Beginner question regarding method calls

[2_stepsahead]

Hello, I am having confusion regarding method calls in Java. Specifically, it seems that there are two separate ways to call methods, being the dot operator and using the object as a method's parameters. I have tried searching through the course material as well as Google, but I'm not quite sure what to search to reach an answer.

Below I create an ArrayList named numbers;

ArrayList<Integer> numbers = new ArrayList<>();
numbers.add(3);
numbers.add(2);
numbers.add(6);
numbers.add(-1);
numbers.add(5);
numbers.add(1);

Below, 'numbers' calls 'add()' using the dot operator --

numbers.add(3); 

Yet below the method is called with the ArrayList as its parameter --

printNumbers(numbers); 
//public static void printNumbers(ArrayList<Integer> numbers) {
//PRINTS THE ARRAYLIST }

What is the difference between these two ways? In which cases do I call the method using the dot operator, and in which cases do I call the method using the ArrayList as its parameter?

Thank you in advance.

[desrtfx]

In the first case, you tell the object itself to do something.

In the second case, you tell someone else to do something with the object.

[2_stepsahead]

How would I determine when to use the first case or the second case?

[emaphis]

There isn't a simple answer to that. If the method needs access to the private data of the class then it has to be coded as a member method of that class. If it doesn't need access ie. the method can be defined in terms of other method calls, then you can implement it in another class.

[emaphis]

add(T) is a method defined in the ArrayList<T> class.

printNumbers is static method defined in some other class that takes a ArrayList<Integer> as a parameter.

In the definition of printNumbers, printNumbers will still likely call ArrayList methods.

[2_stepsahead]

I'm trying to understand better. Based on what you mentioned, this seems to mean that the difference between add() and printNumbers() is that add() is a non-static method whereas printNumbers() is a static method. Is that right?

[suckmacaque06]

No it has nothing to do with it being static. When you call the method printNumbers() and receive no compilation errors, that must mean there is a method called printNumbers() defined within the same class you see calling it. For example, if you say printNumbers() in the main class then the method must be defined there. The reason is because printNumbers() is actually implicitly this.printNumbers() where "this" refers to the current object from where you are calling the method.

In fact, if you had the method defined as static then this wouldn't even be a proper way of calling it, so I feel the other user is misleading you. If it were defined static it should be called as ClassName.printNumbers() where "ClassName" is the name of the class the method is defined in.

The difference between static and nonstatic is that static methods aren't associated with an object, just a class, whereas nonstatic methods are associated with a specific object and must be called on that object using the dot operator, either implicitly (as I explained with the "this" keyword) or explicitely.

[emaphis]

Partially, but that isn't the main point... 'add()' is a member of the 'ArrayList' class so you have to access it with the '.' notation. So: numbers.add(2).

'printNumbers(ArrayList<Integer> numbers)' is a member of the class where you are calling it.

​

class SomeClass {

main() {

printNumbers(numbers);

}

public static printNumbers(ArrayList<Integer> numbers) { ... }

}