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我已經開始學習Java和Android開發，並且作爲第一個練習，我正嘗試爲Android開發一個使用自定義位置服務的Speedometer應用程序，問題似乎是當我初始化OnCreate方法中的按鈕時，這些無法找到。當爲一個按鈕調用setOnClickListener時出現NullPointerException - Android Studio
該應用程序基於滾動模板。
這是onCreate方法：
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_speedometer);
Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
setSupportActionBar(toolbar);
// Create the adapter that will return a fragment for each of the three
// primary sections of the activity.
mSectionsPagerAdapter = new SectionsPagerAdapter(getSupportFragmentManager());
// Set up the ViewPager with the sections adapter.
mViewPager = (ViewPager) findViewById(R.id.container);
mViewPager.setAdapter(mSectionsPagerAdapter);
FloatingActionButton fab = (FloatingActionButton) findViewById(R.id.fab);
fab.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View view) {
Snackbar.make(view, "Replace with your own action", Snackbar.LENGTH_LONG)
.setAction("Action", null).show();
}
});
// ATTENTION: This was auto-generated to implement the App Indexing API.
// See https://g.co/AppIndexing/AndroidStudio for more information.
client = new GoogleApiClient.Builder(this).addApi(AppIndex.API).build();
speedText = (TextView) findViewById(R.id.speedText);
unitText = (TextView) findViewById(R.id.unitText);
startButton = (Button) findViewById(R.id.startButton);
resetButton = (Button) findViewById(R.id.resetButton);
if (!runtimePermissions()) //Will call a method that ckechs Location serivces are permited.
{
enableButtons();
}
}
這是一個似乎會導致問題
private void enableButtons() //using this method will not require the use of two buttons for On/Off
{
startButton.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View view) {
startClickCount++; //Counts how many time the button has been clicked
if (startClickCount % 2 == 0) //If it is even
{
Intent i = new Intent(getApplicationContext(), GPSService.class);
startService(i);
}
else
{
Intent i = new Intent(getApplicationContext(), GPSService.class);
stopService(i);
}
}
});
}
這是我fragment_speedometer.xml編輯eneableButtons（）方法
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:paddingBottom="@dimen/activity_vertical_margin"
android:paddingLeft="@dimen/activity_horizontal_margin"
android:paddingRight="@dimen/activity_horizontal_margin"
android:paddingTop="@dimen/activity_vertical_margin"
tools:context="com.amp.systems.performancespeedometer.Speedometer$PlaceholderFragment">
<Button
android:text="Reset Info"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_alignParentBottom="true"
android:layout_centerHorizontal="true"
android:layout_marginBottom="181dp"
android:id="@+id/resetButton" />
<TextView
android:text="999"
android:textSize="100dp"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:id="@+id/speedText"
android:layout_alignParentTop="true"
android:layout_centerHorizontal="true"
android:layout_marginTop="100dp" />
<TextView
android:text="Km/h"
android:textSize="25dp"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:id="@+id/unitText"
android:layout_below="@+id/speedText"
android:layout_centerHorizontal="true" />
<Button
android:text="Start"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_alignParentBottom="true"
android:layout_alignParentStart="true"
android:layout_marginStart="24dp"
android:layout_marginBottom="65dp"
android:id="@+id/startButton" />
這是日誌deployng當我得到：執行時
E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.amp.systems.performancespeedometer, PID: 4090
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.amp.systems.performancespeedometer/com.amp.systems.performancespeedometer.Speedometer}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' 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$H.handleMessage(ActivityThread.java:1477)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:154)
at android.app.ActivityThread.main(ActivityThread.java:6119)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$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 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' on a null object reference
at com.amp.systems.performancespeedometer.Speedometer.enableButtons(Speedometer.java:160)
at com.amp.systems.performancespeedometer.Speedometer.onCreate(Speedometer.java:154)
at android.app.Activity.performCreate(Activity.java:6679)
at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1118)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2618)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2726)
at android.app.ActivityThread.-wrap12(ActivityThread.java)
at android.app.ActivityThread$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:6119)
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)
<?xml version="1.0" encoding="utf-8"?>
<android.support.design.widget.AppBarLayout
android:id="@+id/appbar"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:paddingTop="@dimen/appbar_padding_top"
android:theme="@style/AppTheme.AppBarOverlay">
<android.support.v7.widget.Toolbar
android:id="@+id/toolbar"
android:layout_width="match_parent"
android:layout_height="?attr/actionBarSize"
android:background="?attr/colorPrimary"
app:layout_scrollFlags="scroll|enterAlways"
app:popupTheme="@style/AppTheme.PopupOverlay">
</android.support.v7.widget.Toolbar>
</android.support.design.widget.AppBarLayout>
<android.support.v4.view.ViewPager
android:id="@+id/container"
android:layout_width="match_parent"
android:layout_height="match_parent"
app:layout_behavior="@string/appbar_scrolling_view_behavior" />
<android.support.design.widget.FloatingActionButton
android:id="@+id/fab"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_gravity="end|bottom"
android:layout_margin="@dimen/fab_margin"
app:srcCompat="@android:drawable/ic_dialog_email" />
該應用程序只是崩潰。
在此先感謝！ :)
你可以發佈完整的'activity_speedometer.xml'嗎？ – MohanadMohie