所以我有一个计算器应用程序。构建并且它可以正常工作但是如果用户输入例如 4+√ 我得到一个错误（如下所示）。有人告诉我，当用户点击 √ 按钮时，我需要查看用户是否已经输入了操作，如果他们已经输入了，那么就什么也不做。虽然我对 Java 很陌生，但我不知道该怎么做。
这是完成所有计算的动作侦听器（我相信代码需要在您看到 if (a.getSource() == textSqrt) 的位置下）：
public class event implements ActionListener {
public void actionPerformed(ActionEvent a) {
String text = a.getActionCommand();
if (text.equals("1")) {
result.setText(result.getText() + "1");
} else if (text.equals("2")) {
result.setText(result.getText() + "2");
} else if (text.equals("3")) {
result.setText(result.getText() + "3");
} else if (text.equals("4")) {
result.setText(result.getText() + "4");
} else if (text.equals("5")) {
result.setText(result.getText() + "5");
} else if (text.equals("6")) {
result.setText(result.getText() + "6");
} else if (text.equals("7")) {
result.setText(result.getText() + "7");
} else if (text.equals("8")) {
result.setText(result.getText() + "8");
} else if (text.equals("9")) {
result.setText(result.getText() + "9");
} else if (text.equals("0")) {
result.setText(result.getText() + "0");
} else if (result.getText().indexOf(".") < 1){
result.setText(result.getText() + ".");
}
String str = result.getText();
textBox = Double.parseDouble(str);
if (a.getSource()
== textAdd) {
op = 1;
firstInput = textBox;
result.setText("");
}
if (a.getSource()
== textSubtract) {
op = 2;
firstInput = textBox;
result.setText("");
}
if (a.getSource()
== textMultiply) {
op = 3;
firstInput = textBox;
result.setText("");
}
if (a.getSource()
== textDivide) {
op = 4;
firstInput = textBox;
result.setText("");
}
if (a.getSource()
== textPercent) {
op = 5;
firstInput = textBox;
result.setText("");
}
if (a.getSource() == textSqrt) {
op = 6;
firstInput = textBox;
answer = Math.sqrt(textBox);
str = Double.toString(answer);
result.setText(str);
}
if (a.getSource()
== textSign) {
double neg;
op = 7;
neg = 0 - textBox;
str = Double.toString(neg);
result.setText(str);
}
if (a.getSource()
== textEqual) {
if (op == 1) {
answer = firstInput + textBox;
str = Double.toString(answer);
result.setText(str);
} else if (op == 2) {
answer = firstInput - textBox;
str = Double.toString(answer);
result.setText(str);
} else if (op == 3) {
answer = firstInput * textBox;
str = Double.toString(answer);
result.setText(str);
} else if (op == 4) {
answer = firstInput / textBox;
str = Double.toString(answer);
result.setText(str);
} else if (op == 5) {
answer = firstInput % textBox;
str = Double.toString(answer);
result.setText(str);
}
}
}
}
这是我按下操作和平方根按钮时得到的错误
Exception in thread "AWT-EventQueue-0" java.lang.NumberFormatException: For input string: "."
at sun.misc.FloatingDecimal.readJavaFormatString(FloatingDecimal.java:1241)
at java.lang.Double.parseDouble(Double.java:540)
at inlab05.InLab05$event.actionPerformed(InLab05.java:215)
at javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:2018)
at javax.swing.AbstractButton$Handler.actionPerformed(AbstractButton.java:2341)
at javax.swing.DefaultButtonModel.fireActionPerformed(DefaultButtonModel.java:402)
at javax.swing.DefaultButtonModel.setPressed(DefaultButtonModel.java:259)
at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(BasicButtonListener.java:252)
at java.awt.Component.processMouseEvent(Component.java:6505)
at javax.swing.JComponent.processMouseEvent(JComponent.java:3321)
at java.awt.Component.processEvent(Component.java:6270)
at java.awt.Container.processEvent(Container.java:2229)
at java.awt.Component.dispatchEventImpl(Component.java:4861)
at java.awt.Container.dispatchEventImpl(Container.java:2287)
at java.awt.Component.dispatchEvent(Component.java:4687)
at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4832)
at java.awt.LightweightDispatcher.processMouseEvent(Container.java:4492)
at java.awt.LightweightDispatcher.dispatchEvent(Container.java:4422)
at java.awt.Container.dispatchEventImpl(Container.java:2273)
at java.awt.Window.dispatchEventImpl(Window.java:2719)
at java.awt.Component.dispatchEvent(Component.java:4687)
at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:735)
at java.awt.EventQueue.access$200(EventQueue.java:103)
at java.awt.EventQueue$3.run(EventQueue.java:694)
at java.awt.EventQueue$3.run(EventQueue.java:692)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:76)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:87)
at java.awt.EventQueue$4.run(EventQueue.java:708)
at java.awt.EventQueue$4.run(EventQueue.java:706)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:76)
at java.awt.EventQueue.dispatchEvent(EventQueue.java:705)
at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:242)
at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:161)
at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:150)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:146)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:138)
at java.awt.EventDispatchThread.run(EventDispatchThread.java:91)
抱歉发了这么长的帖子。希望有人可以帮助我:)