I have a JApplet which has to display 3 Components. (2 JPanels and 1 Paint Method)

问题

I have an assignment in which I have to allow a user to plot a graph using a quadratic equation.

I managed to draw the skeleton of the graph, and now I am trying to display the "control panel" for the user to input the values.

I have 4 files:

• graph.java
• panel.java
• panelB.java
• panelC.java

My problem is when I run the code it is displaying only the panel.java even in the container where it should display the other two panels.

panel.java

import java.awt.Dimension;
import java.awt.Graphics;
import java.awt.*;
import java.awt.geom.*;
import javax.swing.JPanel;


public class panel extends JPanel {

    public panel(){
        this.setBackground(Color.yellow);
    }
}

Can anyone please advise what changes I should do to fix this problem?

I have done some changes to the graph.java file:

import java.awt.BorderLayout;
import java.awt.Container;
import java.awt.Dimension;
import javax.swing.*;

public class GraphApplet extends JApplet{


    public GraphApplet(){

    raph p = new Graph();//graph

    p.setPreferredSize(new Dimension(760,500));
        conn.add(p,BorderLayout.CENTER);
    }

And now all that is being displayed is the graph.

Regarding the other code, I also made some changes to the class names:

gnjk;.java

import java.awt.Dimension;
import java.awt.Graphics;
import java.awt.*;
import java.awt.geom.*;
import javax.swing.JPanel;


public class Graph extends JPanel {

public Graph(){
    this.setBackground(Color.yellow);
    }

    public void paintComponent(Graphics p) {

        super.paintComponent(p);

        Graphics2D graph = (Graphics2D)p;

        this.setBackground(Color.yellow);//set background color.

        int x,y,y1,x1,a,b,p1x,p1y,p2x,p2y;

        int xstart = 7;
        int ystart = 1;

        int xfinish = 3;

.......

bhfvhn.java

import javax.swing.*;

import java.awt.*;

import javax.swing.JPanel;


public class ControlsA extends JPanel{

public void init (Box g) {

    a = Box.createVerticalBox();
    a.add(new JLabel("Please enter the values below:"));
    a.add(new JLabel("h"));


}

}

jknmk.java

import javax.swing.*;

import java.awt.Component;
import java.awt.Dimension;

public class ControlsB extends JPanel{

public void init (Box b) {

    b = Box.createHorizontalBox();
    b.add(new JLabel("a"));
    JTextField f1 = new JTextField("0.0");
    f1.setMaximumSize(new Dimension(100,30));
    b.add(f1);


}
}

---

Here is an updated to my project:

jkl.java

import java.awt.BorderLayout;
import java.awt.Container;

public class GraphApplet extends JApplet{
  public GraphApplet() {
    public void init(){
        SwingUtilities.invokeLater(new Runnable() {

         public void run(){
             Container conn = getContentPane();
             conn.setLayout(new BorderLayout());

             Graph z = new Graph();
             conn.add(p,BorderLayout.CENTER);


             fasfae a = new ControlsA(box1);
             conn.add(a,BorderLayout.LINE_START);


             adsfawef b = new ControlsB(box2);
             conn.add(b,BorderLayout.PAGE_END);
         }
        });
        }
        }




    /*Container conn = getContentPane();
    conn.setLayout(new BorderLayout());     

    Graph p = new Graph();//graph

    p.setPreferredSize(new Dimension(460,560));
    conn.add(p,BorderLayout.CENTER);

    Box a = new Box(BoxLayout.Y_AXIS);

    a.setPreferredSize(new Dimension(50,50));
    conn.add(a,BorderLayout.EAST);

    Box b = new Box(BoxLayout.X_AXIS);

    b.setPreferredSize(new Dimension(201,50));
    conn.add(b,BorderLayout.SOUTH);*/
    //this code is commented not to loose it

vtk.java

import java.awt.Dimension;
import java.awt.Graphics;
import java.awt.*;
import java.awt.geom.*;
import javax.swing.JPanel;


class Graph extends JPanel {

public Graph(){
    this.setBackground(Color.yellow);
}
@Override
public Dimension getPreferredSize(){return (new Dimension(460,560)); }

public void paint(Graphics z) {



        Graphics graph = (Graphics2D)z;


        this.setBackground(Color.yellow).

