如何引用将对象添加到对象内部的ArrayList

Question

In JavaFX I am creating a sports tracking system.

My Object User has many Foods and many Exercises, these are stored as ArrayLists.

public class User {

    private ArrayList<Food> listofFood = new ArrayList<Food>();
    private ArrayList<Exercise> listofExercise = new ArrayList<Exercise>();
    private String um;

    public User(String u) {
        this.um = u;
    }
    public void addNewFood(Food f) {
        listofFood.add(f);
    }
    public void addNewExercise(Exercise ex) {
        listofExercise.add(ex);
    }
}

Food and exercise are similar in structure.

public class Food {
    private String name;

    public Food(String n) {
        this.name = n;
    }
    public void setNameFood(String n) {
        this.name = n;
    }
}

In my MainApp class I create a user.

public class MainApp {
    public User UserLoggedIn;
}

and I pass in an instance of this class using the controller.

public class MainApp {
    MyController c = loader.getController();
    c.setDialogStage(dialogStage);
    c.setApp(this); 
}

And in MyController class I set the application ...

public class MyController { 
    public void setApp(MainApp app) {
        this.mainApp = app;
    }
}

... which I then reference in MyController.

public class MyController { 
    mainApp.UserLoggedIn.addNewFood((new Food(nameField.getText())));
}         

This gives a Null Pointer exception

Answer 1

UserLoggedIn is never instantiated and it will therefore always be null. Make a default constructor for MainApp() that just creates the User object. Then create a getter method called getUser() that returns the user object. Also, it is Java convention to use camel case instead. So, UserLoggedIn should be userLoggedIn.

  public class MainApp 
  {
     private User userLoggedIn;

     public MainApp()
     {
         userLoggedIn = new User("john doe");
     }

     public User getUserLoggedIn()
     {
         return userLoggedIn;
     }
  }

  public class MyController 
  { 
    MainApp mainApp = new MainApp();
    mainApp.getUserLoggedIn().addNewFood(new Food(//enter datum here));
  }