如何在不创建子类和/或继承类的情况下从另一个类调用Object的方法？

Question

I've been learning basic code at a very beginner level. Now I'm finally starting to dabble in actually writing simple programs, and got really stuck.

• I'm writing a simple a program that consists of two classes; People, MainPage.

• Once the program runs, the method openApp() is called in main method from (MainPage Class).

   public static void main(String[] args) { openApp(); } 

• Next, when the openApp() is called, the user has three menus to choose to go to that are selected by entering the corresponding number

  ie 1 = Newsfeed, 2 = Profile or 3 = Friends.

public class MainPage {

public static void openApp() {


    System.out.println("Welcome to App!");
    System.out.println();
    System.out.println("To Select Option for:");
    System.out.println("Newsfeed : 1");
    System.out.println("Profile :  2");
    System.out.println("Friends :  3");
    System.out.println("Enter corresponding number: ");
    int optionSelected = input.nextInt();

    switch (optionSelected) { 

    case 1: System.out.println("NewsFeed");
             break;
    case 2:  System.out.println("Profile");
             break;
    case 3:  System.out.println("Friends");
        break;

        if (optionSelected == 3) {
            people.friend();// Is it possible to write: friend() from "People" Class without extending to People Class
                    }

    }
}

• If User selects "friends" then the program calls a method from
  People Class called friend(People name) in MainPage class that prints out people object's friends.

My Attempt:

  if (optionSelected == 3) {
        people.friend();
                }

The Error I get:

Exception in thread "main" java.lang.Error: Unresolved compilation problem: people cannot be resolved

Problem is I don't want to extend People Class in MainPage and inherit all it's methods, yet I still want to call an Object method from People Class to print people object's friends.

Note: just in case anyone would want to look at the friend(People people) method that is located in the People class:

public void friend(People people) {
    System.out.println(people.friend);

Answer 1

Excellent question format.

You can declare an Object of type People , and use that.

public class MainPage
{
    People people = new People();

    // .. Some code.

    if(optionSelected == 3) {
        people.friend();
    } 
}

Explanation

Your friend method is an instance method . This means that in order to access it, you need to have an instance of the object created. This is done with the new keyword. Secondly, unless People is some form of utility class, then your friend method should probably read more like:

 public void friend()
 {
     System.out.println(this.friend);
 }

And for the sake of good code design, remember that your MainPage class is outputting to the user, so you should return the value rather than print it. Secondly, you should conform to good naming standards, and in Java we use the get prefix when getting a class member.

public void getFriend()
{
    return this.friend;
}

and in the MainPage class, you should print this.

if(optionSelected == 3)
{
   System.out.println(people.getFriend());
}