Applet shows up blank, not initializing?

问题:

Hello everyone my name is Fyree! I have an assignment for an introductory Java course where I need to create a program that takes user input for the beginning values, and all five year values to make a line graph (but that is for another post), and also print the rate. Here is my problem, when I try to run this applet it opens (it compiles without error), but only shows up as a blank screen, and says applet not initialized at the bottom of the applet. I am not sure why this is (currently I am trying to get the rate part of the assignment working where it takes the year five value and beginning value given by the user, and uses them to print out the rate of growth or interest). I want to know why this is happening, and what I need to do to correctly have the rate output on the screen (or under the line graph once I have that put in). Any help would be appreciated as this assignment is due tomorrow!

import java.awt.*;
import javax.swing.*;
import java.awt.event.*;
public class Rate_Graph extends JApplet implements ActionListener
{
JLabel bv, ev1, ev2, ev3, ev4, ev5;
JTextField bv1, ev_1, ev_2, ev_3, ev_4, ev_5;
JButton go, add1, add2, add3, add4, add5;
public void init()
{
    setLayout(new FlowLayout()); //This sets the preferred layout format

    bv = new JLabel("Enter beginning value:"); //Beginning value input
    bv1 = new JTextField(5);

    ev1 = new JLabel("Enter year 1 value:"); //Year 1 value input
    ev_1 = new JTextField(5);

    ev2 = new JLabel("Enter year 2 value:"); //Year 2 value input
    ev_2 = new JTextField(5);

    ev3 = new JLabel("Enter year 3 value:"); //Year 3 value input
    ev_3 = new JTextField(5);

    ev4 = new JLabel("Enter year 4 value:"); //Year 4 value input
    ev_4 = new JTextField(5);

    ev5 = new JLabel("Enter year 5 value:"); //Year 5 value input
    ev_5 = new JTextField(5);

    go = new JButton("Add!"); //Button that implements all the values
    go.addActionListener(this); //So it does something
    add(bv); add(bv1); //Adding all of the different things
    add(ev1); add(ev_1);
    add(ev2); add(ev_2);
    add(ev3); add(ev_3);
    add(ev4); add(ev_4);
    add(ev5); add(ev_5);
    add(go);
}
public void actionPerformed(ActionEvent event)
{
        /*Object src = event.getSource(); 
        if(src==go){
            String strbv = bv1.getText();
            String strev1 = ev_1.getText();
            String strev2 = ev_2.getText();
            String strev3 = ev_3.getText();
            String strev4 = ev_4.getText();
            String strev5 = ev_5.getText();
        }*/
}
int intbv = Integer.parseInt(bv1.getText());
int intev1 = Integer.parseInt(ev_1.getText());
int intev2 = Integer.parseInt(ev_2.getText());
int intev3 = Integer.parseInt(ev_3.getText());
int intev4 = Integer.parseInt(ev_4.getText());
int intev5 = Integer.parseInt(ev_5.getText());
public double computeRate()
{
    double rate = (Math.pow(intev5 / intbv, 1.0 / 5.0) - 1);
    return rate;
}
}

回答1:

In your code you are getting NullPointerException because you are trying to initialize intbv, intbv1, ... by using JLabel's bv, bv1, .... which are not yet initialized.

Just only declare intbv, intbv1, intbv2...(not initialize), and assign them values when any action is performed in actionPerformed.

public void actionPerformed(ActionEvent event) {
    intbv = Integer.parseInt(bv1.getText());
    intev1 = Integer.parseInt(ev_1.getText());
    intev2 = Integer.parseInt(ev_2.getText());
    intev3 = Integer.parseInt(ev_3.getText());
    intev4 = Integer.parseInt(ev_4.getText());
    intev5 = Integer.parseInt(ev_5.getText());
    int sum = intbv + intev1 + intev2 + intev3 + intev4 + intev5;
    System.out.println("Sum            : " + sum);
    System.out.println("Compute rate   : " + computeRate());
}
int intbv;
int intev1;
int intev2;
int intev3;
int intev4;
int intev5;