为什么我不需要创建一个对象？

Question

import javax.swing.JoptionPane;

class Hat
{
  public static void main(String[]args)
   {

      String fn = JOptionPane.showInputDialog("Enter first number");
      String sn = JOptionPane.showInputDialog("Enter second number");

      int num1 = Integer.parseInt(fn);

      int num2 = Integer.parseInt(sn);

      int sum = num1 + num2;

      JOptionPane.showMessageDialog(null,"The answer is " +sum,"the title");
   }
}

Why is it not necessary to create an object in order to use showInputDialog method from the JOptionPane class? Why is null used in showMessageDialog method?

Answer 1

The showMessageDialog method is static and therefore exists on the class not on an instance. Here is a little more explanation about static methods

The first parameter is the parent which specifise relative to which other Frame the OptionPane belongs. If it is null , the OptionPane is independent of any other Frame. See the documentation

> parentComponent - determines the Frame in which the dialog is
> displayed; if null, or if the parentComponent has no Frame, a default
> Frame is used

Answer 2

Because showInputdialog is a static method. It is not logically attached to an object. It is rather a functionality exposed by the class itself.

Answer 3

All methods/variables used in that method are either method scoped or static. JOptionPane.showMessageDialog() for example is static, while num1/num2 are all method scoped.