School, simple app, 14.01.18

// activity_main.xml
--------------------
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical"
    android:id="@+id/llyBack">

    <Button
        android:id="@+id/btnClickMe"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:background="@color/myColor"
        android:textSize="32sp"
        android:text="click me"/>

    <TextView
        android:id="@+id/tvMessgae"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:textSize="28sp"
        android:text="Hello"
        android:gravity="center"
        android:layout_marginTop="16dp"
        />
</LinearLayout>

// activity_main2.xml
---------------------
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:background="@color/myColor"
    android:orientation="vertical">

<Button
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:text="Back to Main Activity"
    android:textSize="24sp"
    android:id="@+id/btnBackToMain"/>
</LinearLayout>

MainActivity.java
-----------------
package com.example.mohamadpc.myapplication140118;

import android.content.Intent;
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.LinearLayout;
import android.widget.TextView;

import java.util.Random;

// step 3
public class MainActivity extends AppCompatActivity implements View.OnClickListener {
    // step 1
    Button btnClick;
    TextView tvMessage;
    LinearLayout llyBack;
    Random rnd = new Random();

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

        // step 2
        btnClick = findViewById(R.id.btnClickMe);
        tvMessage = findViewById(R.id.tvMessgae);
        llyBack = findViewById(R.id.llyBack);

        tvMessage.setText("Wellcome");
        // step 5
        btnClick.setOnClickListener(this);
        tvMessage.setOnClickListener(this);
        llyBack.setOnClickListener(this);
    }

    // step 4
    @Override
    public void onClick(View view) {
        // step 6
        if (view.getId() == R.id.btnClickMe) {
            // when button clicked
            setColor();
        } else if (view.getId()==R.id.tvMessgae){
            // when text view is clicked
            tvMessage.setText("");
        } else {
            // click on background
            Intent i;
             i = new Intent(this,Main2Activity.class);
             startActivity(i);
             // finish();
        }
    }

    private void setColor() {
        int red = rnd.nextInt(256);
        int green = rnd.nextInt(256);
        int blue = rnd.nextInt(256);

        int myColor = Color.rgb(red, green, blue);

        llyBack.setBackgroundColor(myColor);
        tvMessage.setText(myColor + "");
    }
}

//Main2Activity.java
--------------------
package com.example.mohamadpc.myapplication140118;

import android.os.Bundle;
import android.support.v7.app.AppCompatActivity;
import android.view.View;

public class Main2Activity extends AppCompatActivity implements View.OnClickListener {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main2);

        findViewById(R.id.btnBackToMain).setOnClickListener(this);
    }

    @Override
    public void onClick(View view) {
        switch (view.getId()) {
            case R.id.btnBackToMain:
                /* create a new activity
                Intent i;
                i = new Intent(this, MainActivity.class);
                startActivity(i);
                */

                finish();
                break;

        }
    }
}