读取 ArrayList JTextFields [英] reading ArrayList JTextFields

问题描述

我的程序首先提示用户他们想要多少文本字段

My program starts by prompting the user how many text fields they would like to have

public class textEvent1 implements ActionListener {    //action listener for "how many masses?"
            public void actionPerformed (ActionEvent e) {
                n = (int)(Double.parseDouble(massNumField.getText())); 

接下来我创建一个 for 循环来创建标签和文本字段(我已经为其创建了列表，因为我不知道会有多少).有几个列表，但我只举一个例子.

next I create a for loop to create labels and textfields (which I have created lists for because I dont know how many there will be). There are a couple of lists but I will give an example of just one.

ArrayList masses = new ArrayList();

    for(int i=1; i<=n; i++) {                                  //adds text event 2 text to the screen
                        massLabel = new JLabel("How much mass does Mass " +i+ " have? ");
                        massField = new JTextField(5);
                        masses.add(massField);

现在，当我尝试将质量列表的一个元素分配给像这样的变量时，我的问题出现了.

Now my problem appears to come when I try to assign an element of the masses list to a variable like so.

for(int i=1; i<=n; i++) {
                        mass =  Double.parseDouble(((JTextComponent) masses.get(i)).getText());

我尝试了几件事...mass = mass.get(i).....mass = mass.get(i).getText()) 等等.我要么不断收到诸如 Null 指针异常之类的错误，要么说我无法 parseDouble an Object.

I have tried a couple of things...mass = masses.get(i).....mass = masses.get(i).getText()) and so on and so on. I either keep getting errors such as Null pointer exceptions or things saying I cant parseDouble an Object.

这个例子出现的错误如下

There errors that arrise for this example are as below

Exception in thread "AWT-EventQueue-0" java.lang.ArrayIndexOutOfBoundsException: 3 >= 3
    at java.util.Vector.elementAt(Unknown Source)
    at acmx.export.java.util.ArrayList.get(ArrayList.java:54)
    at Orbit$textEvent2.actionPerformed(Orbit.java:151)

第 151 行是

mass =  Double.parseDouble(((JTextComponent) masses.get(i)).getText());

推荐答案

在创建 JTextFields 时，您需要:

When creating the JTextFields, you do:

for(int i=1; i<=n; i++) {
    ...

请注意，List 索引从 0 开始，因此当您使用类似的循环检索项目时，使用 i 作为索引，您正在尝试访问超过最后一项.将阅读循环索引更改为:

Note that List indices start at 0, so when you retrieve the items with a similar loop, using i as the index, you are trying to access one past the last item. Change the reading loop indices to:

for (int i = 0; i < n; i++) {
    ...

或者您可以使用增强 for 循环，除非您需要使用古老的 Java 版本:

Or you could use an enhance for loop, unless you need to use an ancient java version:

for (Object massField : masses) {
    mass =  Double.parseDouble(((JTextComponent) massField).getText());
    ...

(那么你也应该真正使用泛型，如果 Java 版本支持它们).

(Then you should really use generics too, if the java version supports them).

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