程序加入对话框代码LogCat报错空指针

来源：互联网发布：中国印度知乎编辑：程序博客网时间：2024/05/18 19:45

最近在做简易日记本，其中有一步是点击“addButton”（即“添加新日志”键）后弹出对话框提示是否要添加新日志，但是将这部分代码写进程序后，LogCat显示空指针，初学安卓开发，还请各位大神指点~

对话框代码如下：
addBn.setOnClickListener(new OnClickListener() {
public void onClick(View v) {
AlertDialog.Builder builder = new AlertDialog.Builder(MainActivity.this);
builder.setMessage("是否放弃当前日志？");
builder.setPositiveButton("是", new DialogInterface.OnClickListener() {
public void onClick(DialogInterface dialog, int which) {
setContentView(R.layout.activity_main);
}
});
builder.setNegativeButton("否", new DialogInterface.OnClickListener() {
public void onClick(DialogInterface dialog, int which) {
Toast.makeText(MainActivity.this, "您已取消添加新日志！", Toast.LENGTH_LONG).show();
}
});
}
});

LogCat显示如下：
07-30 02:55:36.854: E/AndroidRuntime(393): java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.example.diary/com.example.diary.MainActivity}: java.lang.NullPointerException
07-30 02:55:36.854: E/AndroidRuntime(393): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2585)
07-30 02:55:36.854: E/AndroidRuntime(393): at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2679)
07-30 02:55:36.854: E/AndroidRuntime(393): at android.app.ActivityThread.access$2300(ActivityThread.java:125)
07-30 02:55:36.854: E/AndroidRuntime(393): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2033)
07-30 02:55:36.854: E/AndroidRuntime(393): at android.os.Handler.dispatchMessage(Handler.java:99)
07-30 02:55:36.854: E/AndroidRuntime(393): at android.os.Looper.loop(Looper.java:123)
07-30 02:55:36.854: E/AndroidRuntime(393): at android.app.ActivityThread.main(ActivityThread.java:4627)
07-30 02:55:36.854: E/AndroidRuntime(393): at java.lang.reflect.Method.invokeNative(Native Method)
07-30 02:55:36.854: E/AndroidRuntime(393): at java.lang.reflect.Method.invoke(Method.java:521)
07-30 02:55:36.854: E/AndroidRuntime(393): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:868)
07-30 02:55:36.854: E/AndroidRuntime(393): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:626)
07-30 02:55:36.854: E/AndroidRuntime(393): at dalvik.system.NativeStart.main(Native Method)
07-30 02:55:36.854: E/AndroidRuntime(393): Caused by: java.lang.NullPointerException
07-30 02:55:36.854: E/AndroidRuntime(393): at android.app.Activity.findViewById(Activity.java:1637)
07-30 02:55:36.854: E/AndroidRuntime(393): at com.example.diary.MainActivity.<init>(MainActivity.java:26)
07-30 02:55:36.854: E/AndroidRuntime(393): at java.lang.Class.newInstanceImpl(Native Method)
07-30 02:55:36.854: E/AndroidRuntime(393): at java.lang.Class.newInstance(Class.java:1429)
07-30 02:55:36.854: E/AndroidRuntime(393): at android.app.Instrumentation.newActivity(Instrumentation.java:1021)
07-30 02:55:36.854: E/AndroidRuntime(393): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2577)
07-30 02:55:36.854: E/AndroidRuntime(393): ... 11 more

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