I have a problem with my android studio project

Question

So this code should first create 16 buttons, then select one of them randomly and change its colour to red. Now the user should click on the red button and then a new button becomes red.

In android studio, everything seems to be working, but when I want to open it with the emulator, the app instantly crashes.

I was wondering if someone here knows what to do.

MainActivity.java:

package com.example.sixteenbuttons;

import android.graphics.Color;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.LinearLayout;
import android.widget.Toast;

import java.util.Random;

public class MainActivity extends AppCompatActivity {

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


        LinearLayout layout = findViewById(R.id.linearLayout);
        //aufgabe1
        for (int i = 1; i < 17; i++) {
            final Button button = new Button(this);
            button.setText("Button " + i);
            layout.addView(button);

            //aufgabe2
            button.setOnClickListener(new View.OnClickListener(){
                @Override
                public void onClick(View v) {
                    Toast.makeText(MainActivity.this, "Klick auf " + button.getText(),Toast.LENGTH_LONG).show();
                }
            });
        }
        setRandom();
        setRedButton();
        newRedButton();
    }
    int currentRandom;
    public void setRandom() {
        Random random = new Random();
        LinearLayout layout = findViewById(R.id.linearLayout);
        currentRandom = random.nextInt(layout.getChildCount() - 1);
    }
    public void setRedButton() {
        Button winButton = findViewById(currentRandom);
        winButton.setTextColor(Color.RED);
    }
    public void newRedButton() {
        Button winButton = findViewById(currentRandom);
        winButton.setTextColor(Color.BLACK);
        setRandom();
        setRedButton();
    }
}

activity_main.xml:

<?xml version="1.0" encoding="utf-8"?>
<android.support.constraint.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity">

    <ScrollView
        android:layout_width="match_parent"
        android:layout_height="match_parent">

        <LinearLayout
            android:id="@+id/linearLayout"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:orientation="vertical">

        </LinearLayout>
    </ScrollView>

</android.support.constraint.ConstraintLayout>

AndroidManifest.xml:

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

    <application
        android:allowBackup="true"
        android:icon="@mipmap/ic_launcher"
        android:label="@string/app_name"
        android:roundIcon="@mipmap/ic_launcher_round"
        android:supportsRtl="true"
        android:theme="@style/AppTheme">
        <activity android:name=".MainActivity">
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />
                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>
    </application>

</manifest>

score 1 · Answer 1 · answered Oct 19 '20 at 08:42

1

currentRandom = random.nextInt(layout.getChildCount() - 1);
...
Button winButton = findViewById(currentRandom);

The problem is here. currentRandom is random int between 0 and the amount of your buttons and then you are trying to run findViewById for that random Int. You should use getChildAt() instead of findViewById

Button winButton = (Button)layout.getChildAt(currentRandom);

answered Oct 19 '20 at 08:42

grig

848
6
15

the for the quick answer! i corrected my code with what u suggested, but unfortunately "layout" is red. when hovering over it, android studio tells me: "qualifier must be an expression". – ManManu Oct 19 '20 at 09:03
Did you defined it? `LinearLayout layout = findViewById(R.id.linearLayout);` – grig Oct 19 '20 at 09:05
setRandom and setButton both work perfectly now, thx for your help. newRedButton however does not seem to work at all. normally after clicking on the red button, it should select a new red button, but somehow it doesnt. the button just stays red – ManManu Oct 19 '20 at 09:27

I have a problem with my android studio project

1 Answers1