School, 22.10.2017, first application

activity_main.xml
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<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical"
    android:background="#00999F"
    android:id="@+id/llyBackground">

    <Button
        android:id="@+id/btnSayHello"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:text="say hello"
        android:textColor="#ff00ff"
        android:textSize="28sp"
        android:layout_margin="16dp"
        />

    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:orientation="horizontal"
        android:layout_margin="16dp">

        <Button
            android:id="@+id/btnRed"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:layout_weight="1"
            android:text="Red" />

        <Button
            android:id="@+id/btnGreen"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:layout_weight="1"
            android:text="Green" />

        <Button
            android:id="@+id/btnBlue"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="Blue"

            />
    </LinearLayout>

    <Button
        android:id="@+id/btnQuit"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_margin="16dp"
        android:text="quit"
        android:textColor="#0099ff"
        android:textSize="22sp" />

    <TextView
        android:id="@+id/tvText"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:layout_margin="16dp"
        android:background="#009922"
        android:gravity="center"
        android:text="dogma" />
</LinearLayout>


MainActivity.java
------------------
package com.example.mohamadpc.myfirstapplication;

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

public class MainActivity extends AppCompatActivity implements View.OnClickListener {
    Button b1, b2;
    TextView text1;

    Button btnRed, btnBlue, btnGreen;

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

        b1 = (Button) findViewById(R.id.btnSayHello);
        b2 = (Button) findViewById(R.id.btnQuit);
        text1 = (TextView) findViewById(R.id.tvText);

        btnRed = (Button) findViewById(R.id.btnRed);
        btnGreen = (Button) findViewById(R.id.btnGreen);
        btnBlue = (Button) findViewById(R.id.btnBlue);

        b1.setOnClickListener(this);
        b2.setOnClickListener(this);
        text1.setOnClickListener(this);

        btnRed.setOnClickListener(this);
        btnGreen.setOnClickListener(this);
        btnBlue.setOnClickListener(this);

        findViewById(R.id.llyBackground).setBackgroundColor(Color.YELLOW);
    }

    @Override
    public void onClick(View v) {
        switch (v.getId()) {
            case R.id.btnSayHello:
                text1.setText("Welcome to android");
                text1.setTextSize(10);
                break;
            case R.id.btnQuit:
                this.finish();
                break;
            case R.id.tvText:
                text1.setTextSize(text1.getTextSize() + 1);
                break;

            case R.id.btnBlue:
                text1.setBackgroundColor(Color.BLUE);
                text1.setTextColor(Color.WHITE);
                break;

            case R.id.btnRed:
                text1.setBackgroundColor(Color.RED);
                text1.setTextColor(Color.YELLOW);
                break;

            case R.id.btnGreen:
                text1.setBackgroundColor(Color.GREEN);
                text1.setTextColor(Color.BLUE);
                break;

        }

    }
}