Question

Android newb在这里。编写我的第一个hello world应用程序并且无法通过两个“不幸”错误和崩溃。我猜它是一个空指针异常，但没有任何谷歌搜索让我更好地理解它。非常感谢你。

的AndroidManifest.xml

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.example.eventexample"
    android:versionCode="1"
    android:versionName="1.0" >
    <uses-sdk
        android:minSdkVersion="19"
        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.eventexample.EventExampleActivity"
            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>

EventExampleActivity.java

package com.example.eventexample;

import android.app.Activity;
import android.app.ActionBar;
import android.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.ViewGroup;
import android.os.Build;
import android.widget.Button;
import android.widget.TextView;

public class EventExampleActivity extends Activity {
    @Override
    protected void onStart()
    {
        super.onStart();

        Button button = 
                (Button)findViewById(R.id.myButton);

            button.setOnClickListener(
                new Button.OnClickListener() {
                    public void onClick(View v) {
                        TextView myTextView = (TextView) findViewById(R.id.myTextView);
                        myTextView.setText("Button clicked");
                }
            }
        );
    }
}

activity_event_example.xml

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:id="@+id/myLayout"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent"
    tools:context=".EventExampleActivity" >

    <Button
        android:id="@+id/myButton"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_centerHorizontal="true"
        android:layout_centerVertical="true"
        android:text="@string/mybutton_string" />

    <TextView
        android:id="@+id/myTextView"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_above="@+id/myButton"
        android:layout_centerHorizontal="true"
        android:layout_marginBottom="41dp"
        android:text="@string/mytextview_string"
        android:textAppearance="?android:attr/textAppearanceLarge" />

</RelativeLayout>

控制台转储

[2014-07-06 21:35:36 - EventExample] ------------------------------
[2014-07-06 21:35:36 - EventExample] Android Launch!
[2014-07-06 21:35:36 - EventExample] adb is running normally.
[2014-07-06 21:35:36 - EventExample] Performing com.example.eventexample.EventExampleActivity activity launch
[2014-07-06 21:35:38 - EventExample] Application already deployed. No need to reinstall.
[2014-07-06 21:35:38 - EventExample] Starting activity com.example.eventexample.EventExampleActivity on device 192.168.56.101:5555
[2014-07-06 21:35:38 - EventExample] ActivityManager: WARNING: linker: libdvm.so has text relocations. This is wasting memory and is a security risk. Please fix.
[2014-07-06 21:35:39 - EventExample] ActivityManager: Starting: Intent { act=android.intent.action.MAIN cat=[android.intent.category.LAUNCHER] cmp=com.example.eventexample/.EventExampleActivity }

LogCat转储

07-07 02:35:38.276: E/AndroidRuntime(1681): FATAL EXCEPTION: main
07-07 02:35:38.276: E/AndroidRuntime(1681): Process: com.example.eventexample, PID: 1681
07-07 02:35:38.276: E/AndroidRuntime(1681): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.eventexample/com.example.eventexample.EventExampleActivity}: java.lang.NullPointerException
07-07 02:35:38.276: E/AndroidRuntime(1681):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2195)
07-07 02:35:38.276: E/AndroidRuntime(1681):     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2245)
07-07 02:35:38.276: E/AndroidRuntime(1681):     at android.app.ActivityThread.access$800(ActivityThread.java:135)
07-07 02:35:38.276: E/AndroidRuntime(1681):     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1196)
07-07 02:35:38.276: E/AndroidRuntime(1681):     at android.os.Handler.dispatchMessage(Handler.java:102)
07-07 02:35:38.276: E/AndroidRuntime(1681):     at android.os.Looper.loop(Looper.java:136)
07-07 02:35:38.276: E/AndroidRuntime(1681):     at android.app.ActivityThread.main(ActivityThread.java:5017)
07-07 02:35:38.276: E/AndroidRuntime(1681):     at java.lang.reflect.Method.invokeNative(Native Method)
07-07 02:35:38.276: E/AndroidRuntime(1681):     at java.lang.reflect.Method.invoke(Method.java:515)
07-07 02:35:38.276: E/AndroidRuntime(1681):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:779)
07-07 02:35:38.276: E/AndroidRuntime(1681):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:595)
07-07 02:35:38.276: E/AndroidRuntime(1681):     at dalvik.system.NativeStart.main(Native Method)
07-07 02:35:38.276: E/AndroidRuntime(1681): Caused by: java.lang.NullPointerException
07-07 02:35:38.276: E/AndroidRuntime(1681):     at com.example.eventexample.EventExampleActivity.onStart(EventExampleActivity.java:25)
07-07 02:35:38.276: E/AndroidRuntime(1681):     at android.app.Instrumentation.callActivityOnStart(Instrumentation.java:1171)
07-07 02:35:38.276: E/AndroidRuntime(1681):     at android.app.Activity.performStart(Activity.java:5241)
07-07 02:35:38.276: E/AndroidRuntime(1681):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2168)
07-07 02:35:38.276: E/AndroidRuntime(1681):     ... 11 more

Answer 1

您必须让活动知道要为UI呈现哪个xml。因此，在您的活动中覆盖onCreate方法并添加此调用：setContentView（R.id.activity_event_example）

同时将您在onStart方法中拥有的代码移动到onCreate（它被调用一次，而每次屏幕被调到前台时调用onStart），但调用setContentView后

Answer 2

通常，创建GUI内容的代码应该进入onCreate()，您需要在setContentView()之前调用findViewById()

你的课应该是这样的：

public class EventExampleActivity extends Activity {

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_event_example);
    Button button =(Button)findViewById(R.id.myButton);
    button.setOnClickListener(
            new Button.OnClickListener() {
                public void onClick(View v) {
                    TextView myTextView = (TextView) findViewById(R.id.myTextView);
                    myTextView.setText("Button clicked");
                }
            }
    );
}

}

更新：实际上它不应该读R.id.但R.layout。我编辑了代码，现在它应该是正确的。

Answer 3

试试这个 -

public class EventExampleActivity extends Activity {

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_event_example);
    Button button =(Button)findViewById(R.id.myButton);
    button.setOnClickListener(
            new Button.OnClickListener() {
                public void onClick(View v) {
                    TextView myTextView = (TextView) findViewById(R.id.myTextView);
                    myTextView.setText("Button clicked");
                }
            }
    );
}

}

在您的活动中，您必须覆盖onCreate方法并致电：setContentView(R.layout.activity_event_example)。其他答案部分正确，他们都告诉你写 setContentView（R.id.activity_event_example）; ，而你应该写 setContentView（R.layout.activity_event_example）; 。它应该是 R.layout.your_xml ，因为xmls是在布局类中定义的，而不是在id类中。

如果您在R.java文件中查找，那么您将找到类似

的内容

public static final class layout {
    public static final int your_xml_name=0x7f030000;
   }

所以将id更改为layout。

新手Android 4.4 Hello Word应用程序崩溃问题 - 按钮单击更改/更新TextView

3 个答案: