java - 当我想要执行新任务时，我的 Android 应用程序就会崩溃

Question

大家好，我是 Android 新手，我想制作一个应用程序。类似于每日计划表的东西。问题是这样的，我的应用程序打开，并且有新任务按钮，当我点击按钮添加新任务时，应用程序崩溃了。我在论坛中搜索了类似的问题，但无法获得帮助。

这是我的猫日志文件:

09-27 19:03:46.035: D/dalvikvm(19346): Late-enabling CheckJNI

09-27 19:03:46.440: D/dalvikvm(19346): GC_FOR_ALLOC freed 136K, 4% free 12874K/13319K, paused 31ms

09-27 19:03:46.440: I/dalvikvm-heap(19346): Grow heap (frag case) to 13.127MB for 379472-byte allocation

09-27 19:03:46.465: D/dalvikvm(19346): GC_FOR_ALLOC freed 0K, 4% free 13245K/13703K, paused 20ms

09-27 19:03:46.535: D/AbsListView(19346): Get MotionRecognitionManager

09-27 19:03:49.510: D/CLIPBOARD(19346): Hide Clipboard dialog at Starting input: finished by someone else... !

09-27 19:03:53.625: D/AndroidRuntime(19346): Shutting down VM

09-27 19:03:53.625: W/dalvikvm(19346): threadid=1: thread exiting with uncaught exception (group=0x40c5d1f8)

09-27 19:03:53.635: E/AndroidRuntime(19346): FATAL EXCEPTION: main

09-27 19:03:53.635: E/AndroidRuntime(19346): java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.example.dailyplanner/com.example.dailyplanner.new_task}: java.lang.NullPointerException

09-27 19:03:53.635: E/AndroidRuntime(19346):    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1894)

09-27 19:03:53.635: E/AndroidRuntime(19346):    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:1995)

09-27 19:03:53.635: E/AndroidRuntime(19346):    at android.app.ActivityThread.access$600(ActivityThread.java:128)

09-27 19:03:53.635: E/AndroidRuntime(19346):    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1161)

09-27 19:03:53.635: E/AndroidRuntime(19346):    at android.os.Handler.dispatchMessage(Handler.java:99)

09-27 19:03:53.635: E/AndroidRuntime(19346):    at android.os.Looper.loop(Looper.java:137)

09-27 19:03:53.635: E/AndroidRuntime(19346):    at android.app.ActivityThread.main(ActivityThread.java:4514)

09-27 19:03:53.635: E/AndroidRuntime(19346):    at java.lang.reflect.Method.invokeNative(Native Method)

09-27 19:03:53.635: E/AndroidRuntime(19346):    at java.lang.reflect.Method.invoke(Method.java:511)

09-27 19:03:53.635: E/AndroidRuntime(19346):    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:993)

09-27 19:03:53.635: E/AndroidRuntime(19346):    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:760)

09-27 19:03:53.635: E/AndroidRuntime(19346):    at dalvik.system.NativeStart.main(Native Method)

09-27 19:03:53.635: E/AndroidRuntime(19346): Caused by: java.lang.NullPointerException

09-27 19:03:53.635: E/AndroidRuntime(19346):    at android.app.Activity.findViewById(Activity.java:1851)

09-27 19:03:53.635: E/AndroidRuntime(19346):    at com.example.dailyplanner.new_task.<init>(new_task.java:19)

09-27 19:03:53.635: E/AndroidRuntime(19346):    at java.lang.Class.newInstanceImpl(Native Method)

09-27 19:03:53.635: E/AndroidRuntime(19346):    at java.lang.Class.newInstance(Class.java:1319)

09-27 19:03:53.635: E/AndroidRuntime(19346):    at android.app.Instrumentation.newActivity(Instrumentation.java:1027)

09-27 19:03:53.635: E/AndroidRuntime(19346):    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1885)

09-27 19:03:53.635: E/AndroidRuntime(19346):    ... 11 more

这是 Android list :

<manifest package="com.example.dailyplanner"
android:versionName="1.0" xmlns:android="http://schemas.android.com/apk/res/android" android:versionCode="1">

<uses-sdk
    android:minSdkVersion="9"
    android:targetSdkVersion="15" />

<application
    android:icon="@drawable/ic_launcher"
    android:label="@string/app_name"
    android:theme="@android:style/Theme.Light.NoTitleBar" >
    <activity
        android:name=".MainActivity"
        android:label="@string/title_activity_main"
        android:screenOrientation="portrait" 
        android:configChanges="orientation|keyboardHidden">
        <intent-filter>
            <action android:name="android.intent.action.MAIN" />

            <category android:name="android.intent.category.LAUNCHER" />
        </intent-filter>
    </activity>
    <activity android:name=".new_task"/>
</application>

这是我的 main.java 文件:

package com.example.dailyplanner;

import android.os.Bundle;
import android.app.Activity;
import android.app.ListActivity;
import android.content.Intent;
import android.database.sqlite.SQLiteDatabase;
import android.view.Menu;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;

public class MainActivity extends ListActivity {





@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    //SQLiteDatabase db = openOrCreateDatabase("MyDB", MODE_PRIVATE, null   );
    //db.execSQL("CREATE TABLE IF NOT EXISTS Data1 (Id INT IDENTITY(1,1) NOT NULL, text VARCHAR, time TIME, date DATE)");



    Button nTaskBtn = (Button) findViewById(R.id.nTask);
    nTaskBtn.setOnClickListener(new OnClickListener() {

        public void onClick(View v) {

            Intent nTaskScreen = new Intent(getApplicationContext() , new_task.class);



            startActivity(nTaskScreen);



        }
    });

}

@Override
public boolean onCreateOptionsMenu(Menu menu) {
    getMenuInflater().inflate(R.menu.activity_main, menu);
    return true;
}

这是 new_task.java 文件:

package com.example.dailyplanner;

import android.R.string;
import android.app.Activity;
import android.app.Dialog;
import android.app.TimePickerDialog;
import android.net.UrlQuerySanitizer.ValueSanitizer;
import android.os.Bundle;
import android.renderscript.Sampler.Value;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;
import android.widget.TextView;
import android.widget.TimePicker;

public class new_task extends Activity {

static final int TIME_DIALOG_ID=0;
final TextView timeText = (TextView) findViewById(R.id.timeTxt);
TextView dateText = (TextView) findViewById(R.id.dateTxt);



@Override
protected void onCreate(Bundle savedInstanceState) {
    // TODO Auto-generated method stub
    super.onCreate(savedInstanceState);
    setContentView(R.layout.new_task);

    Button btnSave = (Button) findViewById(R.id.btnSave);
    btnSave.setOnClickListener(new OnClickListener() {

        public void onClick(View v) {




        }
    });



    Button btnSetTime = (Button) findViewById(R.id.btnSetTime);
    btnSetTime.setOnClickListener(new OnClickListener() {

        public void onClick(View v) {

            showDialog(TIME_DIALOG_ID);

        }
        });
}
private TimePickerDialog.OnTimeSetListener mTimeSetListener = new TimePickerDialog.OnTimeSetListener() {

            public void onTimeSet(TimePicker view, int hourOfDay, int minute) {
                timeText.setText(hourOfDay+":"+minute);
                };

};
protected Dialog onCreateDialog(int id){
    switch(id){

    case TIME_DIALOG_ID:
    return new TimePickerDialog(this, mTimeSetListener, 0, 0, true);

    }
    return null;

}

有什么帮助吗？

Answer 1

只有在设置内容 View 后才能使用findViewById。在访问变量之前对变量进行 null 检查也很有用。最后，确保布局文件 R.layout.new_task 包含 ID 为 timeTxt、dateTxt、btnSave 的项目。

我重写了你的一些工作，为你提供了一个例子来说明我的意思:

private TextView timeText;
private TextView dateText;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.new_task);

    timeText = (TextView) findViewById(R.id.timeTxt);
    dateText = (TextView) findViewById(R.id.dateTxt);

    Button btnSave = (Button) findViewById(R.id.btnSave);
    if (btnSave != null) {
        btnSave.setOnClickListener(new OnClickListener() {
            public void onClick(View v) {
            }
        });
    } else {
        Log.e("MyActivity", "btnSave was null :(");
    }
}