Java Pass Method as Parameter

Since Java 8 there is a Function<T, R> interface (docs), which has method

R apply(T t);

You can use it to pass functions as parameters to other functions. T is the input type of the function, R is the return type.

In your example you need to pass a function that takes Component type as an input and returns nothing – Void. In this case Function<T, R> is not the best choice, since there is no autoboxing of Void type. The interface you are looking for is called Consumer<T> (docs) with method

void accept(T t);

It would look like this:

public void setAllComponents(Component[] myComponentArray, Consumer<Component> myMethod) {
    for (Component leaf : myComponentArray) {
        if (leaf instanceof Container) { 
            Container node = (Container) leaf;
            setAllComponents(node.getComponents(), myMethod);
        } 
        myMethod.accept(leaf);
    } 
}

And you would call it using method references:

setAllComponents(this.getComponents(), this::changeColor);
setAllComponents(this.getComponents(), this::changeSize); 

Assuming that you have defined changeColor() and changeSize() methods in the same class.

If your method happens to accept more than one parameter, you can use BiFunction<T, U, R> – T and U being types of input parameters and R being return type. There is also BiConsumer<T, U> (two arguments, no return type). Unfortunately for 3 and more input parameters, you have to create an interface by yourself. For example:

public interface Function4<A, B, C, D, R> {

    R apply(A a, B b, C c, D d);
}