切换活动时android中的空指针异常

Question

我刚开始为 Android 编程，当时我的老板让我为他制作一个移动应用程序，所以我在两天内完成了所有这些，并在我做的过程中自学，主要来自这个网站。当我运行我的程序并单击切换下一个活动的按钮时，我不断收到 nullPointerException

第一个活动：

package com.Android.HelpDeskMobileApp;

import android.app.Activity;
import android.content.Intent;
import android.os.Bundle;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;
public class HelpDeskFront extends Activity{

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.first);
        Button serviceButton = (Button)findViewById(R.id.serviceButton),
                orderButton = (Button)findViewById(R.id.OrderButton),
                programmingButton = (Button)findViewById(R.id.ProgrammingButton);
        serviceButton.setOnClickListener(buttonListener);
        orderButton.setOnClickListener(buttonListener);
        programmingButton.setOnClickListener(buttonListener);
    }
    private OnClickListener buttonListener = new OnClickListener() {

        public void onClick(View v) {
            String serviceType = null;
            Intent i = new Intent(HelpDeskFront.this, Main.class);
            if (v.getId() == R.id.serviceButton)
                serviceType = "service";
            else if (v.getId() == R.id.OrderButton)
                serviceType = "order";
            else if (v.getId() == R.id.ProgrammingButton)
                serviceType = "programming";
            i.putExtra("serviceType", serviceType);
            startActivityForResult(i, Intent.FILL_IN_DATA);
        };
}

按钮导致我的第二个活动：

package com.Android.HelpDeskMobileApp;

import java.util.ArrayList;

import org.apache.http.HttpResponse;
import org.apache.http.NameValuePair;
import org.apache.http.client.HttpClient;
import org.apache.http.client.entity.UrlEncodedFormEntity;
import org.apache.http.client.methods.HttpPost;
import org.apache.http.impl.client.DefaultHttpClient;
import org.apache.http.message.BasicNameValuePair;

import android.app.Activity;
import android.content.Intent;
import android.os.Bundle;
import android.util.Log;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;
import android.widget.EditText;

public class Main extends Activity{

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);
        final Button send = (Button)findViewById(R.id.sendRequest);
        send.setOnClickListener(buttonListener);
    }
    private OnClickListener buttonListener = new OnClickListener() {

        ArrayList<NameValuePair> data = new ArrayList<NameValuePair>(5);
        final EditText caller = (EditText) findViewById(R.id.CallerEnter), 
                 callExt = (EditText) findViewById(R.id.CallNumberEnter),
                 computerName = (EditText) findViewById(R.id.ComputerNameEnter),
                 location = (EditText) findViewById(R.id.LocationEnter),
                 request = (EditText) findViewById(R.id.RequestEnter);

        public void onClick(View v) {
                    Intent i = getIntent();
        final String serviceType = i.getStringExtra("serviceType");
            Intent intent = new Intent(Main.this, HelpDeskEnd.class);
            final String[] email = {"target emails"};
            data.add(new BasicNameValuePair("Caller: ", textToString(caller)));
            data.add(new BasicNameValuePair("Call Ext: ", textToString(callExt)));
            data.add(new BasicNameValuePair("Computer Name: ", textToString(computerName)));
            data.add(new BasicNameValuePair("Locations: ", textToString(location)));
            data.add(new BasicNameValuePair("Request: ", textToString(request)));
            sendData(data);

            String body = data.get(0).toString() + " " + data.get(1).toString() + " " +
                    data.get(2).toString() + " " + data.get(3).toString() + " " + 
                    data.get(4).toString();
            Mail mail = new Mail("an email", "email password");
            mail.setTo(email);
            mail.setBody(body);
            mail.setFrom("an email");
            mail.setSubject(serviceType);
            try {
                mail.send();
            }
            catch (Exception e) {
                 Log.e("Send Mail", e.getMessage(), e);
            }
            intent.putExtra("serivceType", serviceType);
            startActivity(intent);
        }

    };
    private String textToString(EditText x)
    {
        return x.getText().toString();
    }
    private void sendData(ArrayList<NameValuePair> data)
    {
        try {

            HttpClient httpclient = new DefaultHttpClient();
            HttpPost httpPost = new HttpPost("https//www.awebsite.com/phpfile.php");
            httpPost.setEntity(new UrlEncodedFormEntity(data));
            HttpResponse response = httpclient.execute(httpPost);
            Log.i("postData", response.getStatusLine().toString());
        }
        catch (Exception e){
            Log.e("log_tag", "Error: " + e.toString());
        }
    }
}

这可能是一些非常容易解决的问题。谢谢你的帮助。这是我的清单文件，我也不知道它是否有用，但在这里：

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
      package="com.Android.HelpDeskMobileApp"
      android:versionCode="1"
      android:versionName="1.0">
    <uses-sdk android:minSdkVersion="12" />
    <uses-permission android:name="android.permission.INTERNET" />

    <application android:icon="@drawable/logo" android:label="@string/app_name">
        <activity android:name=".HelpDeskFront"
                  android:label="@string/app_name">
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />
                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>
        <activity android:label="@string/app_name" 
                  android:name=".Main">
                  <intent-filter></intent-filter>
        </activity>
        <activity android:label="@string/app_name" 
                  android:name=".HelpDeskEnd">
                  <intent-filter></intent-filter>
        </activity>

    </application>
</manifest>

当我点击按钮从第一个活动转到 .main 活动时发生错误。我已经尝试注释掉代码的不同部分，但我无法弄清楚是什么导致了空指针异常。谢谢。 日志猫：

    07-13 18:37:29.833: ERROR/AndroidRuntime(770): FATAL EXCEPTION: main
07-13 18:37:29.833: ERROR/AndroidRuntime(770): java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.Android.HelpDeskMobileApp/com.Android.HelpDeskMobileApp.Main}: java.lang.NullPointerException
07-13 18:37:29.833: ERROR/AndroidRuntime(770):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1672)
07-13 18:37:29.833: ERROR/AndroidRuntime(770):     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:1764)
07-13 18:37:29.833: ERROR/AndroidRuntime(770):     at android.app.ActivityThread.access$1500(ActivityThread.java:122)
07-13 18:37:29.833: ERROR/AndroidRuntime(770):     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1002)
07-13 18:37:29.833: ERROR/AndroidRuntime(770):     at android.os.Handler.dispatchMessage(Handler.java:99)
07-13 18:37:29.833: ERROR/AndroidRuntime(770):     at android.os.Looper.loop(Looper.java:132)
07-13 18:37:29.833: ERROR/AndroidRuntime(770):     at android.app.ActivityThread.main(ActivityThread.java:4025)
07-13 18:37:29.833: ERROR/AndroidRuntime(770):     at java.lang.reflect.Method.invokeNative(Native Method)
07-13 18:37:29.833: ERROR/AndroidRuntime(770):     at java.lang.reflect.Method.invoke(Method.java:491)
07-13 18:37:29.833: ERROR/AndroidRuntime(770):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:841)
07-13 18:37:29.833: ERROR/AndroidRuntime(770):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:599)
07-13 18:37:29.833: ERROR/AndroidRuntime(770):     at dalvik.system.NativeStart.main(Native Method)
07-13 18:37:29.833: ERROR/AndroidRuntime(770): Caused by: java.lang.NullPointerException
07-13 18:37:29.833: ERROR/AndroidRuntime(770):     at android.app.Activity.findViewById(Activity.java:1744)
07-13 18:37:29.833: ERROR/AndroidRuntime(770):     at com.Android.HelpDeskMobileApp.Main$1.<init>(Main.java:35)
07-13 18:37:29.833: ERROR/AndroidRuntime(770):     at com.Android.HelpDeskMobileApp.Main.<init>(Main.java:31)
07-13 18:37:29.833: ERROR/AndroidRuntime(770):     at java.lang.Class.newInstanceImpl(Native Method)
07-13 18:37:29.833: ERROR/AndroidRuntime(770):     at java.lang.Class.newInstance(Class.java:1301)
07-13 18:37:29.833: ERROR/AndroidRuntime(770):     at android.app.Instrumentation.newActivity(Instrumentation.java:1022)
07-13 18:37:29.833: ERROR/AndroidRuntime(770):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1663)
07-13 18:37:29.833: ERROR/AndroidRuntime(770):     ... 11 more

Answer 1

这是 Main 的第 34 行。 logcat 输出中“Caused by”下的行是相关信息通常所在的位置。

好吧，罪魁祸首应该是初始化器

Intent i = getIntent();
        final String serviceType = i.getStringExtra("serviceType");
        final EditText caller = (EditText) findViewById(R.id.CallerEnter), 
            callExt = (EditText) findViewById(R.id.CallNumberEnter),
            computerName = (EditText) findViewById(R.id.ComputerNameEnter),
            location = (EditText) findViewById(R.id.LocationEnter),
            request = (EditText) findViewById(R.id.RequestEnter);

发生在 Object 构造函数中。由于onClickListener 构造函数将在onCreate() 方法之前运行，getIntent() 将返回 null，并且您不会设置 contentView，因此所有 findViewById() 也将返回 null。

