我正在使用FlowLayout，GridLayout和BorderLayout创建GUI计算器。我有以下代码。
import java.awt.*;
import javax.swing.*;
//Imports visual components for program
import java.awt.event.*;
//Imports functions for providing performing action on object
public class Calc extends JFrame implements ActionListener {
JPanel[] row = new JPanel[5];
//5 panels are created for 5 rows of buttons
JButton[] button = new JButton[19];
String[] buttonString = {"1","2","3","+","4","5","6","-",
"7","8","9","*",".","/","C","rt","%",
"=", "0"};
double[] temporary = {0,0};
//Two memory locations for current number and upcoming number for signs such as *,/,%,+,-
boolean[] sign = new boolean[5];
//5 values for +,-,*,/,% are stored in array because they expect another number upon invocation
JTextArea display = new JTextArea(1,10);
//Creates display with location specified
Calc(){
//Constructor begins here
setResizable(false);
//Sets calculator size to be fixed at 380x250
//5x5 is created and selected for the calculator
for(int a = 0; a < 5; a++)
sign[a] = false;
//Initialises the state of the signs for +,-,*,/,%
JPanel displaypanel = new JPanel();
JPanel first = new JPanel();
JPanel last = new JPanel();
//Create three panels for buttons to be placed in
displaypanel.setLayout(new FlowLayout());
displaypanel.add(display);
//Display is added
first.setLayout(new GridLayout(3,5));
for(int a = 0; a<15; a++)
first.add(button[a]);
//"first" panel is added
last.setLayout(new GridLayout(1,4));
last.add(button[15]);
last.add(button[16]);
last.add(button[17]);
last.add(button[18]);
JFrame window = new JFrame("Task twelve");
window.setLayout(new BorderLayout());
window.add(displaypanel, BorderLayout.PAGE_START);
window.add(first, BorderLayout.CENTER);
window.add(last, BorderLayout.PAGE_END);
window.setVisible(true);
window.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
window.setSize(400, 400);
for(int a = 0; a < 19; a++){
button[a] = new JButton();
button[a].setText(buttonString[a]);
button[a].addActionListener(this);
//Assigns all the numbers and signs for the buttons
}
for(int a = 0; a < 5; a++)
row[a] = new JPanel();
//Initialises JPanel for rows so they can be used
//Assigns size for all buttons and display
display.setEditable(false);
}
public void clear(){
try{
display.setText("");
//Sets the display to be blank
for(int a = 0; a < 5; a++)
sign[a] = false;
//Sets state of all signs to be false
temporary[0] = 0;
temporary[1] = 0;
//Sets temporary values to be 0 as well
} catch(NullPointerException e){
}
}
public void root(){
try{
double temp = Math.sqrt(Double.parseDouble(display.getText()));
//Creates variable that converts the value in display to a double and Sqroots the value
display.setText(Double.toString(temp));
//Converts value in temp to string and copies it to display
} catch(NullPointerException e){
}
}
public void getResult() {
double result = 0;
temporary[1] = Double.parseDouble(display.getText());
String temp0 = Double.toString(temporary[0]);
String temp1 = Double.toString(temporary[1]);
try {
if(temp0.contains("-")) {
String[] temp2 = temp0.split("-", 2);
temporary[0] = (Double.parseDouble(temp2[1]) * -1);
}
if(temp1.contains("-")) {
String[] temp3 = temp1.split("-", 2);
temporary[1] = (Double.parseDouble(temp3[1]) * -1);
}
} catch(ArrayIndexOutOfBoundsException e) {
}
try {
if(sign[0] == true)
//Addition sign
result = temporary[0] + temporary[1];
else if(sign[1] == true)
//Subtraction sign
result = temporary[0] - temporary[1];
else if(sign[2] == true)
//Multiplication sign
result = temporary[0] * temporary[1];
else if(sign[3] == true)
//Division sign
result = temporary[0] / temporary[1];
else if(sign[4] == true)
//Modulus sign
result = temporary[0] % temporary[1];
display.setText(Double.toString(result));
for(int a = 0; a < 5; a++)
sign[a] = false;
//Sets state of all signs to be false after one of them has been invoked
} catch(NumberFormatException e) {
}
}
public void actionPerformed(ActionEvent ae){
if(ae.getSource() == button[0])
display.append("1");
//When "1" is pressed, "1" is inserted to the display
if(ae.getSource() == button[1])
display.append("2");
if(ae.getSource() == button[2])
display.append("3");
if(ae.getSource() == button[3]){
//Addition sign is selected
temporary[0] = Double.parseDouble(display.getText());
sign[0] = true;
display.setText("");
}
if(ae.getSource() == button[4])
display.append("4");
if(ae.getSource() == button[5])
display.append("5");
if(ae.getSource() == button[6])
display.append("6");
if(ae.getSource() == button[7]){
//Subtraction sign is selected
temporary[0] = Double.parseDouble(display.getText());
sign[1] = true;
display.setText("");
}
if(ae.getSource() == button[8])
display.append("7");
if(ae.getSource() == button[9])
display.append("8");
if(ae.getSource() == button[10])
display.append("9");
if(ae.getSource() == button[11]){
//Multiplication sign is selected
temporary[0] = Double.parseDouble(display.getText());
sign[2] = true;
display.setText("");
}
if(ae.getSource() == button[12])
display.append(".");
if(ae.getSource() == button[13]){
//Division sign is selected
temporary[0] = Double.parseDouble(display.getText());
sign[3] = true;
display.setText("");
}
if(ae.getSource() == button[14])
clear();
if(ae.getSource() == button[15])
root();
if(ae.getSource() == button[16]){
//Modulus sign is selected
temporary[0] = Double.parseDouble(display.getText());
sign[4] = true;
display.setText("");
}
if(ae.getSource() == button[17])
getResult();
if(ae.getSource() == button[18])
display.append("0");
}
public static void main(String[] args){
Calc c = new Calc();
}
}
编译这不会导致任何错误。但是，运行该类可以。
Exception in thread"main" java.lang.NullPointerException
at java.awt.Container.addlmpl(Unknown Source)
at java.awt.Container.add(Unknown Source)
at Calc.<init>(Calc.java:43)
at Calc.main(Calc.java:198)
我不理解这些错误，所以我不知道如何解决这个问题。有人可以帮忙吗？
答案 0 :(得分：2)
你在循环button[a] = new JButton(buttonString[a]);中创建了15个按钮，但是你要求按钮[15,16，...]（你没有创建的第16个，第17个......按钮）并且为空。