void testInstanceMethod() {
System.out.println("The instance method in Animal.");
}
}
public class Cat extends Animal {
public void testInstanceMethod() {
System.out.println("The instance method in Cat.");
}
public static void main(String[] args) {
Cat myCat = new Cat();
Animal myAnimal = myCat;
myAnimal.testInstanceMethod();
}
}
Not only is it possible, but it is a key feature in polymorphism an code reusability.
Note, however, that MyClass.computeCode might be final - in this case, it cannot be overridden.
You override methods of classes that you extend. Therefore, in your example your Testing class could override the various existing methods of JDialog. If you wanted to override computeCode() from MyClass (assuming it's not final), you should make Testing extend MyClass.
public class Testing extends MyClass
{
#Override
public int computeCode()
{
return 1;
}
}
You can override a class's method only in a subclass (a class that extends the class whose method you want to override). However, given your skeletal code, you can (within Testing) have a nested class that extends MyClass and force an instance of that nested class into the myClass instance variable... so, the answer must be "yes".
Whether that's the best choice (as opposed to using interfaces, rather than subclassing concrete classes, and relying on Dependency Injection to get the implementations most suited for your testing), that's a different question (and my answer would be, unless you're testing legacy code that you can't seriously refactor until it's well test-covered... then, probably not;-).
See, if you want to override method from MyClass then you need to extend it.
As per your code, it seems you want to make a wrapper wround MyClass.
Wrapper means, calling implemented class method will call method of MyClass.
I am just clearing how wrapping works as below.
public class Testing extends JDialog {
public MyClass myClass;
public Testing() {
}
public void someMethod() {
//Add some more logic you want...
...
..
myClass.computeCode();
}
}
thanks.
The wording of the question is confused and lost.
Here are some key points:
You can't #Override something that you didn't inherit to begin with
You can't #Override something that is final
Here's a small example:
import java.util.*;
public class OverrideExample {
public static void main(String[] args) {
List<String> list = new ArrayList<String>(
Arrays.asList("a", "b", "c")
) {
#Override public String toString() {
return "I'm a list and here are my things : " + super.toString();
}
};
System.out.println(list);
// prints "I'm a list and here are my things : [a, b, c]"
}
}
Here, we have an anonymous class that #Override the toString() method inherited from java.util.ArrayList.
Note that here, it's not class OverrideExample that overrides the ArrayList.toString(); it's the anonymous class that (implicitly) extends ArrayList that does.
All the above answers are valid. But, if you want to extend JDialog but still if you want to override a method of another class it is possible through interfaces. Interfaces won't have method definitions but will have method declarations. More about interfaces, you can read at http://java.sun.com/docs/books/tutorial/java/concepts/interface.html
In your case, you can make use of interface like this
public interface MyInterface{
public void myMethod();
}
public class Testing extends javax.swing.JDialog implements MyIterface{
public void myMethod(){
// code for your method
}
}
Since Testing class has already inherited JDialog, there is no way let it inherit MyClass again unless to implement an interface. What you can do is to use some design pattern. However this is not overriding, since there is no inheritance. The Adapter is the one you need. Again you are losing the flexibility of polymorphism.
public class Testing extends JDialog {
MyClass myClass = new MyClass();
public Testing() {
}
public void methodA(){
myClass.methodA();
}
}
class MyClass {
public void methodA(){}
}
I am trying to wrap my mind around something in java. When I pass an object to another class' method, can I not just call any methods inherent to that object class?
What is the reason code such as the example below does not compile?
Thank you,
class a {
public static void myMethod(Object myObj) {
myObj.testing();
}
}
class b {
public void testing() {
System.out.println ("TESTING!!!");
}
}
class c {
public static void main (String[] args) {
b myB = new b();
a.myMethod(myB);
}
}
Edit: The reason I have left the parameter in myMethod as type Object, is because I would like to be able to pass in a variety of object types, each having a testing() method.
If you would like to pass in a variety of objects with testing() methods, have each object implement a Testable interface:
public interface Testable
{
public void testing()
}
Then have myMethod() take a Testable.
public static void myMethod(Testable testable)
{
testable.testing();
}
Edit: To clarify, implementing an interface means that the class is guaranteed to have the method, but the method can do whatever it wants. So I could have two classes whose testing() methods do different things.
public class AClass implements Testable
{
public void testing()
{
System.out.println("Hello world");
}
}
public class BClass implements Testable
{
public void testing()
{
System.out.println("Hello underworld");
}
}
The problem is that myMethod can't know it's getting a b object until it actually runs. You could pass a String in, for all it knows.
