Android buttons null pointer

Question

I am trying to add an onClickListener to a button, but log cat outputs an error

Here is the log cat

FATAL EXCEPTION: main
                                                                Process: com.pk.a_loner, PID: 10539
                                                                java.lang.RuntimeException: Unable to start activity ComponentInfo{com.pk.a_loner/com.pk.a_loner.main.gui.Connect}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' on a null object reference
                                                                    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2814)
                                                                    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2879)
                                                                    at android.app.ActivityThread.access$900(ActivityThread.java:182)
                                                                    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1475)
                                                                    at android.os.Handler.dispatchMessage(Handler.java:102)
                                                                    at android.os.Looper.loop(Looper.java:145)
                                                                    at android.app.ActivityThread.main(ActivityThread.java:6141)
                                                                    at java.lang.reflect.Method.invoke(Native Method)
                                                                    at java.lang.reflect.Method.invoke(Method.java:372)
                                                                    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1399)
                                                                    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1194)
                                                                 Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' on a null object reference
                                                                    at com.pk.a_loner.main.gui.Connect.init(Connect.java:20)
                                                                    at com.pk.a_loner.main.gui.Connect.onCreate(Connect.java:53)
                                                                    at android.app.Activity.performCreate(Activity.java:6374)
                                                                    at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1119)
                                                                    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2767)
                                                                    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2879) 
                                                                    at android.app.ActivityThread.access$900(ActivityThread.java:182) 
                                                                    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1475) 
                                                                    at android.os.Handler.dispatchMessage(Handler.java:102) 
                                                                    at android.os.Looper.loop(Looper.java:145) 
                                                                    at android.app.ActivityThread.main(ActivityThread.java:6141) 
                                                                    at java.lang.reflect.Method.invoke(Native Method) 
                                                                    at java.lang.reflect.Method.invoke(Method.java:372) 
                                                                    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1399) 
                                                                    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1194)

The code below, should be fine?

package com.pk.a_loner.main.gui;

import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.util.Log;
import android.view.View;
import android.widget.Button;

import com.pk.a_loner.R;
import com.pk.a_loner.main.networking.Broadcast;
import com.pk.a_loner.main.networking.Listen;

public class Connect extends AppCompatActivity {

    private Button button_toggleBroadcast;
    private Button button_listen;

    private void init() {
        this.button_toggleBroadcast = (Button) (findViewById(R.id.button));
        this.button_toggleBroadcast.setOnClickListener(new Button.OnClickListener() {
            @Override
            public void onClick(View v) {
                if (!Broadcast.isInBroadcast()) {
                    if (!Broadcast.isHasInit()) {
                        Broadcast.init();
                    }
                    Broadcast.broadcast();
                } else {
                    Broadcast.halt();
                }
            }
        });
        this.button_listen = (Button) (findViewById(R.id.button2));
        this.button_listen.setOnClickListener(new Button.OnClickListener() {
            @Override
            public void onClick(View v) {
                if (!Listen.isInScan()) {
                    if (!Listen.isHasInit()) {
                        Listen.init();
                    }
                    Listen.scan();
                } else {
                    Listen.halt();
                }
            }
        });
    }

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        Log.v("hello world", "this is a hello");
        this.init();
        setContentView(R.layout.activity_launcher);
    }


}

and here is the xml

<?xml version="1.0" encoding="utf-8"?>
<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=".main.gui.Connect">

    <LinearLayout
        android:orientation="vertical"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:layout_alignParentTop="true"
        android:layout_alignParentStart="true">

        <Button
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="Toggle broadcast"
            android:id="@+id/button" />

        <Button
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="Listen 10 secs"
            android:id="@+id/button2" />
    </LinearLayout>
</RelativeLayout>

I checked the code, I did initialize the buttons before I add the listener, shouldn't be null? buttons also declared in xml.

The app currently crashes on startup.

Any help appreciated, many thanks.

Call `this.init();` after `setContentView(R.layout.activity_launcher);` line. always access Views from layout after set layout from Activity. currently you are accessing Button's before setting layout for Activity, in which both Button is present. — ρяσѕρєя K, Apr 22 '16 at 16:21
vivien_pk, see my comment already explained. let me know if still have any doubt — ρяσѕρєя K, Apr 22 '16 at 16:26
I mean the line this.button_toggleBroadcast = (Button) (findViewById(R.id.button) should already return the button? because the R.id.button is independent of the current class? — christopher_pk, Apr 22 '16 at 16:29
Button with `button ` id is in `activity_launcher ` layout, so we need to tell system, in which layout Button with `button ` id is present. we do it by calling `setContentView ` by passing layout id to it(in your case doing using `setContentView(R.layout.activity_launcher);` line). and after `setContentView ` method we can access Views from layout using `findViewById ` method but using Activity Context. — ρяσѕρєя K, Apr 22 '16 at 16:36

score 2 · Answer 1 · answered Apr 22 '16 at 16:23

Use this in your onCreate() instead:

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    Log.v("hello world", "this is a hello");
    setContentView(R.layout.activity_launcher);
    this.init();
}

You need to initialise your views AFTER your layout has been inflated by setContentView()

score 2 · Answer 2 · answered Apr 22 '16 at 16:23

call setContentView(R.layout.activity_launcher); before init();

@Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        Log.v("hello world", "this is a hello");
        setContentView(R.layout.activity_launcher);
        this.init();

    }

Android buttons null pointer

2 Answers2