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I am trying to create a generic menu over an arbitrary type T implementing a Menuable interface:

public interface Procedure {
    void invoke();
}

public class Menu<T extends Menuable> implements Menuable {
    List<T> items; //The items of the menu
    List<String> itemsDisplay; //A list of String specifying how to display each of the above items
    Procedure procedure; //A method to be performed on the elements of items

    public Menu(List<T> items, List<String> itemsDisplay) {
        this.items = items;
        this.itemsDisplay = itemsDisplay;
    }

    //Returns a String display of the menu
    //Each item is numbered and displayed as specified by the list itemsDisplay
    public String menuDisplay() {

        String s = "";
        
        int i;
        for (i = 1; i <= this.itemsDisplay.size(); i++) {
            s = s + i + "\t" + this.itemsDisplay.get(i-1) + "\n";
        }
  
    return s;
    }
    ...
}

The menu will be printed on a terminal, and when the user chooses an item, I would like the menu to be able to perform any instance method on this item. Basically, here is what I would like to do:

public Menu(List<T> items, List<String> itemsDisplay, Procedure procedure) {
    this.items = items;
    this.itemsDisplay = itemsDisplay;
    Here I would like to assign a method to this.procedure for later use,
    but without specifying on which object to use it yet.
}

public void waitForAction() throws IOException {
        
    //Display the menu to the user
    System.out.println(this.menuDisplay());
    
    BufferedReader inputs = new BufferedReader(new InputStreamReader(System.in));

    int choice = 0;
    //Read inputs until the user enters a valid choice
    while(!(1 <= choice && choice <= this.items.size())) {

        try {
            choice = Integer.parseInt(inputs.readLine());
        } catch(NumberFormatException e) {
        }
    }

    //The item selected is at index (choice - 1) in the list this.items
    Here I would like to use my method on this.items.get(choice - 1).
}

I am new to the idea of using methods as variables / arguments and I don't know how to do that. I heard about lambda expressions and method references in Java, but from what I understood, when using them you have to specify the method you want to use and its instance / parameters at the same time, which is awkward. Am I missing something ?

Konrad Rudolph
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EternalApprentice
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    Why you want to use methods if you can have simple objects? To get methods accessible and operate on them you will need incorporate the reflection mechanism meanwhile you can have some simple interface simmilar to java.lang.Runnable, java.util.function.Function, java.util.function.Consumer, etc. For this particular case I guess using a simple consumer will be fine. – Kamil Jun 15 '21 at 11:01
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    Don’t post all that irrelevant stuff. We don’t need to know how you assemble your menu strings nor how you read an integer from `System.in`. Your question is about that `Procedure`, so the relevant parts would be how you initialize the `procedure` variable and how you invoke its method, yet, you didn’t show anything about it. It would also be interesting how `Menuable` is defined as it seems that there is an overlapping between that interface and `Procedure`… – Holger Jun 22 '21 at 07:31

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