import javax.swing.JOptionPane;

public class Addition{
public static void main(String args[])
{
String firstNumber,
secondNumber;
int number1,
number2,
sum;

JOptionPane.showInputDialog(
"Enter first integer");

number1=Integer.parseInt(firstNumber);
number2=Integer.parseInt(SecondNumber);
sum= number1+number2;

JOptionPane.showMessageDialog(
null,"The sum is"+sum,"Results",
JOptionPane.PLAIN-MESSAGE);

System.exit(0);
}
}




谢谢啦

number2=Integer.parseInt(SecondNumber);这个s要小写
JOptionPane.PLAIN-MESSAGE);那一横应该是下划线
还有就是firstNumber,secondNumber要初始化，或者你把他们定义在main()以外也行

谢谢大哥，回去试试

你的代码编译都通不过

改了之后编译能通过，但是运行了下就有问题：
Exception in thread "main" java.lang.NumberFormatException: For input string: ""
at java.lang.NumberFormatException.forInputString(Unknown Source)
at java.lang.Integer.parseInt(Unknown Source)
at java.lang.Integer.parseInt(Unknown Source)
at src.Addition.main(Addition.java:17)

两个String根本就没有赋值的

我大致理解了一下你的意思，吧程序修改了一下！
import javax.swing.JOptionPane;

public class Addition{
public static void main(String args[])
{
int number1,
number2,
sum=0;

String firstNumber=JOptionPane.showInputDialog(
"Enter first integer");
String secondNumber=JOptionPane.showInputDialog(
"Enter second integer");

number1=Integer.parseInt(firstNumber);
number2=Integer.parseInt(secondNumber);
sum= number1+number2;

JOptionPane.showMessageDialog(
null,"The sum is"+sum,"Results",
JOptionPane.INFORMATION_MESSAGE);

System.exit(0);
}
}

谢谢各位,原来是没有赋值,现在补上已经好了,呵呵