空对象引用上的 java.lang.String.compareToIgnoreCase(java.lang.String)'

Question

我有一个包含两个字符串的数组。我想按字母顺序对其进行排序。所以这是下面的代码。

  ArrayList<String> a = new ArrayList<> ();
                j = (String) b.get ("user");
                a.add (j);
                chatroomname.setText (j);
                a.add (Fusername);
                Log.i ("Size Of A ", String.valueOf (a.size ()));
                Collections.sort (a, new Comparator<String> () {
                    @Override
                    public int compare(String o1, String o2) {
                        return o1.compareToIgnoreCase (o2);
                    }
                });

我还在控制台上记录了数组的大小。它显示 2。哪个是正确的。所以我不明白为什么我会得到这个空指针异常。

这是我的错误日志:

java.lang.RuntimeException: Unable to start activity ComponentInfo{com.buckydroid.anonchat/com.buckydroid.anonchat.ChatRoom}: java.lang.NullPointerException: Attempt to invoke virtual method 'int java.lang.String.compareToIgnoreCase(java.lang.String)' on a null object reference
                                                                             at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2665)
                                                                             at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2726)
                                                                             at android.app.ActivityThread.-wrap12(ActivityThread.java)
                                                                             at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1477)
                                                                             at android.os.Handler.dispatchMessage(Handler.java:102)
                                                                             at android.os.Looper.loop(Looper.java:154)
                                                                             at android.app.ActivityThread.main(ActivityThread.java:6126)
                                                                             at java.lang.reflect.Method.invoke(Native Method)
                                                                             at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:886)
                                                                             at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:776)
                                                                          Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'int java.lang.String.compareToIgnoreCase(java.lang.String)' on a null object reference
                                                                             at com.buckydroid.anonchat.ChatRoom$5.compare(ChatRoom.java:244)
                                                                             at com.buckydroid.anonchat.ChatRoom$5.compare(ChatRoom.java:242)
                                                                             at java.util.TimSort.countRunAndMakeAscending(TimSort.java:351)
                                                                             at java.util.TimSort.sort(TimSort.java:216)
                                                                             at java.util.Arrays.sort(Arrays.java:1523)
                                                                             at java.util.Collections.sort(Collections.java:238)
                                                                             at com.buckydroid.anonchat.ChatRoom.onCreate(ChatRoom.java:241)
                                                                             at android.app.Activity.performCreate(Activity.java:6679)
                                                                             at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1119)
                                                                             at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2618)
                                                                             at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2726) 
                                                                             at android.app.ActivityThread.-wrap12(ActivityThread.java) 
                                                                             at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1477) 
                                                                             at android.os.Handler.dispatchMessage(Handler.java:102) 
                                                                             at android.os.Looper.loop(Looper.java:154) 
                                                                             at android.app.ActivityThread.main(ActivityThread.java:6126) 
                                                                             at java.lang.reflect.Method.invoke(Native Method) 
                                                                             at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:886) 
                                                                             at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:776) 

谢谢!!!

Answer 1

如果异常来自您粘贴的代码，请不仅检查a数组的大小，还要检查其内容。它可能存储 2 个值，但它们可能为 null。