Multiple inheritance for an anonymous class
JavaMultiple InheritanceAnonymous ClassJava Problem Overview
How can an anonymous class implement two (or more) interfaces? Alternatively, how can it both extend a class and implement an interface? For example, I want to create an object of anonymous class that extends two interfaces:
// Java 10 "var" is used since I don't know how to specify its type
var lazilyInitializedFileNameSupplier = (new Supplier<String> implements AutoCloseable)() {
private String generatedFileName;
@Override
public String get() { // Generate file only once
if (generatedFileName == null) {
generatedFileName = generateFile();
}
return generatedFileName;
}
@Override
public void close() throws Exception { // Clean up
if (generatedFileName != null) {
// Delete the file if it was generated
generatedFileName = null;
}
}
};
Then I can use it in a try-with-resources block as AutoCloseable
as lazily-initialized utility class:
try (lazilyInitializedFileNameSupplier) {
// Some complex logic that might or might not
// invoke the code that creates the file
if (checkIfNeedToProcessFile()) {
doSomething(lazilyInitializedFileNameSupplier.get());
}
if (checkIfStillNeedFile()) {
doSomethingElse(lazilyInitializedFileNameSupplier.get());
}
}
// By now we are sure that even if the file was generated, it doesn't exist anymore
I don't want to create an inner class because I'm absolutely sure that this class won't be used anywhere except the method I need to use it in (and I also might want to use local variables declared in that method that might be of var
type).
Java Solutions
Solution 1 - Java
Anonymous classes must extend or implement something, like any other Java class, even if it's just java.lang.Object
.
For example:
Runnable r = new Runnable() {
public void run() { ... }
};
Here, r
is an object of an anonymous class which implements Runnable
.
An anonymous class can extend another class using the same syntax:
SomeClass x = new SomeClass() {
...
};
What you can't do is implement more than one interface. You need a named class to do that. Neither an anonymous inner class, nor a named class, however, can extend more than one class.
Solution 2 - Java
An anonymous class usually implements an interface:
new Runnable() { // implements Runnable!
public void run() {}
}
JFrame.addWindowListener( new WindowAdapter() { // extends class
} );
If you mean whether you can implement 2 or more interfaces, than I think that's not possible. You can then make a private interface which combines the two. Though I cannot easily imagine why you would want an anonymous class to have that:
public class MyClass {
private interface MyInterface extends Runnable, WindowListener {
}
Runnable r = new MyInterface() {
// your anonymous class which implements 2 interaces
}
}
Solution 3 - Java
I guess nobody understood the question. I guess what this guy wanted was something like this:
return new (class implements MyInterface {
@Override
public void myInterfaceMethod() { /*do something*/ }
});
because this would allow things like multiple interface implementations:
return new (class implements MyInterface, AnotherInterface {
@Override
public void myInterfaceMethod() { /*do something*/ }
@Override
public void anotherInterfaceMethod() { /*do something*/ }
});
this would be really nice indeed; but that's not allowed in Java.
What you can do is use local classes inside method blocks:
public AnotherInterface createAnotherInterface() {
class LocalClass implements MyInterface, AnotherInterface {
@Override
public void myInterfaceMethod() { /*do something*/ }
@Override
public void anotherInterfaceMethod() { /*do something*/ }
}
return new LocalClass();
}
Solution 4 - Java
Anonymous classes always extend superclass or implements interfaces. for example:
button.addActionListener(new ActionListener(){ // ActionListener is an interface
public void actionPerformed(ActionEvent e){
}
});
Moreover, although anonymous class cannot implement multiple interfaces, you can create an interface that extends other interface and let your anonymous class to implement it.
Solution 5 - Java
// The interface
interface Blah {
void something();
}
...
// Something that expects an object implementing that interface
void chewOnIt(Blah b) {
b.something();
}
...
// Let's provide an object of an anonymous class
chewOnIt(
new Blah() {
@Override
void something() { System.out.println("Anonymous something!"); }
}
);
Solution 6 - Java
An anonymous class is extending or implementing while creating its object For example :
Interface in = new InterFace()
{
..............
}
Here anonymous class is implementing Interface.
Class cl = new Class(){
.................
}
here anonymous Class is extending a abstract Class.