java - Intent 不起作用，单击按钮2时，应用程序崩溃

Question

我无法在 Activity 之间切换，当我单击 button2 时，应用程序崩溃了。

MainActivity.Java

<pre>package com.example.againtry;

    import java.util.Random;

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

    public class MainActivity extends Activity {
    /** Called when the activity is first created. */
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        // Declare our View Variables and assign them the Views from the layout file

        final TextView answerLabel = (TextView) findViewById(R.id.textView1);
        Button getAnswerButton = (Button) findViewById(R.id.button1);

        getAnswerButton.setOnClickListener(new View.OnClickListener() {

    @Override
    public void onClick(View arg0) {
    // The Button was clicked, so the update the answer label with an answer

         String answer = "";
    // Randomly select one of three answers: Yes, No or May be

    // Update the label with our dynamic answer

    Random randomGenerator = new Random(); // Construct a new Random number Generator
    int randomNumber = randomGenerator.nextInt(3);
    answer = Integer.toString(randomNumber);
    // update the label with our dynamic answer

    answerLabel.setText(answer);
    }});

        Button btnSimple= (Button) findViewById(R.id.button2);
        btnSimple.setOnClickListener(new View.OnClickListener() {

    @Override
    public void onClick(View v) {

    // From One Activity to another
    Intent intent = new Intent(MainActivity.this, SendMessageActivity.class);
    startActivity(intent);

    }});
    }



    @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;
    }}</pre>

这是我的第二个 Activity

SendMessageActivity.Java
<pre>package com.example.againtry;

    import android.app.Activity;
    import android.os.Bundle;
    import android.telephony.SmsManager;
    import android.view.Menu;
    import android.view.View;
    import android.widget.Button;
    import android.widget.EditText;
    import android.widget.Toast;

    public class SendMessageActivity extends Activity {
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.send_message);

        final EditText editSMS = (EditText) findViewById(R.id.editText1);
        final EditText editPhoneNum = (EditText) findViewById(R.id.phoneNum);
        Button getSendButton = (Button) findViewById(R.id.button1);



        getSendButton.setOnClickListener(new View.OnClickListener() {

    @Override
    public void onClick(View v) {
    String phoneNo = editPhoneNum.getText().toString();
    String sms = editSMS.getText().toString();

    try {
    SmsManager smsManager = SmsManager.getDefault();
    smsManager.sendTextMessage(phoneNo, null, sms, null, null);
    Toast.makeText(getApplicationContext(), "SMS Sent!",
    Toast.LENGTH_LONG).show();
    } catch (Exception e) {
    Toast.makeText(getApplicationContext(),
    "SMS faild, please try again later!",
    Toast.LENGTH_LONG).show();
    e.printStackTrace();
    }

    }
    });

    }


    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
    // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.send_message, menu);
        return true;
    }
    }</pre>


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

    <uses-sdk
        android:minSdkVersion="10"
        android:targetSdkVersion="10" />
    <uses-permission android:maxSdkVersion="10" android:name="android.permission.READ_CONTACTS"/>
    <uses-permission android:maxSdkVersion="10" android:name="android.permission.SEND_SMS"/>
    <uses-permission android:maxSdkVersion="10" android:name="android.permission.READ_SMS"/>

    <application
        android:allowBackup="true"
        android:icon="@drawable/ic_launcher"
        android:label="@string/app_name"
        android:theme="@style/AppTheme" >
        <activity
            android:name="com.example.againtry.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="com.example.againtry.SendMessageActivity"
            android:label="@string/title_activity_send_message" >
        </activity>
    </application>

    </manifest></pre>

activity_main<pre>

    <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:gravity="bottom"
    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=".MainActivity" >

    <Button
        android:id="@+id/button1"
        style="@android:style/ButtonBar"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_alignBottom="@+id/textView1"
        android:layout_centerHorizontal="true"
        android:layout_marginBottom="70dp"
        android:text="Enlighten Me!" />

     <TextView
        android:id="@+id/textView1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignParentTop="true"
        android:layout_centerHorizontal="true"
        android:layout_marginTop="177dp"
        android:textSize="32sp" />

       <Button
        android:id="@+id/button2"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_above="@+id/button1"
        android:layout_alignLeft="@+id/button1"
        android:layout_marginBottom="44dp"
        android:text="Message" />

    </RelativeLayout></pre>

send_message.xml<pre>

    <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=".SendMessageActivity" >

    <EditText
        android:id="@+id/editText1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignParentBottom="true"
        android:layout_alignParentLeft="true"
        android:ems="10"
        android:hint="Enter your message" >

        <requestFocus />
    </EditText>

    <Button
        android:id="@+id/sendButton"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignBaseline="@+id/editText1"
        android:layout_alignBottom="@+id/editText1"
        android:layout_alignParentRight="true"
        android:text="@string/button_send" />

    <TextView
        android:id="@+id/messageView1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignLeft="@+id/editText1"
        android:layout_alignParentTop="true"
        android:layout_marginTop="77dp"
        android:text="TextView" />

    <EditText
        android:id="@+id/phoneNum"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignLeft="@+id/messageView1"
        android:layout_alignParentTop="true"
        android:layout_marginTop="18dp"
        android:text="Enter Phone Number" android:inputType="number"/>
    </RelativeLayout></pre>

Answer 1

在 SendMessageActivity 的布局(即 send_message.xml)中，没有 ID 为“button1”的按钮。所以在做的时候:

Button getSendButton = (Button) findViewById(R.id.button1);

findViewById 返回 null，因此 getSendButton.setOnClickListener(/***/); 抛出 NullPointerException .

应该是:

Button getSendButton = (Button) findViewById(R.id.sendButton);