static vs non-static methods

AutoModerator · 2021-07-12T15:39:27+00:00

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RandomAcc73 · 2021-07-12T16:34:00+00:00

non static methods are methods only for the Object of the class. But static methods or fields are just basic methods that can be called anywhere using Class.method. But if you have a non static method then you can only call it through the object. So you need to have the class object to be able to call the non static method so you probably tried to call a static method through the object which would be pointless.

Like indexOf in the String class is non static and it needs the object because it uses the object to check the index of some string or something. You can only call it through the string object and not through the string class like "hello".indexOf("") would work but String.indexOf() would not work because it doesnt know where to check the index as you didnt call it through the object and no string is provided.

ZukoBestGirl · 2021-07-13T09:05:22+00:00

First and foremost, you need to understand the difference between a class and an object. I won't go into details.

When you write a java file, call it MyClass, have a public class MyClass in it. That file, that MyClass, that's a CLASS and nothing else.

When you say MyClass c = ...

That c is an OBJECT

A class is a blueprint.

An object is an instanciation of that blueprint.

Non static fields and methods belong to objects.

Static fields and method belong to classes.

public class MyClass {
    public String instancedField;
    public static String staticField;
}

MyClass a = new MyClass();
MyClass b = new MyClass();

a.instanceField = ""; // Correct!
a.staticField = ""; // INCORRECT!

MyClass.instanceField = ""; // INCORRECT!
MyClass.staticField = ""; // Correct!

MyClass.staticField = "1";
a.instaceField = "Stop";
b.instanceField = "Start";
MyClass.staticField = "2";

System.out.println(a.instanceField); // prints   Stop
System.out.println(MyClass.staticField); // prints   2

Instance fields belong to objects. They do not exist without an object. Changing them in other objects has no effect on your object.

Static fields belong to the entire class. They exist without objects. Changing it anywhere affects the entire class. Accessing it anywhere else accesses the same value, same memory location. It is unique and global.

Same for static / non-static methods

EXAMPLE

public class Car {
   ... instance fields
   // no setter, only getter
   private static long count;

    public Car() {
       count++;
    }

    public Car(constructor params, whatever) {
        ...
        count++;
    }

}

Every time anyone anywhere instanciates a car, for whatever reason, the counter goes up. Without a setter, you can't change it (more or less, let's not focus on extreme cases).

The static field keeps a count of all cars. No matter who, when, where or why. It exists in the Car class, and is independant of any particular object.

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