添加到ArrayList[long post]时发生java NullPointerException

3 周，1 日 Questions & Answers 129

我正在为一个类项目创建一个正则表达式解析器，但遇到了一个无法解决的问题。每当它遇到一个带括号的组时，它都应该向groupsArrayList添加一个占位符，然后从字符串中捕获一个组。问题是，每当它试图添加到此ArrayList时，都会抛出NullPointerException。下面是Segment构造函数，其中发生了以下情况：

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
    }
}

注意RegexGUI.input指的是Regex的相关实例，其构造函数在这里：

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

正如您所看到的，groups显然是在构造函数中初始化的。而且，Segment构造函数只在Regex类中的tokenize()方法中调用，而该方法只由Regex构造函数调用，因此我无法看出问题仅仅是groups尚未初始化

整个stacktrace如下所示：

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());

我不知道是否需要发布，但下面是每个类的实例和类变量：

Regex类：

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

Segment类：

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类：

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

如果你们能给我任何帮助，我将不胜感激。我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，试图找出在初始化groups之前调用Segment构造函数的任何方法，但我看不到任何可能的方法

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添加到ArrayList[long post]时发生java NullPointerException

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