Multiple inheritance for an anonymous class

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).

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.

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
   }

 }

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.

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();
}