Question

我正在为类项目创建一个正则表达式解析器，我遇到了一个我无法弄清楚如何修复的问题。每当遇到带括号的组时，都应该在groups ArrayList中添加一个占位符，然后可以从String中捕获一个组。问题是它每次尝试添加到此ArrayList时都会抛出NullPointerException。这是Segment构造函数，发生这种情况：

public Segment(String value, boolean capture) throws ParseException {
    values = new ArrayList<>();
    parse(value);
    minimum = 1;
    maximum = 1;
    matchNot = false;
    captureGroup = capture;
    if (capture) {
        RegexGUI.input.groups.add(""); //error occurs on this line
    }
}

请注意，RegexGUI.input是指Regex的相关实例，其构造函数位于此处：

public Regex(String value) throws ParseException {
    this.value = value;
    segments = new ArrayList<>();
    groups = new ArrayList<>();
    anchorStart = false;
    anchorEnd = false;
    tokenize(value, true, false);
}

如您所见，groups在构造函数中已明确初始化。此外，Segment构造函数仅在tokenize()类的Regex方法中调用，该方法仅由Regex构造函数调用，因此没有可能的方法我可以看到问题只是groups尚未初始化。

整个堆栈跟踪在这里：

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
    at regexparser.Segment.<init>(Segment.java:39)
    at regexparser.Regex.tokenize(Regex.java:57)
    at regexparser.Regex.<init>(Regex.java:27)
    at regexparser.RegexGUI.actionPerformed(RegexGUI.java:80)
    at javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:2022)
    at javax.swing.AbstractButton$Handler.actionPerformed(AbstractButton.java:2348)
    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:6535)
    at javax.swing.JComponent.processMouseEvent(JComponent.java:3324)
    at java.awt.Component.processEvent(Component.java:6300)
    at java.awt.Container.processEvent(Container.java:2236)
    at java.awt.Component.dispatchEventImpl(Component.java:4891)
    at java.awt.Container.dispatchEventImpl(Container.java:2294)
    at java.awt.Component.dispatchEvent(Component.java:4713)
    at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4888)
    at java.awt.LightweightDispatcher.processMouseEvent(Container.java:4525)
    at java.awt.LightweightDispatcher.dispatchEvent(Container.java:4466)
    at java.awt.Container.dispatchEventImpl(Container.java:2280)
    at java.awt.Window.dispatchEventImpl(Window.java:2750)
    at java.awt.Component.dispatchEvent(Component.java:4713)
    at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:758)
    at java.awt.EventQueue.access$500(EventQueue.java:97)
    at java.awt.EventQueue$3.run(EventQueue.java:709)
    at java.awt.EventQueue$3.run(EventQueue.java:703)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:76)
    at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:86)
    at java.awt.EventQueue$4.run(EventQueue.java:731)
    at java.awt.EventQueue$4.run(EventQueue.java:729)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:76)
    at java.awt.EventQueue.dispatchEvent(EventQueue.java:728)
    at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:201)
    at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:116)
    at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:105)
    at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:101)
    at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:93)
    at java.awt.EventDispatchThread.run(EventDispatchThread.java:82)

这是stacktrace中提到的tokenize()中的代码行：

segments.add(new Segment(value.substring(0, value.indexOf(")")+1), true));

这是stacktrace中提到的actionPerformed()中的代码行：

input = new Regex(regexInput.getText());

我不知道是否有必要发帖，但这是我的每个类的实例和类变量：

Regex上课：

String value;
ArrayList<Segment> segments;
ArrayList<String> groups;
boolean anchorStart;
boolean anchorEnd;

Segment上课：

ArrayList<String> values;
int minimum;
int maximum;
boolean matchNot;
boolean captureGroup;

final static String UPPERCASE = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
final static String LOWERCASE = "abcdefghijklmnopqrstuvwxyz";
final static String DIGITS = "0123456789";
final static String[] SETS = {UPPERCASE, LOWERCASE, DIGITS};
final static String WORD = UPPERCASE + LOWERCASE + DIGITS + "_";

final static String WHITESPACE = " \r\n\t\f";

RegexGUI上课：

Container contentPane;
JPanel panel;
GridBagLayout gridbag;
GridBagConstraints c;
JLabel regexPrompt, stringPrompt, result;
JTextField regexInput;
JTextArea stringInput;
JButton test;
static Regex input;
boolean match;

我很感激你们能给我的任何帮助。我已经尝试查找可能导致此异常的原因，但我能找到的唯一答案是groups未初始化。我已经搜索了我的代码，试图弄清楚在Segment初始化之前可以调用groups构造函数的方法，但我看不出任何可行的方法。

Answer 1

你的代码是一个问题22

这一行：

input = new Regex(regexInput.getText());

将创建Regex对象，然后分配它。

但是，在创建Regex实例时，您尝试在实际完成之前引用Regex.input实例，因此它实际上尚未分配，实际上仍为null。

一个相当快速的解决方法是在构造对象后调用tokenize方法。因此，从构造函数中删除tokenize调用，然后运行类似：

input = new Regex(regexInput.getText());
input.tokenize(...);

添加到ArrayList时发生NullPointerException [long post]

1 个答案: