我无法理解为什么会发生NullPointerException

Question

    package Sound;

    import javax.swing.*;

    public class practice_1 {
        JFrame frame;
        public static void main(String[] args) 
        {
            practice_1 gogo=new practice_1();

            gogo.go();
        }
        public void go()
        {
            frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

            JButton button =new JButton("click me");
            frame.getContentPane().add(button);

            frame.setSize(300,300);
            frame.setVisible(true);
        }
    }


this is error code 
Exception in thread "main" java.lang.NullPointerException
    at Sound.practice_1.go(practice_1.java:16)
    at Sound.practice_1.main(practice_1.java:12)

我无法理解为什么会发生NUllPointerException。我对NUllPointerException的了解是，当我使用nullpointer对象时，它会发生NUllPointerException。是吗？