我是java的新手，所以我已经开始制作一个应用程序，通过预先添加和减去按钮找到TOTAL。我在FRAGMENT_MAIN.XML中定义了按钮和文本视图，并在main activity.JAVA中定义了它们的功能。在主activity.java中定义按钮的功能后，我的程序中出现了logcat错误，不幸的是，应用程序在Android模拟器中停止运行。我使用的是android sdk 22.6.4版，eclipse ide for java develpers版本是2.0.2.20140224-0000，eclipse平台版本是4.3.2.v2.140221-1852。
我的MainActivity.java
package com.counterehr.abhi;
public class MainActivity extends ActionBarActivity {
int counter;
Button add, sub;
TextView display;
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
counter = 0;
add = (Button) findViewById(R.id.bAdd);
sub = (Button) findViewById(R.id.bSub);
display = (TextView) findViewById(R.id.tvDisplay);
add.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
// TODO Auto-generated method stub
counter++;
display.setText("Your total is " + counter);
}
});
sub.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
// TODO Auto-generated method stub
counter--;
display.setText("Your total is " + counter);
}
});
if (savedInstanceState == null) {
getSupportFragmentManager().beginTransaction()
.add(R.id.container, new PlaceholderFragment())
.commit();
}
}
@Override
public boolean onCreateOptionsMenu(Menu menu) {
// Inflate the menu; this adds items to the action bar if it is present.
getMenuInflater().inflate(R.menu.main, menu);
return true;
}
@Override
public boolean onOptionsItemSelected(MenuItem item) {
// Handle action bar item clicks here. The action bar will
// automatically handle clicks on the Home/Up button, so long
// as you specify a parent activity in AndroidManifest.xml.
int id = item.getItemId();
if (id == R.id.action_settings) {
return true;
}
return super.onOptionsItemSelected(item);
}
/**
* A placeholder fragment containing a simple view.
*/
public static class PlaceholderFragment extends Fragment {
public PlaceholderFragment() {
}
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
View rootView = inflater.inflate(R.layout.fragment_main, container, false);
return rootView;
}
}
}
和FRAGMENT_MAIN.XM L是
FRAGMENT_MAIN.XML
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="fill_parent"
android:layout_height="fill_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.counterehr.abhi.MainActivity$PlaceholderFragment" >
<TextView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="Your total is 0"
android:textSize="45sp"
android:layout_gravity="center"
android:gravity="center"
android:id="@+id/tvDisplay"/>
<Button
android:id="@+id/bAdd"
android:layout_width="250dp"
android:layout_height="wrap_content"
android:layout_below="@+id/tvDisplay"
android:layout_centerHorizontal="true"
android:layout_marginTop="46dp"
android:text="Add one"
android:textSize="20sp" />
<Button
android:id="@+id/bSub"
android:layout_width="250dp"
android:layout_height="wrap_content"
android:layout_alignLeft="@+id/bAdd"
android:layout_centerVertical="true"
android:text="Subtract one"
android:textSize="20sp" />
</RelativeLayout>
在运行应用程序时，我收到了logcat错误。
logcat错误
07-01 18:16:17.963: E/Trace(833): error opening trace file: No such file or directory (2)
07-01 18:16:19.675: D/dalvikvm(833): GC_FOR_ALLOC freed 80K, 9% free 2419K/2644K, paused 356ms, total 356ms
07-01 18:16:19.704: I/dalvikvm-heap(833): Grow heap (frag case) to 3.415MB for 960016-byte allocation
07-01 18:16:19.802: D/dalvikvm(833): GC_FOR_ALLOC freed 1K, 7% free 3355K/3584K, paused 97ms, total 97ms
07-01 18:16:20.002: D/dalvikvm(833): GC_CONCURRENT freed <1K, 7% free 3355K/3584K, paused 9ms+33ms, total 201ms
07-01 18:16:20.382: D/dalvikvm(833): GC_CONCURRENT freed <1K, 6% free 3779K/4004K, paused 5ms+4ms, total 52ms
07-01 18:16:20.543: D/AndroidRuntime(833): Shutting down VM
07-01 18:16:20.543: W/dalvikvm(833): threadid=1: thread exiting with uncaught exception (group=0x40a71930)
07-01 18:16:20.583: E/AndroidRuntime(833): FATAL EXCEPTION: main
07-01 18:16:20.583: E/AndroidRuntime(833): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.counterehr.abhi/com.counterehr.abhi.MainActivity}: java.lang.NullPointerException
07-01 18:16:20.583: E/AndroidRuntime(833): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2180)
07-01 18:16:20.583: E/AndroidRuntime(833): at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2230)
07-01 18:16:20.583: E/AndroidRuntime(833): at android.app.ActivityThread.access$600(ActivityThread.java:141)
07-01 18:16:20.583: E/AndroidRuntime(833): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1234)
07-01 18:16:20.583: E/AndroidRuntime(833): at android.os.Handler.dispatchMessage(Handler.java:99)
07-01 18:16:20.583: E/AndroidRuntime(833): at android.os.Looper.loop(Looper.java:137)
07-01 18:16:20.583: E/AndroidRuntime(833): at android.app.ActivityThread.main(ActivityThread.java:5041)
07-01 18:16:20.583: E/AndroidRuntime(833): at java.lang.reflect.Method.invokeNative(Native Method)
