Question

我正在开发一个简单的应用程序来了解Android，但每当我尝试运行它时，我都会收到此错误：

11-28 14:51:34.536: E/AndroidRuntime(272): FATAL EXCEPTION: main
11-28 14:51:34.536: E/AndroidRuntime(272): java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{hu.mdpr.wk1/hu.mdpr.wk1.MainActivity}: java.lang.NullPointerException
11-28 14:51:34.536: E/AndroidRuntime(272):  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2585)
11-28 14:51:34.536: E/AndroidRuntime(272):  at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2679)
11-28 14:51:34.536: E/AndroidRuntime(272):  at android.app.ActivityThread.access$2300(ActivityThread.java:125)
11-28 14:51:34.536: E/AndroidRuntime(272):  at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2033)
11-28 14:51:34.536: E/AndroidRuntime(272):  at android.os.Handler.dispatchMessage(Handler.java:99)
11-28 14:51:34.536: E/AndroidRuntime(272):  at android.os.Looper.loop(Looper.java:123)
11-28 14:51:34.536: E/AndroidRuntime(272):  at android.app.ActivityThread.main(ActivityThread.java:4627)
11-28 14:51:34.536: E/AndroidRuntime(272):  at java.lang.reflect.Method.invokeNative(Native Method)
11-28 14:51:34.536: E/AndroidRuntime(272):  at java.lang.reflect.Method.invoke(Method.java:521)
11-28 14:51:34.536: E/AndroidRuntime(272):  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:868)
11-28 14:51:34.536: E/AndroidRuntime(272):  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:626)
11-28 14:51:34.536: E/AndroidRuntime(272):  at dalvik.system.NativeStart.main(Native Method)
11-28 14:51:34.536: E/AndroidRuntime(272): Caused by: java.lang.NullPointerException
11-28 14:51:34.536: E/AndroidRuntime(272):  at android.app.Activity.findViewById(Activity.java:1637)
11-28 14:51:34.536: E/AndroidRuntime(272):  at hu.mdpr.wk1.MainActivity.<init>(MainActivity.java:12)
11-28 14:51:34.536: E/AndroidRuntime(272):  at java.lang.Class.newInstanceImpl(Native Method)
11-28 14:51:34.536: E/AndroidRuntime(272):  at java.lang.Class.newInstance(Class.java:1429)
11-28 14:51:34.536: E/AndroidRuntime(272):  at android.app.Instrumentation.newActivity(Instrumentation.java:1021)
11-28 14:51:34.536: E/AndroidRuntime(272):  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2577)
11-28 14:51:34.536: E/AndroidRuntime(272):  ... 11 more

根据大多数关于这个问题的线索，这通常意味着我的Manifest存在问题，但我找不到任何东西。

我的宣言：

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="hu.mdpr.wk1"
    android:versionCode="1"
    android:versionName="1.0" >

    <uses-sdk
        android:minSdkVersion="8"
        android:targetSdkVersion="18" />

    <application
        android:allowBackup="true"
        android:icon="@drawable/ic_launcher"
        android:label="@string/app_name"
        android:theme="@style/AppTheme" >
        <activity
            android:name=".MainActivity"
            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>

我的活动：

package hu.mdpr.wk1;

import android.os.Bundle;
import android.app.Activity;
import android.view.Menu;
import android.view.View;
import android.widget.Button;
import android.widget.TextView;

public class MainActivity extends Activity {
    private int presses = 0;
    private final Button b = (Button)findViewById(R.id.button1);
    private final TextView v = (TextView)findViewById(R.id.textView1);

    @Override 
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        b.setOnClickListener(new ButtonClickHandler());
    }


    @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;
    }

    public class ButtonClickHandler implements View.OnClickListener
    {
        @Override
        public void onClick(View source)
        {
            presses++;
            v.setText("button pressed "+presses+" times.");
        }
    }

}

我已经尝试更改导出顺序，清理项目和更改API级别，但都没有解决问题。请帮帮我。

编辑：事实证明，将按钮设为最终，并尝试在onCreate之外填充变量导致了问题。谢谢所有回答的人。

Answer 1

private final Button b = (Button)findViewById(R.id.button1);
private final TextView v = (TextView)findViewById(R.id.textView1);

您尝试在尚未创建的活动中查找视图在findViewById()

之后致电setContentView()

Answer 2

变化

private int presses = 0;
private final Button b = (Button)findViewById(R.id.button1);
private final TextView v = (TextView)findViewById(R.id.textView1);

@Override 
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    b.setOnClickListener(new ButtonClickHandler());
}

到

private int presses = 0;
private Button b;
private TextView v;

@Override 
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    b = (Button)findViewById(R.id.button1);
    v = (TextView)findViewById(R.id.textView1);

    b.setOnClickListener(new ButtonClickHandler());
}

Answer 3

您尚未创建活动时创建视图：

这样做：

package hu.mdpr.wk1;

import android.os.Bundle;
import android.app.Activity;
import android.view.Menu;
 import android.view.View;
import android.widget.Button;
import android.widget.TextView;

public class MainActivity extends Activity {
private int presses = 0;
private  Button b; 
private TextView v ;

@Override 
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
      b = (Button)findViewById(R.id.button1);
      v = (TextView)findViewById(R.id.textView1);
    b.setOnClickListener(new ButtonClickHandler());
}


@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;
}

public class ButtonClickHandler implements View.OnClickListener
{
    @Override
    public void onClick(View source)
    {
        presses++;
        v.setText("button pressed "+presses+" times.");
    }
}

}

Answer 4

创建全局按钮顶部的声明

private Button b;
private TextView v;

oncreate（）方法中的引用

b = (Button)findViewById(R.id.button1);
v = (TextView)findViewById(R.id.textView1);

Android应用程序因无法实例化活动ComponentInfo而关闭

4 个答案: