awt - Show a panel in CardLayout in Java -

i wrote following code showing 3 panels depending upon of 3 buttons clicked @ bottom.

import java.awt.borderlayout; import java.awt.button; import java.awt.cardlayout; import java.awt.color; import java.awt.frame; import java.awt.panel; import java.awt.event.actionevent; import java.awt.event.actionlistener; import java.awt.event.windowadapter; import java.awt.event.windowevent;  public class cart extends frame implements actionlistener {  panel cardpanel; panel firstp, secondp, thirdp; panel buttonp;  button b1, b2, b3;  cardlayout clayout;  cart() {     cardpanel = new panel();     clayout = new cardlayout();     cardpanel.setlayout(clayout);      firstp = new panel();     firstp.setbackground(color.blue);      secondp = new panel();     secondp.setbackground(color.red);      thirdp = new panel();     thirdp.setbackground(color.yellow);      b1 = new button("first");     b2 = new button("second");     b3 = new button("third");      b1.addactionlistener(this);     b2.addactionlistener(this);     b3.addactionlistener(this);      buttonp = new panel();     buttonp.add(b1);     buttonp.add(b2);     buttonp.add(b3);      add(buttonp, borderlayout.south);     add(cardpanel, borderlayout.center);      cardpanel.add("first", firstp);     cardpanel.add("second", secondp);     cardpanel.add("third", thirdp);      setsize(400, 500);     setvisible(true);      addwindowlistener(new windowadapter() {         public void windowclosing(windowevent e) {             system.exit(0);         }      });  }// constructor ends here  public void actionperformed(actionevent e) {      if (e.getsource() == b1) {         clayout.show(cardpanel, "firstp");          system.out.print("first");     }      else if (e.getsource() == b2) {         clayout.show(cardpanel,"secondp");          system.out.print("second");     }      else if (e.getsource() == b3) {         clayout.show(cardpanel, "thirdp");      }  }  public static void main(string args[]) {     new cart();  }   } 

the methods handling 3 buttons called,and print terminal button clicked panels don't change.it's firstp that's visible.