java - JPanel How to create an instance/object -

i keep mixing jframe , jpanel. have code compiles nothings happens , program ends.

this homework assignment requires draw triangle. add button, when button pressed triangle flip upside down. press again return first location.

any assistance great. countless rough draft , i'm trying initiate object view.

import java.awt.*; import javax.swing.*; import java.awt.event.*; import java.awt.geom.line2d; import java.awt.color;    public class triangle extends jpanel implements actionlistener  {    jbutton button = new jbutton("just flip it");     public triangle()    {       add(button);       button.addactionlistener(this);    }     public void actionperformed(actionevent e)    {     }    @override    public void paintcomponent(graphics g)    {       super.paintcomponent(g);       g.drawline(125,75,100,200);       g.drawline(125,75,150,200);       g.drawline(100,200,150,200);      }     public static void main(string[] args)    {        triangle frame = new triangle();       frame.setsize(400, 400);       frame.setvisible(true);     }  } 

think of jpanel piece of paper , jframe book.

if never add piece of paper book, never shown.

so, here steps program running:

• create jframe hold jpanel (triangle) inside it
• add logic in actionlistener have boolean flag, changed true or false depending on current state , based on repaint pane
• have logic inside paintcomponent(...) method draws triangle upside down or upside based on state of boolean flag above

so, above in main(...) method should have

jframe frame = new jframe("my frame"); //initialize triangle here frame.add(triangle); frame.setdefaultcloseoperation(jframe.exit_on_close); frame.pack(); frame.setvisible(true); 

your actionperformed() method should this:

public void actionperformed(actionevent e) {     clicked = !clicked;     repaint(); } 

and paintcomponent():

super.paintcomponent(); if (clicked) {     //draw upside } else {     //draw upside down } 

as we're using pack() instead of setsize(...) method, should override getpreferredsize() method of our jpanel well:

@override public dimension getpreferredsize() {     return new dimension(400, 400); } 

otherwise small jframe (and it's practice way)

also don't forget place program on edt (as show in point #2 of answer)

as side note, might want move use shape's api recommended @madprogrammer in this answer

both links above have cool examples , tips, check them out!