在JPanel上绘制字符串

Question

I'm having a bit of trouble. I'm trying to draw a String (toString()) from a paint method (Graphics g), I have two main issues I believe:

1) When Repaint is called, it appears to not respond.

2) When I try to call from an obj. made in a constructor, I get an 'Applet' not initialized error.

It's probably, a starters mistake, but any help is REALLY appreciated.. This is my first time posting on this site.

Here's my code for the applet in question:

 import java.awt.*;
 import javax.swing.*;
 import javax.swing.event.*; 
 import java.awt.event.*;
 import java.util.*;
 import java.lang.*;

 /**
 *
 *
 */
public class GradeBookApp extends JApplet
implements ActionListener
{
private JRadioButton jrbadd,jrbdel,jrbupdate,jrbsortgrade,jrbsortname;
    private JPanel select,drawselect,infocore;
    private JButton operate;
    private JLabel nameprint;
    private int numstu;
    private Student[] list; // Not set in init..


   public void init(){

   JPanel select = new JPanel();
   select.setLayout(new GridLayout(2,3));


  JPanel drawselect = new JPanel();
  drawselect.setBackground(Color.pink);


  JPanel infocore = new JPanel();
  infocore.setBackground(Color.red);


  jrbadd = new JRadioButton("Add Student ");
  select.add(jrbadd);

  jrbdel = new JRadioButton("Delete Student ");
  select.add(jrbdel);

  jrbupdate = new JRadioButton("Update Student ");
  select.add(jrbupdate);

  jrbsortgrade = new JRadioButton("Sort: Grade");
  select.add(jrbsortgrade);

  jrbsortname = new JRadioButton("Sort: Name");
  select.add(jrbsortname);

  operate = new JButton("Start... ");
  operate.addActionListener(this);
  operate.setBackground(Color.magenta);
  select.add(operate);




  ButtonGroup gr = new ButtonGroup();
  gr.add(jrbadd);
  gr.add(jrbdel);
  gr.add(jrbupdate);
  gr.add(jrbsortgrade);
  gr.add(jrbsortname);


  Container c= getContentPane();

  c.add(select,BorderLayout.NORTH);   
  c.add(drawselect,BorderLayout.CENTER);
  c.add(infocore,BorderLayout.SOUTH);





}

public void actionPerformed(ActionEvent e){
   if(operate.isSelected()){
    repaint();
  }
}

public void StudentList(){

 list = new Student[30];

 numstu = 9;

 list[0] = new Student("Moba Fett",77.5,'M',1981);
 list[1] = new Student("Marion Silver",45.3,'F',2000);
 list[2] = new Student("Tom Highlander",99.1,'M',1986);
 list[3] = new Student("Ross Scott",74.2,'M',1971);
 list[4] = new Student("Jon Smith",53.1,'M',1945);
 list[5] = new Student("Jade Cerion",100.0,'F',2007);
 list[6] = new Student("Jorge Bush",0,'M',2008);
 list[7] = new Student("Tom Fulp",89.2,'M',1981);
 list[8] = new Student("Rose DimWitt",0.2,'F',1912);
 list[9] = new Student("Arya Stark",91.9,'F',8763);



}

public void delete(){



}

public void update(){

}

public void add(){

}

public void sortgrade(){

}

public void sortname(){

}


 public void paint (Graphics g){

    super.paint(g);


    int i = 0; // local
    int x = 100; // local
    int y = 100; // local

 // numstu called from private list...9



   for(i=0;i<=numstu;i++){

  if(list[i].GetGPA()>65){    
   g.setColor(Color.green);
   g.drawString(list[i].toStr(),x,y);


  }
 else if(list[i].GetGPA()<65){
    g.setColor(Color.red);
    g.drawString(list[i].toStr(),x,y);


    }

   y=y+10;

   }



 }



}

And here is the constructor:

public class Student
 {
private String name;
private double gpa;
private char gender;
private int year;

public Student(){
    name = "John Doe";
    gpa = 100;
    gender = 'u';
    year = 1972;
}

public Student(String n,double gp,char g,int y){
    name=n;
    gpa=gp;
    gender=g;
    year=y;

}

public String GetName(){
    return name;
}

public double GetGPA(){
    return gpa; 
}

public char GetGender(){
   return gender; 
}

public int GetYear(){
    return year;
}

public void ChangeName(String n){
    name = n;
}

public void ChangeGpa(double gp){
    gpa = gp;
}

public void ChangeGender(char g){
    gender = g;
}

public void ChangeYear(int y){
    year = y; 
}

  public String toStr(){
    String f = name.substring(0,name.indexOf(" "));
    String l = name.substring(name.indexOf(" "), name.length());

    String n =(l+","+f+" "+gender+" "+year+" "+gpa);

    return n;
}

private int find(int amountofstudents,Student[] list){


   return 1337; // unf.
}

}

Answer 1

There are several problems here :

1. The student list is never initialized because you never call your method StudentList() . You can call it in the init() method for example. By the way, initStudentList() would be a better name.

2. The variable numstu should be initialized to 10, because there are ten students here (from index 0 to 9)

3. The for loop should be like this (we loop from 0 to 9) : for(i=0;i<numstu;i++){

Once these operations are done, the students are correctly displayed on the JPanel.