Understand the 'static' keyword in Java

We can think of static is something related to the class itself rather than to an instance of the class.

1. Class Variables

public class Bicycle {      
  private int cadence;
  private int gear;
  private int speed;
  private int id;
  private static int numberOfBicycles = 0;
      
  public Bicycle(int startCadence, int startSpeed, int startGear){
      gear = startGear;
      cadence = startCadence;
      speed = startSpeed;

      // increment number of Bicycles
      // and assign ID number
      id = ++numberOfBicycles;
  }

  // new method to return the ID instance variable
  public int getID() {
      return id;
  }
      ...
}

The variable numberOfBicycles is common to all objects/instance. Or we can say that all objects of the class share this variable.

Class variables are referenced by the class name itself: Bicycle.numberOfBicycles

Not recommended to write class variables like this: myBike.numberOfBicycles

2. Class Methods

public static int getNumberOfBicycles() {
    return numberOfBicycles;
}

A common usage of static methods is to access static fields.

The main method in Java is typically declared with static modifier. This allows the Java Virtual Machine (JVM) to invoke the main method without needing to create an instance of the class containing the main method.

public static void main(String[] args)

Note: Class methods cannot access instance variable or instance methods directly. And it cannot use the this keyword.

3. Constant

static final double PI = 3.141592653589793;

static + final can be used to define constants.

Note: If a primitive type or a string is defined as a constant then the value is known at compile time.

Refs:

https://docs.oracle.com/javase/tutorial/java/javaOO/classvars.html