Question

我真的不知道这有什么不对... 应用程序只应在单击按钮时更改文本视图的文本，在本例中为开始按钮。

当我使用虚拟机启动应用程序时，它会崩溃。

我的代码

package com.example.projectiii;

import android.support.v7.app.ActionBarActivity;
import android.support.v7.app.ActionBar;
import android.support.v4.app.Fragment;
import android.os.Bundle;
import android.view.LayoutInflater;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.view.View.OnClickListener;
import android.view.ViewGroup;
import android.widget.Button;
import android.widget.TextView;
import android.os.Build;

public class MainActivity extends ActionBarActivity {

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

    if (savedInstanceState == null) {
        getSupportFragmentManager().beginTransaction()
                .add(R.id.container, new PlaceholderFragment()).commit();
    }


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

        @Override
        public void onClick(View view) {
            // TODO Auto-generated method stub

            TextView word = (TextView) findViewById(R.id.display);
            word.setText("Project");

        }

    });
}

@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;
    }
}

}

我的xml

<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.example.projectiii.MainActivity$PlaceholderFragment" >

<EditText
    android:id="@+id/text"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_below="@+id/display"
    android:layout_centerHorizontal="true"
    android:layout_marginTop="46dp"
    android:ems="10" >

    <requestFocus />
</EditText>

<Button
    android:id="@+id/btStart"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_alignLeft="@+id/text"
    android:layout_below="@+id/text"
    android:layout_marginTop="42dp"
    android:text="Start" />

<Button
    android:id="@+id/btFinish"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_alignBottom="@+id/btStart"
    android:layout_alignRight="@+id/text"
    android:text="Finish" />

<Chronometer
    android:id="@+id/time"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_alignParentTop="true"
    android:layout_alignRight="@+id/text"
    android:layout_marginTop="40dp"
    android:text="Chronometer" />

<TextView
    android:id="@+id/display"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_below="@+id/time"
    android:layout_centerHorizontal="true"
    android:layout_marginTop="66dp"
    android:text="Text"
    android:textSize="30sp" />

我的logcat

05-03 15:06:48.720: E/AndroidRuntime(921): FATAL EXCEPTION: main
05-03 15:06:48.720: E/AndroidRuntime(921): Process: com.example.projectiii, PID: 921
05-03 15:06:48.720: E/AndroidRuntime(921): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.projectiii/com.example.projectiii.MainActivity}: java.lang.NullPointerException
05-03 15:06:48.720: E/AndroidRuntime(921):  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2195)
05-03 15:06:48.720: E/AndroidRuntime(921):  at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2245)
05-03 15:06:48.720: E/AndroidRuntime(921):  at android.app.ActivityThread.access$800(ActivityThread.java:135)
05-03 15:06:48.720: E/AndroidRuntime(921):  at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1196)
05-03 15:06:48.720: E/AndroidRuntime(921):  at android.os.Handler.dispatchMessage(Handler.java:102)
05-03 15:06:48.720: E/AndroidRuntime(921):  at android.os.Looper.loop(Looper.java:136)
05-03 15:06:48.720: E/AndroidRuntime(921):  at android.app.ActivityThread.main(ActivityThread.java:5017)
05-03 15:06:48.720: E/AndroidRuntime(921):  at java.lang.reflect.Method.invokeNative(Native Method)
05-03 15:06:48.720: E/AndroidRuntime(921):  at java.lang.reflect.Method.invoke(Method.java:515)
05-03 15:06:48.720: E/AndroidRuntime(921):  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:779)
05-03 15:06:48.720: E/AndroidRuntime(921):  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:595)
05-03 15:06:48.720: E/AndroidRuntime(921):  at dalvik.system.NativeStart.main(Native Method)
05-03 15:06:48.720: E/AndroidRuntime(921): Caused by: java.lang.NullPointerException
05-03 15:06:48.720: E/AndroidRuntime(921):  at com.example.projectiii.MainActivity.onCreate(MainActivity.java:33)
05-03 15:06:48.720: E/AndroidRuntime(921):  at android.app.Activity.performCreate(Activity.java:5231)
05-03 15:06:48.720: E/AndroidRuntime(921):  at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1087)
05-03 15:06:48.720: E/AndroidRuntime(921):  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2159)
05-03 15:06:48.720: E/AndroidRuntime(921):  ... 11 more
05-03 15:06:48.760: W/ActivityManager(385):   Force finishing activity com.example.projectiii/.MainActivity

Answer 1

在此处绑定TextView的视图：

setContentView(R.layout.activity_main);
TextView word = (TextView) findViewById(R.id.display);

Answer 2

确保您发布的xml是activity_main，而不是fragment_main。您不需要片段，因此请删除所有与片段相关的代码。

Answer 3

您显示的布局xml是片段（因为它不包含R.id.容器）。您正在尝试访问活动中Fragment布局中定义的按钮元素，该按钮元素为您提供空指针。尝试访问片段类中的按钮，它将解决您的错误。

Android应用在启动时崩溃

3 个答案: