10K's

Static fields

• There is only one single copy of each static field.
• It belongs to class not any individual object.
• the fields are shared by all instances.

static constants

• What more common is static constan.

static Methods

• Methods that not operate on objects.

• You can think it as it does not have a this parameter.

• So it can only access static fields(because it does not operate on objects).

• An object can invoke a static method. It's legal. But this can be confusing because the static method has no reletaive with the object.

  Use class name rather than objects to invoke static methods.

• Use case

  • When all the parameters are explicit passed and the method do not need fields from objects.
  • When only static fields needs to be accessed.

Factory Methods

TO BE ADDED.

The main Method

• The reason main is static is that it doesn't have to create(access) a object to be called. (When a program start, there is no objects.)

• Every class has a main method where you could add some domo codes.

  Run with java XXX,XXX is the class name.

  And if the class belongs to a larger application, the demo code won't be executed. (looks like Python's __main__)

An Example

/**
 * This program demonstrates static methods.
 * @version 1.02 2008-04-10
 * @author Cay Horstmann
 */
public class StaticTest {
    public static void main(String[] args) {
        // fill the staff array with three Employee objects
        var staff = new Employee[3];

        staff[0] = new Employee("Tom", 40000);
        staff[1] = new Employee("Dick", 60000);
        staff[2] = new Employee("Harry", 65000);

        // print out information about all Employee objects
        for (Employee e : staff) {
            System.out.println("name=" + e.getName() + ",id=" + e.getId()
                + ",salary=" + e.getSalary());
        }

        int n = Employee.getNextId(); // calls static method
        System.out.println("Next available id=" + n);
    }
}


public class Employee {
    private static int nextId = 1;

    private String name;
    private double salary;
    private int id;

    public Employee(String n, double s) {
        name = n;
        salary = s;
        id = advanceId();
    }

    public String getName() {
        return name;
    }

    public double getSalary() {
        return salary;
    }

    public int getId() {
        return id;
    }

    public static int advanceId() {
        int r = nextId; // obtain next available id
        nextId++;
        return r;
    }

    public static void main(String[] args) // unit test
    {
        var e = new Employee("Harry", 50000);

        System.out.println(e.getName() + " " + e.getSalary());
    }
}