使用JTextField计算用户输入？

Question

    import java.awt.*;
    import javax.swing.JButton;
    import javax.swing.JComboBox;
    import javax.swing.JFrame;
    import javax.swing.JLabel;
    import javax.swing.JPanel;
    import javax.swing.JTextArea;

    public class ComputeGrades {

    public static void main(String []args) {
        //create a frame
        JFrame frame = new JFrame("Computation of Grades");
        frame.setSize(800,800);
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setVisible(true);

        //add a panel
        JPanel panel = new JPanel();
        panel.setLayout(null);
        panel.setVisible(true);
        frame.add(panel);

        //Enter Full Name : JTextArea
        JLabel fName = new JLabel("Enter Full Name : ");
        fName.setBounds(0,0,100,100);
        fName.setVisible(true);
        panel.add(fName);

        JTextArea fullName = new JTextArea("ln,fn,mi.");
        fullName.setBounds(140,40,200,20);
        fullName.setVisible(true);
        panel.add(fullName);
        //Select Section : JComboBox
        JLabel sec = new JLabel("Select Section : ");
        sec.setBounds(0,30,100,100);
        sec.setVisible(true);
        panel.add(sec);

        JComboBox section = new JComboBox();
        section.setBounds(140,70,150,20);
        section.addItem("IT201M");
        section.addItem("BT403A");
        section.addItem("BT201M");
        section.addItem("BT201P");
        section.addItem("CT201A");
        section.addItem("BT203A");
        section.addItem("BT204A");
        section.addItem("BE201M");
        section.addItem("IT301P");
        section.addItem("BT403M");
        section.addItem("BS401P");
        section.setVisible(true);
        panel.add(section);
        //Enter Prelim Grade : JTextArea
        JLabel pre = new JLabel("Enter Prelim Grade : ");
        pre.setBounds(0,100,200,20);
        pre.setVisible(true);
        panel.add(pre);

        JTextArea prelim = new JTextArea("--.--");
        prelim.setBounds(140,100,150,20);
        prelim.setVisible(true);
        panel.add(prelim);
        //Enter Midterm Grade : JTextArea
        JLabel mid = new JLabel("Enter Midterm Grade : ");
        mid.setBounds(0,130,200,20);
        mid.setVisible(true);
        panel.add(mid);

        JTextArea midterm = new JTextArea("--.--");
        midterm.setBounds(140,130,150,20);
        midterm.setVisible(true);
        panel.add(midterm);
        //Enter Pre-final Grade : JTextArea
        JLabel prefi = new JLabel("Enter Pre-Finals Grade : ");
        prefi.setBounds(0,160,200,20);
        prefi.setVisible(true);
        panel.add(prefi);

        JTextArea prefinals = new JTextArea("--.--");
        prefinals.setBounds(140,160,150,20);
        prefinals.setVisible(true);
        panel.add(prefinals);
        //Enter Finals Grade : JTextArea
        JLabel fi = new JLabel("Enter Finals Grade : ");
        fi.setBounds(0,190,200,20);
        fi.setVisible(true);
        panel.add(fi);

        JTextArea finals = new JTextArea("--.--");
        finals.setBounds(140,190,150,20);
        finals.setVisible(true);
        panel.add(finals);
        //Compute for the General Average : JButton
        JLabel com = new JLabel("Click the Compute button for the General Average");
        com.setBounds(0,220,400,20);
        com.setVisible(true);
        panel.add(com);

        JButton compute = new JButton("Compute");
        compute.setBounds(0,250,100,30);
        compute.setVisible(true);
        panel.add(compute);


        }
    }

我们的课程作业是关于成绩的计算。

  

创建一个Java GUI程序，用于计算第二个JFrame的用户输入的共同平均值。用户将在第二帧输入他的预赛，中期，预赛和决赛等级，他的总平均值将出现。

我刚刚在这里刚刚开始使用Java进行面向对象编程的编程，我只想问一下，我应该使用哪个组件＆＃34; best＆＃34;作为我已创建的JTextArea的替换，以便它可以读取数字（小数）以及我应该使用哪些事件？