07-01 18:16:20.583: E/AndroidRuntime(833): at java.lang.reflect.Method.invoke(Method.java:511)
07-01 18:16:20.583: E/AndroidRuntime(833): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:793)
07-01 18:16:20.583: E/AndroidRuntime(833): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:560)
07-01 18:16:20.583: E/AndroidRuntime(833): at dalvik.system.NativeStart.main(Native Method)
07-01 18:16:20.583: E/AndroidRuntime(833): Caused by: java.lang.NullPointerException
07-01 18:16:20.583: E/AndroidRuntime(833): at com.counterehr.abhi.MainActivity.onCreate(MainActivity.java:33)
07-01 18:16:20.583: E/AndroidRuntime(833): at android.app.Activity.performCreate(Activity.java:5104)
07-01 18:16:20.583: E/AndroidRuntime(833): at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1080)
07-01 18:16:20.583: E/AndroidRuntime(833): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2144)
07-01 18:16:20.583: E/AndroidRuntime(833): ... 11 more
07-01 18:16:23.873: I/Process(833): Sending signal. PID: 833 SIG: 9
请！有人帮助我解决这个问题，我尝试了所有可能的事情，但没有得到解决方案。
答案 0 :(得分：0)
问题是您正在混合片段和活动，因此，处理活动中的事件，即使这些视图显示在片段中。基本上你的意思是在Activity中有一个FrameLayout，就是这样 - 逻辑在片段中处理。
以下是一个例子：
public class ContainerActivity extends ActionBarActivity
{
@Override
public void onCreate(Bundle saveInstanceState)
{
super.onCreate(saveInstanceState);
this.setContentView(R.layout.activity_container);
getSupportActionBar().setDisplayHomeAsUpEnabled(true);
if (saveInstanceState == null)
{
getSupportFragmentManager().beginTransaction()
.add(R.id.activity_container_container, new ExampleFragment())
.addToBackStack(null)
.commit();
}
getSupportFragmentManager().addOnBackStackChangedListener(new OnBackStackChangedListener()
{
public void onBackStackChanged()
{
int backCount = getSupportFragmentManager().getBackStackEntryCount();
if (backCount == 0)
{
finish();
}
}
});
}
}
activity_container.xml：
<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="vertical" >
<FrameLayout
android:id="@+id/activity_container_container"
android:layout_width="match_parent"
android:layout_height="match_parent" />
</RelativeLayout>
ExampleFragment：
public class ExampleFragment extends Fragment implements View.OnClickListener
{
private Button btnOne;
private Button btnTwo;
private Button btnThree;
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState)
{
View rootView = inflater.inflate(R.layout.fragment_example, container, false);
btnOne = (Button) rootView.findViewById(R.id.example_button_one);
btnTwo = (Button) rootView.findViewById(R.id.example_button_two);
btnThree = (Button) rootView.findViewById(R.id.example_button_three);
btnOne.setOnClickListener(this);
btnTwo.setOnClickListener(this);
btnThree.setOnClickListener(this);
return rootView;
}
@Override
public void onClick(View v)
{
if (btnOne == v)
{
Toast.makeText(getActivity(), "One.", Toast.LENGTH_LONG).show();
}
else if (btnTwo == v)
{
Toast.makeText(getActivity(), "Two.", Toast.LENGTH_LONG).show();
}
else if (btnThree == v)
{
Toast.makeText(getActivity(), "Three.", Toast.LENGTH_LONG).show();
}
}
}
fragment_example.xml：
<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent" >
<Button
android:id="@+id/example_button_one"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:layout_alignParentTop="true"
android:layout_centerHorizontal="true"
android:layout_marginTop="30dp"
android:text="@string/hello"
android:layout_marginLeft="20dp"
android:layout_marginRight="20dp"/>
<Button
android:id="@+id/example_button_two"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_alignLeft="@+id/example_button_one"
android:layout_alignRight="@+id/example_button_one"
android:layout_below="@+id/example_button_one"
android:layout_marginTop="30dp"
android:text="@string/hello" />
<Button
android:id="@+id/example_button_three"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_alignLeft="@+id/example_button_two"
android:layout_alignRight="@+id/example_button_two"
android:layout_below="@+id/example_button_two"
android:layout_marginTop="30dp"
android:text="@string/hello" />
</RelativeLayout>
Android的的Manifest.xml：
<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
package="com.example"
android:versionCode="1"
android:versionName="1.0" >
<uses-sdk
android:minSdkVersion="8"
android:targetSdkVersion="19" />
<application
android:allowBackup="true"
android:icon="@drawable/ic_launcher"
android:label="@string/app_name"
android:theme="@style/AppTheme" >
<activity
android:name="com.example.ContainerActivity"
android:label="@string/app_name">
<intent-filter>
<action android:name="android.intent.action.MAIN" />
<category android:name="android.intent.category.LAUNCHER" />
</intent-filter>
</activity>
</application>
</manifest>
这应该是一个有效的例子，它展示了如何使用Activity来显示Fragment，并处理该Fragment中的事件。它目前没有显示Fragment如何与Activity通信以更改为另一个Fragment，这是由接口完成的，您可以在onAttach（）回调方法中创建变量并存储它。
的“温和精致”版本中加入了这一点