Not able to refer a method using composition in java

Why is this allowed:

import java.util.ArrayList;

public class BurgerClass {

	private DefaultMenuClass defaultMenu;
	ArrayList<String> burgerMenuItems = new ArrayList<String>();
	
	public void displayBurgerMenu() {
		
		burgerMenuItems = defaultMenu.getDefaultBurgerMenu();
		for (String burger : burgerMenuItems) {
			System.out.println(burger + "\n");
		}

	}
}

And this isn't:

import java.util.ArrayList;

public class BurgerClass {

	private DefaultMenuClass defaultMenu;
	ArrayList<String> burgerMenuItems = new ArrayList<String>();
	burgerMenuItems = defaultMenu.getDefaultBurgerMenu(); //error here
	public void displayBurgerMenu() {
		
		
		for (String burger : burgerMenuItems) {
			System.out.println(burger + "\n");
		}

	}
}

To answer the specific question, the reason why the first is "allowed" and the second isn't ——

Neither one is "allowed" or "disallowed"

In both cases, as Mureinik says, defaultMenu is never initialized. In the first case it doesn't matter, because displayBurgerMenu() is never called, so you never reach the line burgerMenuItems = defaultMenu.getDefaultBurgerMenu(); which would cause an error.

In the second case you do attempt to call it, at the line you've marked //error here so you get (I assume^*) a NullPointerException.

Either way, you need to initialize defaultMenu to be something that is non-null before you attempt to call a method on it.

You need to do something along the lines of
private DefaultMenuClass defaultMenu = new DefaultMenuClass(<some-params?>);

or have some other way to get a DefaultMenuClass object. It's impossible to tell how that should happen given only the code you've presented in the question.

Separately, and not having anything to do with your error, it's pointless to do both
ArrayList<String> burgerMenuItems = new ArrayList<String>();
and then
burgerMenuItems = defaultMenu.getDefaultBurgerMenu();

You initialize burgerMenuItems to an empty ArrayList then you immediately throw that list away by setting burgerMenuItems to the list returned by .getDefaultBurgerMenu() (if there had been no error and something was returned)

* You really should say what error you are getting in your question, and not leave us to assume something.

@Data
@Builder
class BurgerClass {
    private DefaultMenuClass defaultMenu;

    public void displayBurgerMenu() {
        List<String> burgerMenuItems = defaultMenu.getDefaultBurgerMenu();
        for (String burger : burgerMenuItems) {
            System.out.println(burger + "\n");
        }
    }
}

@Data
@Builder
class DefaultMenuClass {
    List<String> defaultBurgerMenu;

    public DefaultMenuClass(List<String> defaultBurgerMenu) {
        this.defaultBurgerMenu = defaultBurgerMenu;
    }
}

class Main {
    public static void main(String[] args) {
        BurgerClass obj = BurgerClass.builder().defaultMenu(DefaultMenuClass.builder().defaultBurgerMenu(Arrays.asList(new String[]{"Item1", "Item2"})).build()).build();
        obj.displayBurgerMenu();
    }
}

As you can able to see in above code,i am initialize all attribute of BurgerClass by providing builder pattern.

Also you can initialize dafaultMenu attributes using constructor. And while creating an instance of BurgerClass pass value of DefaultMenuClass.

 public BurgerClass(DefaultMenuClass defaultMenu) {
        this.defaultMenu = defaultMenu;
    }

Just run above code, you will get an idea.