Android Studio: App quits, but Build is without errors

DasMatti :

I wanted to create an App, which parses a website and gives an output. But when I start the app in the Emulator the app quits. I don't know where the error is.

I've got the Debug-Info, but I don't know what to do with it. When I google it, I can get no information about this.

My Java-File:

package osa.madsters.radiolohro902;

import android.annotation.SuppressLint;

import androidx.appcompat.app.ActionBar;
import androidx.appcompat.app.AppCompatActivity;

import android.os.Bundle;
import android.os.Handler;
import android.view.MotionEvent;
import android.view.View;
import android.widget.Button;
import android.widget.ImageButton;
import android.widget.TextView;

import org.jsoup.Jsoup;
import org.jsoup.nodes.Document;
import org.jsoup.nodes.Element;
import org.jsoup.select.Elements;

import java.io.IOException;




/**
 * An example full-screen activity that shows and hides the system UI (i.e.
 * status bar and navigation/system bar) with user interaction.
 */
public class Livestream extends AppCompatActivity {

    private ImageButton getBtn;
    private TextView result;


    /**
     * Whether or not the system UI should be auto-hidden after
     * {@link #AUTO_HIDE_DELAY_MILLIS} milliseconds.
     */
    private static final boolean AUTO_HIDE = true;

    /**
     * If {@link #AUTO_HIDE} is set, the number of milliseconds to wait after
     * user interaction before hiding the system UI.
     */
    private static final int AUTO_HIDE_DELAY_MILLIS = 3000;

    /**
     * Some older devices needs a small delay between UI widget updates
     * and a change of the status and navigation bar.
     */
    private static final int UI_ANIMATION_DELAY = 300;
    private final Handler mHideHandler = new Handler();
    private View mContentView;
    private final Runnable mHidePart2Runnable = new Runnable() {
        @SuppressLint("InlinedApi")
        @Override
        public void run() {
            // Delayed removal of status and navigation bar

            // Note that some of these constants are new as of API 16 (Jelly Bean)
            // and API 19 (KitKat). It is safe to use them, as they are inlined
            // at compile-time and do nothing on earlier devices.
            mContentView.setSystemUiVisibility(View.SYSTEM_UI_FLAG_LOW_PROFILE
                    | View.SYSTEM_UI_FLAG_FULLSCREEN
                    | View.SYSTEM_UI_FLAG_LAYOUT_STABLE
                    | View.SYSTEM_UI_FLAG_IMMERSIVE_STICKY
                    | View.SYSTEM_UI_FLAG_LAYOUT_HIDE_NAVIGATION
                    | View.SYSTEM_UI_FLAG_HIDE_NAVIGATION);
        }
    };
    private View mControlsView;
    private final Runnable mShowPart2Runnable = new Runnable() {
        @Override
        public void run() {
            // Delayed display of UI elements
            ActionBar actionBar = getSupportActionBar();
            if (actionBar != null) {
                actionBar.show();
            }
            mControlsView.setVisibility(View.VISIBLE);
        }
    };
    private boolean mVisible;
    private final Runnable mHideRunnable = new Runnable() {
        @Override
        public void run() {
            hide();
        }
    };
    /**
     * Touch listener to use for in-layout UI controls to delay hiding the
     * system UI. This is to prevent the jarring behavior of controls going away
     * while interacting with activity UI.
     */
    private final View.OnTouchListener mDelayHideTouchListener = new View.OnTouchListener() {
        @Override
        public boolean onTouch(View view, MotionEvent motionEvent) {
            if (AUTO_HIDE) {
                delayedHide(AUTO_HIDE_DELAY_MILLIS);
            }
            return false;
        }
    };

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        setContentView(R.layout.activity_livestream);

        mVisible = true;
        mControlsView = findViewById(R.id.fullscreen_content_controls);
        mContentView = findViewById(R.id.fullscreen_content);


        // Set up the user interaction to manually show or hide the system UI.
        mContentView.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                toggle();
            }
        });

        // Upon interacting with UI controls, delay any scheduled hide()
        // operations to prevent the jarring behavior of controls going away
        // while interacting with the UI.
        findViewById(R.id.dummy_button).setOnTouchListener(mDelayHideTouchListener);

        setContentView(R.layout.activity_main);
        result = (TextView) findViewById(R.id.result);
        getBtn = (ImageButton) findViewById(R.id.getBtn);
        getBtn.setOnClickListener(new View.OnClickListener() {

            public void onClick(View view) {
                getWebsite();
            }
        });
    }

    @Override
    protected void onPostCreate(Bundle savedInstanceState) {
        super.onPostCreate(savedInstanceState);

        // Trigger the initial hide() shortly after the activity has been
        // created, to briefly hint to the user that UI controls
        // are available.
        delayedHide(100);
    }

    private void toggle() {
        if (mVisible) {
            hide();
        } else {
            show();
        }
    }

    private void hide() {
        // Hide UI first
        ActionBar actionBar = getSupportActionBar();
        if (actionBar != null) {
            actionBar.hide();
        }
        mControlsView.setVisibility(View.GONE);
        mVisible = false;

        // Schedule a runnable to remove the status and navigation bar after a delay
        mHideHandler.removeCallbacks(mShowPart2Runnable);
        mHideHandler.postDelayed(mHidePart2Runnable, UI_ANIMATION_DELAY);
    }

