Beginner Error with Onclicklistener

Question

I’m starting now in development for android. I’m trying to implement an onclicklistener on a button and it doesn’t work at all, I’ve followed step by step the videos lessons here, youtube, and always the error when compiling, what can it be? below is my java code, xml and logcat:

Code

package com.example.treino;

import android.support.v7.app.ActionBarActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.TextView;

public class MainActivity extends ActionBarActivity {

    Button button;
    TextView textView;

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

        button = (Button) findViewById(R.id.button1);
        textView = (TextView) findViewById(R.id.textView1);
        button.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                textView.setText("wow!");
            }
        });
    }
}

XML

<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="com.example.treino.MainActivity$PlaceholderFragment" >

    <TextView
        android:id="@+id/textView1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="@string/hello_world" />

    <Button
        android:id="@+id/button1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_below="@+id/textView1"
        android:layout_centerHorizontal="true"
        android:layout_marginTop="59dp"
        android:text="Button" />

</RelativeLayout>

Log

05-26 19:22:33.703: D/AndroidRuntime(25773): Shutting down VM
05-26 19:22:33.703: W/dalvikvm(25773): threadid=1: thread exiting with uncaught exception (group=0x41f1dd40)
05-26 19:22:33.706: E/AndroidRuntime(25773): FATAL EXCEPTION: main
05-26 19:22:33.706: E/AndroidRuntime(25773): Process: com.example.treino, PID: 25773
05-26 19:22:33.706: E/AndroidRuntime(25773): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.treino/com.example.treino.MainActivity}: java.lang.NullPointerException
05-26 19:22:33.706: E/AndroidRuntime(25773):    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2209)
05-26 19:22:33.706: E/AndroidRuntime(25773):    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2269)
05-26 19:22:33.706: E/AndroidRuntime(25773):    at android.app.ActivityThread.access$800(ActivityThread.java:139)
05-26 19:22:33.706: E/AndroidRuntime(25773):    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1210)
05-26 19:22:33.706: E/AndroidRuntime(25773):    at android.os.Handler.dispatchMessage(Handler.java:102)
05-26 19:22:33.706: E/AndroidRuntime(25773):    at android.os.Looper.loop(Looper.java:136)
05-26 19:22:33.706: E/AndroidRuntime(25773):    at android.app.ActivityThread.main(ActivityThread.java:5102)
05-26 19:22:33.706: E/AndroidRuntime(25773):    at java.lang.reflect.Method.invokeNative(Native Method)
05-26 19:22:33.706: E/AndroidRuntime(25773):    at java.lang.reflect.Method.invoke(Method.java:515)
05-26 19:22:33.706: E/AndroidRuntime(25773):    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:785)
05-26 19:22:33.706: E/AndroidRuntime(25773):    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:601)
05-26 19:22:33.706: E/AndroidRuntime(25773):    at dalvik.system.NativeStart.main(Native Method)
05-26 19:22:33.706: E/AndroidRuntime(25773): Caused by: java.lang.NullPointerException
05-26 19:22:33.706: E/AndroidRuntime(25773):    at com.example.treino.MainActivity.onCreate(MainActivity.java:28)
05-26 19:22:33.706: E/AndroidRuntime(25773):    at android.app.Activity.performCreate(Activity.java:5248)
05-26 19:22:33.706: E/AndroidRuntime(25773):    at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1110)
05-26 19:22:33.706: E/AndroidRuntime(25773):    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2173)
05-26 19:22:33.706: E/AndroidRuntime(25773):    ... 11 more
05-26 19:22:35.598: I/Process(25773): Sending signal. PID: 25773 SIG: 9
05-26 19:25:13.048: D/AndroidRuntime(26448): Shutting down VM
05-26 19:25:42.291: D/AndroidRuntime(26858): Shutting down VM
05-26 19:25:42.291: W/dalvikvm(26858): threadid=1: thread exiting with uncaught exception (group=0x41f1dd40)
05-26 19:25:42.294: E/AndroidRuntime(26858): FATAL EXCEPTION: main
05-26 19:25:42.294: E/AndroidRuntime(26858): Process: com.example.treino, PID: 26858
05-26 19:25:42.294: E/AndroidRuntime(26858): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.treino/com.example.treino.MainActivity}: java.lang.NullPointerException
05-26 19:25:42.294: E/AndroidRuntime(26858):    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2209)
05-26 19:25:42.294: E/AndroidRuntime(26858):    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2269)
05-26 19:25:42.294: E/AndroidRuntime(26858):    at android.app.ActivityThread.access$800(ActivityThread.java:139)
05-26 19:25:42.294: E/AndroidRuntime(26858):    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1210)
05-26 19:25:42.294: E/AndroidRuntime(26858):    at android.os.Handler.dispatchMessage(Handler.java:102)
05-26 19:25:42.294: E/AndroidRuntime(26858):    at android.os.Looper.loop(Looper.java:136)
05-26 19:25:42.294: E/AndroidRuntime(26858):    at android.app.ActivityThread.main(ActivityThread.java:5102)
05-26 19:25:42.294: E/AndroidRuntime(26858):    at java.lang.reflect.Method.invokeNative(Native Method)
05-26 19:25:42.294: E/AndroidRuntime(26858):    at java.lang.reflect.Method.invoke(Method.java:515)
05-26 19:25:42.294: E/AndroidRuntime(26858):    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:785)
05-26 19:25:42.294: E/AndroidRuntime(26858):    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:601)
05-26 19:25:42.294: E/AndroidRuntime(26858):    at dalvik.system.NativeStart.main(Native Method)
05-26 19:25:42.294: E/AndroidRuntime(26858): Caused by: java.lang.NullPointerException
05-26 19:25:42.294: E/AndroidRuntime(26858):    at com.example.treino.MainActivity.onCreate(MainActivity.java:23)
05-26 19:25:42.294: E/AndroidRuntime(26858):    at android.app.Activity.performCreate(Activity.java:5248)
05-26 19:25:42.294: E/AndroidRuntime(26858):    at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1110)
05-26 19:25:42.294: E/AndroidRuntime(26858):    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2173)
05-26 19:25:42.294: E/AndroidRuntime(26858):    ... 11 more
05-26 19:25:43.807: I/Process(26858): Sending signal. PID: 26858 SIG: 9

Answer 1

Solved

As Piovezan said it was only to exchange activity_main for fragment_main in the method below:

setContentView(R.layout.activity_main);

Answer 2

I believe your mistake of NullPointerException be on the line where you have that statement : Button button1 = (Button)findViewById(R.id.button1);

You probably forgot to declare a button in your xml file with the name button1.

In this question /a/15440/8372 I explain about the button. In case you manage to solve your problem post here saying you got it.