正确的方法是：

private OnClickListener buttonListener = new OnClickListener() {

        ArrayList<NameValuePair> data = new ArrayList<NameValuePair>(5);

        public void onClick(View v) {
                    Intent i = getIntent();
        final String serviceType = i.getStringExtra("serviceType");
        final EditText caller = (EditText) findViewById(R.id.CallerEnter), 
            callExt = (EditText) findViewById(R.id.CallNumberEnter),
            computerName = (EditText) findViewById(R.id.ComputerNameEnter),
            location = (EditText) findViewById(R.id.LocationEnter),
            request = (EditText) findViewById(R.id.RequestEnter);

            Intent intent = new Intent(Main.this, HelpDeskEnd.class);
            final String[] email = {"target emails"};
            data.add(new BasicNameValuePair("Caller: ", textToString(caller)));
            data.add(new BasicNameValuePair("Call Ext: ", textToString(callExt)));
            data.add(new BasicNameValuePair("Computer Name: ", textToString(computerName)));
            data.add(new BasicNameValuePair("Locations: ", textToString(location)));
            data.add(new BasicNameValuePair("Request: ", textToString(request)));
            sendData(data);

            String body = data.get(0).toString() + " " + data.get(1).toString() + " " +
                    data.get(2).toString() + " " + data.get(3).toString() + " " + 
                    data.get(4).toString();
            Mail mail = new Mail("an email", "email password");
            mail.setTo(email);
            mail.setBody(body);
            mail.setFrom("an email");
            mail.setSubject(serviceType);
            try {
                mail.send();
            }
            catch (Exception e) {
                 Log.e("Send Mail", e.getMessage(), e);
            }
            intent.putExtra("serivceType", serviceType);
            startActivity(intent);
        }

    };

Answer 2

HelpDeskFront.startActivity(intent); 解决问题了吗？

Answer 3

私有 OnClickListener buttonListener = new OnClickListener() {

    public void onClick(View v) {
        String serviceType = null;
        Intent i = new Intent(HelpDeskFront.this, Main.class);
        switch (v.getId()) {
        case R.id.serviceButton :
            serviceType = "service";
            break;
        case R.id.OrderButton :
            serviceType = "order";
            break;
        case R.id.ProgrammingButton :
            serviceType = "programming";
            break;
        }
        i.putExtra("serviceType", serviceType);
        startActivity(i);// change this to startActivityForResult(yourintent,Intent.FILL_IN_DATA)
    }
};

乍一看只是一个想法

Answer 4

尝试像这样使用 if(view==programmingButton)else if(view==another button) 而不是 switch，您能否发布 logcat 并告诉哪些代码（作为 onclick 侦听器或第二个活动的一部分）为空指针异常...

Answer 5

错误在第 34 行

final String serviceType = i.getStringExtra("serviceType");

这意味着“i”可能没有正确初始化。

我会尝试这样做，以便上下文正确。

Intent i = Main.this.getIntent();

或者您可以像在其他活动中那样创建一个新意图。

Answer 6

您的应用在其主要活动中的启动方式存在很多问题！

您需要在 onStart() 方法中而不是在 onCreate() 方法中设置监听器。每次显示活动时并不总是调用 onCreate()。

您无需使用开关、if/else 或其他任何复杂的方法来找出单击了哪个按钮...每个按钮都有自己的 onClickListener()。在监听器中调用一个私有方法来启动新的活动。

使用 putExtra() 通过 Intent 传递您想要的任何数据。

这是一个例子......

public class MainActivity extends Activity {

    @Override
    public void onCreate( final Bundle savedInstanceState ) {
        super.onCreate( savedInstanceState );
        this.setContentView( R.layout.main );
    }

    @Override
    protected void onStart() {
        super.onStart();

        final Button _serviceButton = (Button)this.findViewById( R.id.buttonService );
        final Button _orderButton = (Button)this.findViewById( R.id.buttonOrder );
        final Button _progButton = (Button)this.findViewById( R.id.buttonProgramming );

        _serviceButton.setOnClickListener( new OnClickListener() {

            @Override
            public void onClick( final View v ) {
                MainActivity.this.doSomeButtonClick( "service" );
            }
        } );

        _orderButton.setOnClickListener( new OnClickListener() {

            @Override
            public void onClick( final View v ) {
                MainActivity.this.doSomeButtonClick( "order" );
            }
        } );

        _progButton.setOnClickListener( new OnClickListener() {

            @Override
            public void onClick( final View v ) {
                MainActivity.this.doSomeButtonClick( "programming" );
            }
        } );
    }

    private void doSomeButtonClick( final String type ) {
        final Intent intent = new Intent( MainActivity.this, Activity1.class );
        intent.putExtra( "serviceType", type );
        this.startActivity( intent );
    }
}