Question

"Polygon Drawer
Write an applet that lets the user click on six points. After the sixth point is clicked, the
applet should draw a polygon with a vertex at each point the user clicked."

import java.awt.*;
import java.awt.event.MouseListener;
import java.awt.event.MouseEvent;
import java.applet.*;
public class PolygonDrawer extends Applet implements MouseListener{
//Declares variables
   private int[] X, Y;
   private int ptCT;
   private final static Color polygonColor = Color.BLUE;
//Color stuff
   public void init() {
       setBackground(Color.BLACK);
       addMouseListener(this);
       X = new int [450];
       Y = new int [450];
       ptCT = 0;
   }
   public void paint (Graphics g) {
       g.setColor(Color.WHITE);
       g.drawRect(0, 0, getSize().width-1, getSize().height-1);
   }
   private void putLine(int X1, int Y1, int X2, int Y2) {
       Graphics g = getGraphics();
       g.drawLine(X1, Y1, X2, Y2);
       g.dispose();
   }
   private void putPolygon() {
       if(ptCT<2)
           return;
       Graphics g = getGraphics();
       if (ptCT==2) {
           g.drawLine(X[0], Y[0], X[1], Y[1]);
       }
       else {
           g.setColor(Color.BLUE);
           g.fillPolygon(X, Y, ptCT);
           g.setColor(Color.BLACK);
           g.drawPolygon(X,Y,ptCT);
       }
       g.dispose();
   }
   //Mouse clicking
   public void MousePressed(MouseEvent evt) {
       if(evt.isShiftDown()) {
           ptCT = 0;
           repaint();
       }
       else if (ptCT>0&&(Math.abs(X[0]-evt.getX())<=2)&&(Math.abs(Y[0]-evt.getY())<=2)){
           putPolygon();
           ptCT=0;
       }
       else if(evt.isMetaDown()||ptCT==450) {
           putPolygon();
           ptCT=0;
       }
       else {
           X[ptCT]=evt.getX();
           Y[ptCT]=evt.getY();
           ptCT++;
           if(ptCT>=2) {
               putLine(X[ptCT-1],Y[ptCT-2],X[ptCT-1],Y[ptCT-1]);
           }
       }
   }
   public void MouseReleased(MouseEvent evt) {}
   public void MouseClicked(MouseEvent evt) {}
   public void MouseEntered(MouseEvent evt) {}
   public void MouseExited(MouseEvent evt) {}
}

I'm having issues with the "public class PolygonDrawer extends Applet implements MouseListener". How can I fix it?

Answer 1

Since your class implements MouseListener it should override every one of the methods in that interface. Even if it won't be used, you need to provide it, so just make it empty:

import java.awt.*;
import java.awt.event.MouseListener;
import java.awt.event.MouseEvent;
import java.applet.*;
public class PolygonDrawer extends Applet implements MouseListener{
//Declares variables
   private int[] X, Y;
   private int ptCT;
   private final static Color polygonColor = Color.BLUE;
//Color stuff
   public void init() {
       setBackground(Color.BLACK);
       addMouseListener(this);
       X = new int [450];
       Y = new int [450];
       ptCT = 0;
   }
   public void paint (Graphics g) {
       g.setColor(Color.WHITE);
       g.drawRect(0, 0, getSize().width-1, getSize().height-1);
   }
   private void putLine(int X1, int Y1, int X2, int Y2) {
       Graphics g = getGraphics();
       g.drawLine(X1, Y1, X2, Y2);
       g.dispose();
   }
   private void putPolygon() {
       if(ptCT<2)
           return;
       Graphics g = getGraphics();
       if (ptCT==2) {
           g.drawLine(X[0], Y[0], X[1], Y[1]);
       }
       else {
           g.setColor(Color.BLUE);
           g.fillPolygon(X, Y, ptCT);
           g.setColor(Color.BLACK);
           g.drawPolygon(X,Y,ptCT);
       }
       g.dispose();
   }
   //Mouse clicking
   public void mousePressed(MouseEvent evt) {
       if(evt.isShiftDown()) {
           ptCT = 0;
           repaint();
       }
       else if (ptCT>0&&(Math.abs(X[0]-evt.getX())<=2)&&(Math.abs(Y[0]-evt.getY())<=2)){
           putPolygon();
           ptCT=0;
       }
       else if(evt.isMetaDown()||ptCT==450) {
           putPolygon();
           ptCT=0;
       }
       else {
           X[ptCT]=evt.getX();
           Y[ptCT]=evt.getY();
           ptCT++;
           if(ptCT>=2) {
               putLine(X[ptCT-1],Y[ptCT-2],X[ptCT-1],Y[ptCT-1]);
           }
       }
   }
   public void mouseReleased(MouseEvent evt) {}
   public void mouseClicked(MouseEvent evt) {}
   public void mouseEntered(MouseEvent evt) {}
   public void mouseExited(MouseEvent evt) {}
}

i highlighted the issue. it's a case-sensitive issue. function name should start with small letter.

check this url: https://docs.oracle.com/javase/7/docs/api/java/awt/event/MouseListener.html