Change it to
public static void myMethod(b myObj) {
myObj.testing();
}
and it should work.
Update of the question:
Edit: The reason I have left the parameter in myMethod as type Object, is because I would like to be able to pass in a variety of object types, each having a testing() method.
As Amanda S and several others have said, this is a perfect case for an interface. The way to do this is to create an interface which defines the testing() method and change myMethod to take objects implementing that interface.
An alternative solution (without interfaces) would be to reflectively discover if the object has a testing() method and call it, but this is not recommended and not needed for a such a simple case.
What you are talking about is duck typing. Java doesn't have duck typing.
Therefore you need to define an interface that all the classes with a testing() method implement.
e.g:
public interface Testable
{
public void testing()
}
class B implements Testable
{
public void testing() {
System.out.println ("TESTING!!!");
}
}
class A {
public static void myMethod(Testable myObj) {
myObj.testing();
}
}
Your issue is a classic argument in favor of an interface. You want as generic as possible, yet you want every object you pass to have a testing() method. I suggest something along the lines of the following:
public interface Testable
{
public void testing();
}
public class A
{
public static void myMethod(Testable myObj)
{
myObj.testing();
}
}
public class B implements Testable
{
public void testing()
{
System.out.println("This is class B");
}
}
public class C implements Testable
{
public void testing()
{
System.out.println("This is class C");
}
}
public class Test
{
public static void main (String[] args)
{
B myB = new B();
C myC = new C();
A.myMethod(myB); // "This is class B"
A.myMethod(myC); // "This is class C"
}
}
Because you're passing in an Object (b inherit from Object). Object doesn't have testing, b does.
You can either pass in b or cast the object to b before calling the method.
EDIT
To pass in a generic class that implements that method: you'll want to make an interface that has the method signature and pass in the interface type instead of Object. All objects that you pass in must implement the interface.
You can only access the members that are visible for the type of reference you have to the object.
In the case of myMethod(Object myObj) that means only the members defined in Object, so in class a the members of class b will not be visible.
If you changed the definition of a.myMethod to be public static void myMethod(b myObj) you would then be able to see the testing method on the instance of b while in myMethod.
update based on clarification:
In that case defining an interface for all of them to implement is likely what you want.
public interface Testable {
public void testing();
}
public class a {
public static void myMethod(Testable myObj) {
myObj.testing();
}
}
public class b implements Testable {
public void testing () {
System.out.println("TESTING!!!");
}
}
Why can’t java find my method?
Because of the way Java was designed.
Java is "statically typed" that means objects types are checked during compilation.
In Java you can invoke a method only if that method belongs to that type.
Since this verification is made during compilation and the Object type does not have the "testing()" method, the compilation fails ( even though if at runtime the objects do have that method". This is primarily for safety.
The workaround as described by others will require you to create a new type, where you can tell the compiler
"Hey, the instances of this type will respond the the testing method"
If you want to pass a variety of objects and keep it very generic, one way is having those objects to implement and interface.
public interface Testable {
public void testing();
}
class A implements Testable { // here this class commits to respond to "testing" message
public void testing() {
}
}
class B implements Testable { // B "is" testable
public void testing() {
System.out.println("Testing from b");
}
}
class C implements Testable { // C is... etc.
public void testing() {
//....
}
}
Later somewhere else
public void doTest( Testable object ) {
object.testing();
}
doTest( new A() );
doTest( new B() );
doTest( new C() );
The "OTHER" way to do this, in java is invoking the methods reflectively, but I'm not sure if that's what you need, for the code is much more abstract when you do it that way, but that's how automated testing frameworks (and a lot of other frameworks such as Hibernate) do actually work.
I hope this help you to clarify the reason.
If you REALLY, REALLY want to keep the parameter as abstract as possible, you should consider reflection API. That way, you can pass whatever object you want and dynamically execute the method you want. You can take a look at some examples.
It's not the only way, but it might be a valid alternative depending on your problem.
Keep in mind that reflection is way slower than calling your methods directly. You might consider using an interface as well, such as the one on Amanda's post.