问题
I have started to work on my first Java program, which is a simple calculator, however I get an error claiming that my array is out of bounds. I have tried to debug it to see where and why that is so, as well as following the code on paper, both of which display the results that I would expect and wish for. Therefore, I can not see where the problem actually lies. The code is not complete.
According to the debugger, the error occurs at this line:
answer = outputNum.get(operationIndex) * outputNum.get(operationIndex+1);
Here is the main part of the code that I currently have:
private class ButtonHandler implements ActionListener {
/*
* "outputNum" stores the numbers entered in order.
* "orderOfOperations" stores all numbers in order after multiplication and division has been taken care of.
* "operationList" stores the mathematical operations entered in order.
* "currentNum" stores the most recently inputed numbers.
* "currentString" stores the most recently inputed string of numbers.
* "answer" stores temporary values and ultimately the answer to the inputed equation.
* "start" stores whether or not the equals button has been pressed and a new equation has started.
*/
ArrayList <Double> outputNum = new ArrayList <Double> (0);
ArrayList <Double> orderOfOperations = new ArrayList <Double> (0);
ArrayList <String> operationList = new ArrayList <String> (0);
Double currentNum = 0.0;
String currentString = "0";
Double answer = 0.0;
boolean start = false;
public void actionPerformed(ActionEvent event) {
if(event.getSource() == equalsBtn) {
/* Takes the current numbers string and convert it into a double.
* Let the order of operations be the inputed order for now.
* "operationIndex" to get values inside of the unchanging arrays "outputNum" and "operationList".
* "orderIndex" to get values inside of the changing array "orderOfOperations".
*/
currentNum = Double.parseDouble(currentString);
outputNum.add(currentNum);
orderOfOperations = outputNum;
int operationIndex = 0;
int orderIndex = 0;
//Cycle through the mathematical operations that occur in the current equation.
for(String operation : operationList) {
if(operation == "x" || operation == "÷") {
// Multiply/divide numbers with the multiply/divide operations together.
if(operation == "x") {
answer = outputNum.get(operationIndex) * outputNum.get(operationIndex+1);
}
else {
answer = outputNum.get(operationIndex) / outputNum.get(operationIndex+1);
}
// Replace numbers multiplied/divided in the above statement with the new answer.
orderOfOperations.remove(orderIndex);
orderOfOperations.set(orderIndex, answer);
orderIndex++;
}
operationIndex++;
}
// Reset variables to the default values for the new equation.
System.out.println(orderOfOperations);
outputString = answer.toString();
answer = 0.0;
currentNum = 0.0;
currentString = "0";
outputNum.clear();
operationList.clear();
orderOfOperations.clear();
start = true;
}
else if(event.getSource() == additionBtn || event.getSource() == subtractionBtn || event.getSource() == multiplicationBtn || event.getSource() == divisionBtn) {
// Sets the text fields text blank if this is the start of a new equation.
if(start) {
outputString = "";
start = false;
}
/*
* Takes the current numbers string and convert it into a double.
* Add the mathematical operation and reset the current number.
*/
currentNum = Double.parseDouble(currentString);
outputNum.add(currentNum);
operationList.add(event.getActionCommand());
currentString = "0";
outputString = String.format("%s%s", outputString, event.getActionCommand());
}
else {
// Sets the text fields text blank if this is the start of a new equation.
if(start) {
outputString = "";
start = false;
}
// Adds the button value to the text field text and the current number being inputed.
currentString = String.format("%s%s", currentString, event.getActionCommand());
outputString = String.format("%s%s", outputString, event.getActionCommand());
}
outputScreen.setText(outputString);
}
}
Here is the error message that I get:
Exception in thread "AWT-EventQueue-0" java.lang.IndexOutOfBoundsException: Index: 2, Size: 2
at java.util.ArrayList.RangeCheck(ArrayList.java:547)
at java.util.ArrayList.get(ArrayList.java:322)
at MainWindow$ButtonHandler.actionPerformed(MainWindow.java:141)
at javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:2028)
at javax.swing.AbstractButton$Handler.actionPerformed(AbstractButton.java:2351)
at javax.swing.DefaultButtonModel.fireActionPerformed(DefaultButtonModel.java:387)
at javax.swing.DefaultButtonModel.setPressed(DefaultButtonModel.java:242)
at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(BasicButtonListener.java:236)
at java.awt.Component.processMouseEvent(Component.java:6414)
at javax.swing.JComponent.processMouseEvent(JComponent.java:3275)
at java.awt.Component.processEvent(Component.java:6179)
at java.awt.Container.processEvent(Container.java:2084)
at java.awt.Component.dispatchEventImpl(Component.java:4776)
at java.awt.Container.dispatchEventImpl(Container.java:2142)
at java.awt.Component.dispatchEvent(Component.java:4604)
at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4618)
at java.awt.LightweightDispatcher.processMouseEvent(Container.java:4279)
at java.awt.LightweightDispatcher.dispatchEvent(Container.java:4209)
at java.awt.Container.dispatchEventImpl(Container.java:2128)
at java.awt.Window.dispatchEventImpl(Window.java:2492)
at java.awt.Component.dispatchEvent(Component.java:4604)
at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:717)
at java.awt.EventQueue.access$400(EventQueue.java:82)
at java.awt.EventQueue$2.run(EventQueue.java:676)
at java.awt.EventQueue$2.run(EventQueue.java:674)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.AccessControlContext$1.doIntersectionPrivilege(AccessControlContext.java:86)
at java.security.AccessControlContext$1.doIntersectionPrivilege(AccessControlContext.java:97)
at java.awt.EventQueue$3.run(EventQueue.java:690)
at java.awt.EventQueue$3.run(EventQueue.java:688)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.AccessControlContext$1.doIntersectionPrivilege(AccessControlContext.java:86)
at java.awt.EventQueue.dispatchEvent(EventQueue.java:687)
at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:296)
at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:211)
at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:201)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:196)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:188)
at java.awt.EventDispatchThread.run(EventDispatchThread.java:122)
回答1:
When operationIndex is equal to the last element index in outputNum, then operationIndex + 1 will be greater than the last element index: hence your exception
You would be better off using a for loop that started at index 1, and doing:
// assuming that operationList and outputNum always
// have the same number of elements
for (int i = 1; i < operationList.size(); i++) {
...
answer = outputNum.get(i - 1) * outputNum.get(i);
...
}
or, alternatively, use your current loop, but do:
// operationIndex++; // remove this and use ..
if (++operationIndex >= outputNum.size()) break;
回答2:
You are only ever adding one item to your list:
currentNum = Double.parseDouble(currentString);
outputNum.add(currentNum);
Then you try to access what seems to be a second object at index operationindex +1 which doesn't exist:
answer = outputNum.get(operationIndex) * outputNum.get(operationIndex+1);
来源：https://stackoverflow.com/questions/28074277/array-index-out-of-bounds-java