Force close error on setOnClickListener (Example from hello Android book section 3.5)

问题

I am having problem in starting a new activity on clicking a button, here's my code:

package test.project;

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

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

  View aboutButton = findViewById(R.id.about_content);
        aboutButton.setOnClickListener(this);
    }
public void onClick(View v) {
        switch (v.getId()) {
        case R.id.about_content:
        Intent i = new Intent(this, testit.class);
        startActivity(i);
        break;
        // More buttons go here (if any) ...
        }
    }
}

Can anyone please help me correct this error

Error Line

aboutButton.setOnClickListener(this);

Main.xml file

<?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="vertical" >

    <TextView
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        android:text="@string/hello" />

    <Button
        android:id="@+id/button1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Click here" android:layout_gravity="center"
android:text="Click here" android:layout_gravity="center" android:layout_marginTop="30dip"/>
</LinearLayout>

XML file containing about_content is

<?xml version="1.0" encoding="utf-8"?>

<ScrollView
    xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent"
    android:padding="10dip" >
    <TextView
        android:id="@+id/about_content"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="@string/about_text" />
</ScrollView>

about_content is already defined here

<TextView
            android:id="@+id/about_content"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="@string/about_text" />

回答1:

Well, you didn't post your logcat output, but since this is such a common beginner's mistake, I'm going to take a wild guess and say that you are probably getting a NullPointerException.

Your call to findViewById is probably returning null, which means that the system could not find the view associated with the id given by R.id.about_content. I would double check your XML layout for typos.

回答2:

Odds are you don't have anything with the id about_content in main.xml, which will create a NullPointerException.

Also, if aboutButton is supposed to be a traditional Button, then you should use this:

Button aboutButton = (Button) findViewById(R.id.about_content);

Addition

Since aboutButton is a TextView, use this:

TextView aboutButton = (TextView) findViewById(R.id.about_content);

but this TextView must be in the layout passed to setContentView() or findViewById() will return null.

回答3:

That is because the "main.xml" which you have set your content view ... does not contain the about_content TextView, its in the other xml which you have posted ...

Note: You can access only those R.id's which are present in your setContentView(R.layout.yourlayout) xml ...

回答4:

you make setContentView(R.layout.main); but main.xml does not include View have id = R.id.about_content. If you raplace by findViewById(R.id.button1); It will work.

回答5:

This is the solution for

Button aboutButton = (Button)findViewById(R.id.about_content);

And dont forget to add testit Activity in Android Manifest

来源：https://stackoverflow.com/questions/10780305/force-close-error-on-setonclicklistener-example-from-hello-android-book-section