android应用程序中的NULL POINTER EXCEPTION

Question

This is a simple activity which takes the message from user and displays it in the second activity. From nowhere I am getting a java.lang Null pointer exception.

The data in the file is as follows :

// activity_main.xml //这是主要活动的xml文件

<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=".MainActivity" >

    <TextView
        android:id="@+id/textView1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignParentLeft="true"
        android:layout_alignParentTop="true"
        android:layout_marginTop="29dp"
        android:text="Enter the message"
        android:textSize="18dp" />

    <EditText
        android:id="@+id/editText1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignParentLeft="true"
        android:layout_alignParentRight="true"
        android:layout_below="@+id/textView1"
        android:layout_marginTop="38dp"
        android:ems="10" >

        <requestFocus />
    </EditText>

    <Button
        android:id="@+id/button1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_below="@+id/editText1"
        android:layout_centerHorizontal="true"
        android:layout_marginTop="96dp"
        android:text="send"
        android:onClick="sendmessage" />

</RelativeLayout>

// MainActivity.java //这是主活动的代码，它创建一个intent并将输入的值传递给显示活动

package com.example.simpleactivity;

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

public class MainActivity extends Activity {

    public final static String EXTRA_MESSAGE = "com.example.simpleactivity.MESSAGE";
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
    }

    public void sendmessage(View view) {

        EditText textmessage = (EditText) findViewById(R.id.editText1);
        String message = textmessage.getText().toString();
        Intent intent = new Intent(this, DisplayMessageActivity.class);
        intent.putExtra(EXTRA_MESSAGE, message);
        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;
    }

}

// activity_display_message.xml //这是显示活动的xml文件

<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=".DisplayMessageActivity" >

    <TextView
        android:id="@+id/textView2"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignParentLeft="true"
        android:layout_alignParentTop="true"
        android:layout_marginLeft="58dp"
        android:layout_marginTop="110dp"
        android:text="Large Text"
        android:textAppearance="?android:attr/textAppearanceLarge" />

</RelativeLayout>

// DisplayMessageActivity.java    //这是获取和意图的显示活动，并显示额外的消息

package com.example.simpleactivity;

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

public class DisplayMessageActivity extends Activity {

    TextView tv;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        //setContentView(R.layout.activity_display_message);

        Intent intent = getIntent();
        String message = intent.getStringExtra(MainActivity.EXTRA_MESSAGE);
        tv = (TextView) findViewById(R.id.textView2);
        tv.setText(message);
        setContentView(tv);

    }

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

}

// logcat如下：

08-09 18:23:08.812: I/Choreographer(1520): Skipped 37 frames!  The application may be doing too much work on its main thread.

08-09 18:23:08.942: D/gralloc_goldfish(1520): Emulator without GPU emulation detected.

08-09 18:23:13.262: D/AndroidRuntime(1520): Shutting down VM

08-09 18:23:13.262: W/dalvikvm(1520): threadid=1: thread exiting with uncaught exception 
(group=0x414c4700)

08-09 18:23:13.402: E/AndroidRuntime(1520): FATAL EXCEPTION: main

08-09 18:23:13.402: E/AndroidRuntime(1520): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.simpleactivity/com.example.simpleactivity.DisplayMessageActivity}: java.lang.NullPointerException

08-09 18:23:13.402: E/AndroidRuntime(1520):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2211)

08-09 18:23:13.402: E/AndroidRuntime(1520):     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2261)

08-09 18:23:13.402: E/AndroidRuntime(1520):     at android.app.ActivityThread.access$600(ActivityThread.java:141)

08-09 18:23:13.402: E/AndroidRuntime(1520):     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1256)

08-09 18:23:13.402: E/AndroidRuntime(1520):     at android.os.Handler.dispatchMessage(Handler.java:99)

08-09 18:23:13.402: E/AndroidRuntime(1520):     at android.os.Looper.loop(Looper.java:137)

08-09 18:23:13.402: E/AndroidRuntime(1520):     at android.app.ActivityThread.main(ActivityThread.java:5103)

08-09 18:23:13.402: E/AndroidRuntime(1520):     at java.lang.reflect.Method.invokeNative(Native Method)

08-09 18:23:13.402: E/AndroidRuntime(1520):     at java.lang.reflect.Method.invoke(Method.java:525)

08-09 18:23:13.402: E/AndroidRuntime(1520):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:737)

08-09 18:23:13.402: E/AndroidRuntime(1520):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:553)

08-09 18:23:13.402: E/AndroidRuntime(1520):     at dalvik.system.NativeStart.main(Native Method)

08-09 18:23:13.402: E/AndroidRuntime(1520): Caused by: java.lang.NullPointerException

08-09 18:23:13.402: E/AndroidRuntime(1520):     at com.example.simpleactivity.DisplayMessageActivity.onCreate(DisplayMessageActivity.java:20)

08-09 18:23:13.402: E/AndroidRuntime(1520):     at android.app.Activity.performCreate(Activity.java:5133)

08-09 18:23:13.402: E/AndroidRuntime(1520):     at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1087)

08-09 18:23:13.402: E/AndroidRuntime(1520):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2175)

08-09 18:23:13.402: E/AndroidRuntime(1520):     ... 11 more

08-09 18:23:17.812: I/Process(1520): Sending signal. PID: 1520 SIG: 9

Answer 1

使用DisplayMessageActivity onCreate()方法：

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

        Intent intent = getIntent();
        String message = intent.getStringExtra(MainActivity.EXTRA_MESSAGE);
        tv = (TextView) findViewById(R.id.textView2);
        tv.setText(message);
        setContentView(tv);

    }

取消注释第一个setContentView()并在上面评论setContentView()，如下所示：

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

        Intent intent = getIntent();
        String message = intent.getStringExtra(MainActivity.EXTRA_MESSAGE);
        tv = (TextView) findViewById(R.id.textView2);
        tv.setText(message);
        //setContentView(tv);

    }