        int x,y,y1,x1,a,b,p1x,p1y,p2x,p2y;

        //line co-ordinates
        //the numbers represent the number of boxes on the graph
        int xstart = 7;
        int ystart = 1;

        int x = 3;
        int y = 9;

        //other variables
        int i = 0;
        int i2 = 0;
        int m = 0;
        int n = 0;
        int m2 = 0;
        int n2 = 0;
        int f2 = 0;
        int g2 = 1;

        //ranges
        int f = 5;
        int g = -5;

        //change -ve num to +ve
        int g3 = Math.abs(g);

        int a1 = g3 + f;
        int b1 = a1;

        a = (Height);
        f = (Width);

        }
    }
}

// 6 variables the user has to input

}
@Override
public Dimension getPreferredSize() {return (new Dimension(200,100));}
}

nllkl.java

 @Override
 public Dimension getPreferredSize(){return (new Dimension(201,50));}
}

Still no improvement. I cannot understand what is going on.

回答1:

Please try to run this code now, tell me is this closer to what you wanted ? if so do let me know, so that I can delete my answer. Do watch the changes I had done :

• in your PanelC class, where instead of using init() method, I made the constructor.
• Inside your Graph class< i had removed all setPreferredSizes(...) calls, and instead I had overridden getPreferredSize() for each Class extending JPanel. For this visit each Class Panel, PanelB and PanelC.
• Lastly I had changed the Dimension values supplied to the JPanel, which is to go to the LEFT/LINE_START side, to something that can be seen
• BEST CHANGE is the use of EDT - Event Dispatch Thread, seems like you really not aware of Concurrency in Swing

---

import java.awt.*;
import java.awt.geom.*;
import javax.swing.*;

public class Graph extends JApplet{

    public void init(){

        SwingUtilities.invokeLater(new Runnable()
        {
            public void run()
            {
                Container conn = getContentPane();
                conn.setLayout(new BorderLayout());

                Panel p = new Panel();//graph
                conn.add(p,BorderLayout.CENTER);

                Box box1 = new Box(BoxLayout.Y_AXIS);
                PanelB a = new PanelB(box1);//vertical
                conn.add(a,BorderLayout.LINE_START);

                Box box2 = new Box(BoxLayout.X_AXIS);
                PanelC b = new PanelC(box2);//horizontal
                conn.add(b,BorderLayout.PAGE_END);
            }
        });
    }
}

class Panel extends JPanel {

    public Panel(){
        this.setBackground(Color.yellow);
    }

    @Override
    public Dimension getPreferredSize()
    {   
        return (new Dimension(460,560));
    }

    public void paintComponent(Graphics p) {

        super.paintComponent(p);

        Graphics2D graph = (Graphics2D)p;

        Dimension appletSize = this.getSize();
        int appletHeight = (int)(appletSize.height);
        int appletWidth = appletSize.width;

        this.setBackground(Color.yellow);//set background color.

        int x,y,y1,x1,a,b,p1x,p1y,p2x,p2y;

        //line co-ordinates
        //the numbers represent the number of boxes on the graph
        int xstart = 7;
        int ystart = 1;

        int xfinish = 3;
        int yfinish = 9;

        //other variables
        int i = 0;
        int i2 = 0;
        int m = 0;
        int n = 0;
        int m2 = 0;
        int n2 = 0;
        int f2 = 0;
        int g2 = 1;

        //ranges
        int f = 5;
        int g = -5;

        //change -ve num to +ve
        int g3 = Math.abs(g);

        int a1 = g3 + f;
        int b1 = a1;

        y1 = (appletHeight);
        x1 = (appletWidth);
        y = (appletHeight / 2);
        x = (appletWidth / 2);
        a = (appletWidth / a1);
        b = (appletHeight / b1);

        int d = (appletWidth / a1);
        int e = (appletHeight / b1);

        /**
         to determine the
         ammount of pixles there
         is in each box of the
         graph, both y-axis and 
         x-axis
         */
        int xbox = x1 / 10;
        int ybox = y1 / 10;

        //line variables
        //the xstart, ystart, etc represent the number of boxes

        //top point of the line on the graph
        p1x = xbox * xstart;//start x
        p1y = ybox * ystart;//start y

        //lowwer point of the line on the graph
        p2x = xbox * xfinish;//finish x
        p2y = ybox * yfinish;//finish y

        //draw y-axis numbers 
        //(+ve)
        while(f != 0){
            String s = String.valueOf(f);
            p.drawString(s,(x + 5),m + 13);
            m = m + b;
            f = f - 1;
        }
        //(-ve)
        m2 = y;
        while(f2 != g-1){
            String u = String.valueOf(f2);
            p.drawString(u,(x + 5),m2 - 3);
            m2 = m2 + b;
            f2 = f2 - 1;
        }
        //draw x-axis numbers.
        //(-ve)
        while(g != 0){
            String t = String.valueOf(g);
            p.drawString(t,n,y - 5);
            n = n + a;
            g = g + 1;
        }
        //(+ve)
        n2 = x + a;
        while(g2 != g3+1){
            String vw = String.valueOf(g2);
            p.drawString(vw,n2 -10,y - 5);
            n2 = n2 + a;
            g2 = g2 + 1;
        }

        BasicStroke aLine2 = new BasicStroke(1.0F,
           BasicStroke.CAP_ROUND,BasicStroke.JOIN_ROUND);
        graph.setStroke(aLine2);

        //notch on numbers and grid lines
        //left to right, top to bottom notches
        int v2 = -5;
        int v5 = 0;
        while(i <= a1-1){
            p.setColor(Color.lightGray);//lightgray line
            p.drawLine(a,0,a,y1);//vertical lightgray
            p.drawLine(0,b,x1,b);//horizontal lightgray
            a = a + d;
            b = b + e;
            i = i + 1;
        }
        //notches
        while(i2 <= a1){
            p.setColor(Color.blue);//notch color
            p.drawString("x",v2+2,y+3);//xaxis
            p.drawString("x",x-4,v5+4);//yaxis
            v5 = v5 + e;
            v2 = v2 + d;
            i2 = i2 + 1;
        }

        //draws the border of the graph
        p.setColor(Color.black);
        Rectangle2D.Float rect = new Rectangle2D.Float(0,0,x1,y1);
        BasicStroke aLine = new BasicStroke(2.5F, 
               BasicStroke.CAP_ROUND, BasicStroke.JOIN_ROUND);
        graph.setStroke(aLine);
        graph.draw(rect);

        //draw cross
        BasicStroke aLine3 = new BasicStroke(2.5F,
                     BasicStroke.CAP_ROUND, BasicStroke.JOIN_ROUND);
        graph.setStroke(aLine3);
        p.drawLine(x,0,x,y1); //vertical line
        p.drawLine(0,y,x1,y); //horizontal line

        //display the value of graph width and graph height
        String aw = String.valueOf(x1);
        p.drawString("Graph Width = ", 50,90);
        p.drawString(aw,150,90);
        p.drawString("Graph Height = ", 50,110);
        String ah = String.valueOf(y1);
        p.drawString(ah,156,110);

        //draw line on graph

        BasicStroke aLine4 = new BasicStroke(1.5F,
                         BasicStroke.CAP_ROUND, BasicStroke.JOIN_ROUND);
        graph.setStroke(aLine4);
        p.setColor(Color.red);

        if(p1x <= x1 && p2x <= x1 && p1y <= y1 && p2y <= y1){
            p.drawLine(p1x,p1y,p2x,p2y);
            Color c = new Color(0,0,0);
            p.setColor(c);
            p.drawString("X", p1x-4,p1y+4);
            p.drawString("X", p2x-4,p2y+4);
        }
        else{
            p.setColor(Color.black);
            p.drawRect(48,34,223,35);
            p.setColor(Color.white);
            p.fillRect(49,35,222,34);
            p.setColor(Color.red);
            p.drawString("Wrong co-ordinates!!!", 50,50);
            p.drawString("Values exceede applet dimensions.", 50,65);
        }
    }
}

class PanelB extends JPanel{

    public PanelB (Box a) {

        a = Box.createVerticalBox();
        a.add(new JLabel("Please enter the values below:"));
        a.add(new JLabel("a"));
        JTextField g1 = new JTextField("0.0");
        g1.setMaximumSize(new Dimension(100,30));
        a.add(g1);
        a.add(new JLabel("b"));
        JTextField g2 = new JTextField("0.0");
        g2.setMaximumSize(new Dimension(100,30));
        a.add(g2);
        a.add(new JLabel("c"));
        JTextField g3 = new JTextField("0.0");
        g3.setMaximumSize(new Dimension(100,30));
        a.add(g3);
        a.add(new JLabel("d"));
        JTextField g4 = new JTextField("0.0");
        g4.setMaximumSize(new Dimension(100,30));
        a.add(g4);
        a.add(new JButton("Plot"));
        a.add(new JButton("Refine"));
        add(a);
    }

    @Override
    public Dimension getPreferredSize()
    {   
        return (new Dimension(200,100));
    }
}

class PanelC extends JPanel{

    public PanelC (Box b) {

        b = Box.createHorizontalBox();
        b.add(new JLabel("a"));
        JTextField f1 = new JTextField("0.0");
        f1.setMaximumSize(new Dimension(100,30));
        b.add(f1);
        b.add(new JLabel("b"));
        JTextField f2 = new JTextField("0.0");
        f2.setMaximumSize(new Dimension(100,30));
        b.add(f2);
        b.add(new JLabel("c"));
        JTextField f3 = new JTextField("0.0");
        f3.setMaximumSize(new Dimension(100,30));
        b.add(f3);
        b.add(new JLabel("d"));
        JTextField f4 = new JTextField("0.0");
        f4.setMaximumSize(new Dimension(100,30));
        b.add(f4);
        b.add(new JButton("Plot"));
        b.add(new JButton("Refine"));
        add(b);
    }

    @Override
    public Dimension getPreferredSize()
    {   
        return (new Dimension(201,50));
    }
}

Here is the HTML file I used :

<html>
    <p> This file launches the 'Graph' applet: Graph.class! </p>  
    <applet code= "Graph.class" height = 550 width = 1000>
        No Java?!
    </applet>
 </html>

Here is the output I am getting :

回答2:

The main problems in that code are:

1. The applet is adding 3 instances of panel as opposed to one each of panel, panelB & panelC.
2. Neither panelB & panelC ever adds the Box to the panel, so it will not appear.
3. You intimate in the code that panelB should be vertically aligned, which means it would better fit in the LINE_START (WEST) of the BorderLayout, as opposed to the NORTH.
4. public void paint(Graphics p) {.. is wrong for panel. Since panel is a Swing JPanel it should be public void paintComponent(Graphics p) {..

Once those things are attended to, this is how it might appear.

Other problems.

• The only code that needs to extend a class is for the applet itself and panel. In fact, even the panel could be changed to show a BufferedImage (the graph) inside a JLabel
• The panelB and panelC are entirely redundant, just add a Box directly to the layout area of the parent component needed.
• The nomenclature is wrong.
  1. Java class names should be EachWordUpperCase
  2. Use meaningful class names - the applet might be GraphApplet, the graphing area Graph, ..I am not sure what to call the last 2 panels, since they carry identical components. If there were but one, I might call it Controls as a class (which is overkill for this), or controls if it were an instance of a plain JPanel or Box.
• None of the calls to set maximum or preferred size are recommended in this situation. The only case that can be made for it is in the preferred size of the graph itself, but since this appears in the CENTER of applet, a size will be suggested by the applet width/height specified in HTML, minus the natural size of the other components (the CENTER component will get 'the rest of the space').

Update

..how am I going to change the code in my applet so that the applet adds one of each panel(?)

Change:

panel a = new panel();//vertical

To:

panelB a = new panelB(new Box(BoxLayout.Y_AXIS));//vertical

Is the simplest way. Note that it changes if you decide to add the Box directly.

Box a = new Box(BoxLayout.Y_AXIS);//vertical

来源：https://stackoverflow.com/questions/11062249/i-have-a-japplet-which-has-to-display-3-components-2-jpanels-and-1-paint-metho