    @SuppressLint("InlinedApi")
    private void show() {
        // Show the system bar
        mContentView.setSystemUiVisibility(View.SYSTEM_UI_FLAG_LAYOUT_FULLSCREEN
                | View.SYSTEM_UI_FLAG_LAYOUT_HIDE_NAVIGATION);
        mVisible = true;

        // Schedule a runnable to display UI elements after a delay
        mHideHandler.removeCallbacks(mHidePart2Runnable);
        mHideHandler.postDelayed(mShowPart2Runnable, UI_ANIMATION_DELAY);
    }

    /**
     * Schedules a call to hide() in delay milliseconds, canceling any
     * previously scheduled calls.
     */
    private void delayedHide(int delayMillis) {
        mHideHandler.removeCallbacks(mHideRunnable);
        mHideHandler.postDelayed(mHideRunnable, delayMillis);
    }


    private void getWebsite() {
        new Thread(new Runnable() {
            @Override
            public void run() {
                final StringBuilder builder = new StringBuilder();

                try {
                    Document doc = Jsoup.connect("http://www.ssaurel.com/blog").get();
                    String title = doc.title();
                    Elements links = doc.select("a[href]");

                    builder.append(title).append("\n");

                    for (Element link : links) {
                        builder.append("\n").append("Link : ").append(link.attr("href"))
                                .append("\n").append("Text : ").append(link.text());
                    }
                } catch (IOException e) {
                    builder.append("Error : ").append(e.getMessage()).append("\n");
                }

                runOnUiThread(new Runnable() {
                    @Override
                    public void run() {
                        result.setText(builder.toString());
                    }
                });
            }
        }).start();
    }

}

My XML-File is:

<?xml version="1.0" encoding="utf-8"?>
<FrameLayout 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:background="#FF8F06"
    tools:context=".Livestream">

    <!-- The primary full-screen view. This can be replaced with whatever view
         is needed to present your content, e.g. VideoView, SurfaceView,
         TextureView, etc. -->

    <!-- This FrameLayout insets its children based on system windows using
         android:fitsSystemWindows. -->
    <RelativeLayout
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:fitsSystemWindows="true">

        <ImageButton
            android:id="@+id/getBtn"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_centerHorizontal="true"
            android:layout_marginTop="500dp"
            android:background="@color/colorAccent"
            android:src="@drawable/ic_play_arrow_white_24dp"
            android:text="Get website" />

        <TextView
            android:id="@+id/result"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:layout_centerHorizontal="true"
            android:layout_marginVertical="460dp"
            android:layout_marginLeft="20dp"
            android:layout_marginRight="20dp"
            android:text="@string/abspielentext"
            android:textAlignment="center"
            android:textSize="17sp" />
    </RelativeLayout>

        <LinearLayout
            android:id="@+id/fullscreen_content_controls"
            style="?metaButtonBarStyle"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:layout_gravity="bottom|center_horizontal"
            android:background="@color/black_overlay"
            android:orientation="horizontal"
            tools:ignore="UselessParent">

            <Button
                android:id="@+id/dummy_button"
                style="@style/Widget.AppCompat.Button.Small"
                android:layout_width="0dp"
                android:layout_height="wrap_content"
                android:layout_weight="1"
                android:text="@string/dummy_button" />

        </LinearLayout>

        <TextView
            android:id="@+id/fullscreen_content"
            android:layout_width="match_parent"
            android:layout_height="match_parent"
            android:gravity="center"
            android:keepScreenOn="true"
            android:textColor="#FFFFFF"
            android:textSize="50sp"
            android:textStyle="bold" />


</FrameLayout>

The Error in the debug, when I start the app:

E/AndroidRuntime: FATAL EXCEPTION: main
    Process: osa.madsters.radiolohro902, PID: 6215
    java.lang.RuntimeException: Unable to start activity ComponentInfo{osa.madsters.radiolohro902/osa.madsters.radiolohro902.Livestream}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.ImageButton.setOnClickListener(android.view.View$OnClickListener)' on a null object reference
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3270)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3409)
        at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:83)
        at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:135)
        at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:95)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2016)
        at android.os.Handler.dispatchMessage(Handler.java:107)
        at android.os.Looper.loop(Looper.java:214)
        at android.app.ActivityThread.main(ActivityThread.java:7356)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:492)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:930)
     Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.ImageButton.setOnClickListener(android.view.View$OnClickListener)' on a null object reference
        at osa.madsters.radiolohro902.Livestream.onCreate(Livestream.java:133)
        at android.app.Activity.performCreate(Activity.java:7802)
        at android.app.Activity.performCreate(Activity.java:7791)
        at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1299)
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3245)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3409) 
        at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:83) 
        at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:135) 
        at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:95) 
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2016) 
        at android.os.Handler.dispatchMessage(Handler.java:107) 
        at android.os.Looper.loop(Looper.java:214) 
        at android.app.ActivityThread.main(ActivityThread.java:7356) 
        at java.lang.reflect.Method.invoke(Native Method) 
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:492) 
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:930)

92AlanC :

You're calling setContentView at 2 different points and for different layouts in your onCreate method.

The exception you're getting states that you're trying to set an OnClickListener to a null ImageButton, which in your case is getBtn.

In your case you're calling setContentView(R.layout.activity_livestream), which represents the layout you want and then you're calling setContentView(R.layout.activity_main). After that you're trying to set the OnClickListener to getBtn and you're getting the error because getBtn doesn't belong to activity_main.

So to fix your problem just get rid of the call to setContentView(R.layout.activity_main)