Question

我在这里和谷歌搜索了我的问题但收效甚微。我已经接近了，但还没找到我的问题。所以我是Android开发的新手，我无法弄清楚我在哪里错了。我正在developer.android.com网站上为Starting Another Activity做教程。

这是我的MainActivity.java：

package com.example.myfirstapp;

import android.os.Bundle;
import android.app.Activity;
import android.view.Menu;
import android.view.View;
import android.content.Intent;
import android.widget.EditText;

public class MainActivity extends Activity {

    public final static String EXTRA_MESSAGE = "com.example.myfirstapp.MESSAGE";

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


    @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;
    }

    /** Called when the user clicks the Send button */
    public void sendMessage(View view){
        Intent intent = new Intent(this, DisplayMessageActivity.class);
        EditText editText = (EditText)findViewById(R.id.edit_message);
        String message = editText.getText().toString();
        intent.putExtra(EXTRA_MESSAGE, message);
        startActivity(intent);
    }
}

IDE中的行String message = editText.getText().toString();表示它可能会抛出Java.lang.NullPointerException。当我运行应用程序时，它会加载，然后崩溃。

这是我的activity_main.xml：

<LinearLayout 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:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    android:paddingBottom="@dimen/activity_vertical_margin"
    tools:context=".MainActivity">




    <EditText android:id="@+id/edit_message"
              android:layout_width="wrap_content"
              android:layout_height="wrap_content"
              android:hint="@string/edit_message" />

    <Button
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="@string/button_send"
            android:onClick="sendMessage" />

    <activity
            android:name="com.example.myfirstapp.DisplayMessageActivity"
            android:label="@string/title_activity_display_message"
            android:parentActivityName="com.example.myfirstapp.MainActivity" >
        <meta-data
                android:name="android.support.PARENT_ACTIVITY"
                android:value="com.example.myfirstapp.MainActivity" />
    </activity>
</LinearLayout>

这是我的DisplayMessageActivity.java：

package com.example.myfirstapp;

import android.annotation.SuppressLint;
import android.app.Activity;
import android.content.Intent;
import android.os.Bundle;
import android.support.v4.app.NavUtils;
import android.view.MenuItem;
import android.widget.TextView;


/**
 * Created by rvance on 5/16/13.
 */
public class DisplayMessageActivity extends Activity {
    @SuppressLint("NewApi")
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        // Get the message from the intent
        Intent intent = getIntent();
        String message = intent.getStringExtra(MainActivity.EXTRA_MESSAGE);

        TextView textView = new TextView(this);
        textView.setTextSize(40);
        textView.setText(message);

        // Set the text view as the activity layout
        setContentView(textView);
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        switch (item.getItemId()) {
            case android.R.id.home:
                NavUtils.navigateUpFromSameTask(this);
                return true;
        }
        return super.onOptionsItemSelected(item);
    }
}

这是我的AndroidManifest.xml文件：

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

    <uses-sdk
        android:minSdkVersion="8"
        android:targetSdkVersion="17" />

    <application
        android:allowBackup="true"
        android:icon="@drawable/ic_launcher"
        android:label="Russell's First App"
        android:theme="@style/AppTheme" >
        <activity
            android:name="com.example.myfirstapp.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>
        <activity android:name=".DisplayMessageActivity" android:label="activity_display_message"/>
    </application>

</manifest>

我觉得问题出现在MainActivity类的sendMessage（）方法中。特别是行String message = editText.getText().toString();

/** Called when the user clicks the Send button */
public void sendMessage(View view){
    Intent intent = new Intent(this, DisplayMessageActivity.class);
    EditText editText = (EditText)findViewById(R.id.edit_message);
    String message = editText.getText().toString(); // The line that crashes is here
    intent.putExtra(EXTRA_MESSAGE, message);
    startActivity(intent);
}

最后，我毫不怀疑我搞砸了某个地方，但我看不到哪里。任何帮助深表感谢。

Answer 1

在DisplayMessageActivity尝试：

String message = intent.getExtras().getString(MainActivity.EXTRA_MESSAGE);

而不是你的：

String message = intent.getStringExtra(MainActivity.EXTRA_MESSAGE);

我在执行android教程源代码时遇到NullPointerException

1 个答案: