在JPanel上繪制字符串

Question

我有點麻煩。 我正在嘗試從繪畫方法（圖形g）繪制一個字符串（toString（）），我相信有兩個主要問題：

1）調用“重新繪制”時，它似乎沒有響應。

2）當我嘗試從obj調用時。 在構造函數中進行的操作，我收到一個“ Applet”未初始化的錯誤。

這可能是一個入門錯誤，但真的很感謝任何幫助。.這是我第一次在此網站上發布。

這是我所討論的applet的代碼：

 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;

   }



 }



}

這是構造函數：

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

這里有幾個問題：

1. 永遠不會初始化學生列表，因為您永遠不會調用方法StudentList() 。 例如，您可以在init()方法中調用它。 順便說一句， initStudentList()將是一個更好的名稱。

2. 變量numstu應該初始化為10，因為這里有10個學生（從索引0到9）

3. for循環應如下所示（我們從0循環到9）： for(i=0;i<numstu;i++){

完成這些操作后，學生將正確顯示在JPanel上。