startActivity（）导致Android崩溃

Question

So I just started learning to develop Android apps, and I have a programming background (Python mainly), so I somewhat know what I'm doing. I'm having a problem with using startActivity(). I've commented code to figure out exactly where the error gets thrown, and it happens right as startActivity() is encountered. The error I get is from the emulator and it is just a popup window that says, "Unfortunately, Test has stopped." (Test is my program name) after I click my button. My code is this

package com.test.test;

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

public class TestActivity extends Activity {

public final static String EXTRA_MESSAGE = "com.test.test.MESSAGE";

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

    public class DisplayMessageActivity extends Activity {
        @Override
        public void onCreate(Bundle savedInstanceState){
        super.onCreate(savedInstanceState);
        }
    }

    public void sendMessage(View view) {
        setContentView(R.layout.main);
        Intent intent = new Intent(this, DisplayMessageActivity.class);
        EditText editText = (EditText) findViewById(R.id.textBox1);
        String message = editText.getText().toString();
        intent.putExtra(EXTRA_MESSAGE, message);
        startActivity(intent);
    }

}

Now I know that won't do anything yet, but why is it crashing? My XML:

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="fill_parent"
android:layout_height="fill_parent"
android:orientation="horizontal" >

<EditText android:id="@+id/textBox1"
    android:layout_weight="1"
    android:layout_width="0dp"
    android:layout_height="wrap_content"
    android:hint="@string/textBox1Hint" />
<Button 
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:text="@string/button1"
    android:onClick="sendMessage" />   

</LinearLayout>

My manifest:

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

<uses-sdk android:minSdkVersion="15" />

<application
    android:icon="@drawable/ic_launcher"
    android:label="@string/app_name" >
    <activity
        android:name=".TestActivity"
        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" >
    </activity>

</application>
</manifest>

I saw that a lot of other people were having problems with this because they had not declared their activities in the manifest, but I think I have done so properly. Any help would be so greatly appreciated.

Answer 1

One possible problem is that

EditText editText = (EditText) findViewById(R.id.textBox1);

returns null because you dont set anywhere the layout for your activity (with setContentView )

If your editetext is included in view (passed to sendMessage ), you can find it with

EditText editText = (EditText) view.findViewById(R.id.textBox1);

Answer 2

I think problem is you are on DisplayMessageActivity and starting the same activity. What you need to do is start test activity and call DisplayMessageActivity from intent.

Answer 3

the problem is this,
you declare in the manifest that the main class is TestActivity:

<activity
    android:name=".TestActivity"
    android:label="@string/app_name" >

but you wanted to start DisplayMessageActivity, so, change this to the following:

public class TestActivity extends Activity {
@Override

Answer 4

I can't understand what you are actually trying to achieve by starting the same activity again. Try this,

    public class DisplayMessageActivity extends Activity {
    private  String message ;
        @Override
        public void onCreate(Bundle savedInstanceState){
            super.onCreate(savedInstanceState);
setLayout(R.layout.<name of the xml file> ); // change the name if it is not in layout folder
           EditText editText = (EditText) findViewById(R.id.textBox1);
           String message = editText.getText().toString();
           Button submitBtn = (Button) findViewById(R.id.button1);
           submitBtn.setOnClickListener(new OnClickListener() {

            @Override
            public void onClick(View v) {
                     Toast toast = Toast.makeText(DisplayMessageActivity.this,
                                message,
                                Toast.LENGTH_SHORT);
                        toast.show();
    }});
        }
    }

The above code simply displays the String, This is just a sample code... Hope that helps!!!

EDIT To run this code you must also add id tag to your Button Tag inside XML file having value button1..

Answer 5

Actullay you have created the Activity inside activity. Which is not much pref-able but if want to stick with that then you have to change the android:name=".DisplayMessageActivity" to

Answer 6

Use separate files for TestActivity and DisplayMessageActivity classes. In your given code you are not specifying a layout for your DisplayMessageActivity activity using setContentView .