Example ViewGroup and View

actvity_main.xml

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout 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"
    android:orientation="vertical"
    android:padding="16dp"
    tools:context=".MainActivity">

    <TextView
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:id="@+id/title"
        android:gravity="center_horizontal"
        android:hint="*****"
        android:textSize="24sp"
        />

    <EditText
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_marginTop="10dp"
        android:hint="Enter Text Here"
        android:id="@+id/input"/>



    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_marginTop="10dp"
        android:orientation="horizontal">

        <Button
            android:layout_width="0dp"
            android:layout_height="wrap_content"
            android:layout_weight="1"
            android:text="clear"
            android:layout_marginRight="5dp"
            android:id="@+id/clear"/>

        <Button
            android:layout_width="0dp"
            android:layout_height="wrap_content"
            android:layout_weight="1"
            android:layout_marginLeft="5dp"
            android:text="Add"
            android:onClick="addbutton"
            android:id="@+id/add"/>

    </LinearLayout>


</LinearLayout>

MainActivity

package com.example.arslan.bs_app;

import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.TextView;
import android.widget.Toast;

public class MainActivity extends AppCompatActivity {

    // views
    private TextView titleTextView;
    private Button addButton ,clearButton;
    private EditText inputEditText;

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

        titleTextView= findViewById(R.id.title);
        addButton= findViewById(R.id.add);
        clearButton= findViewById(R.id.clear);
        inputEditText =findViewById(R.id.input);

        addButton.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {

                if(inputEditText.getText().toString().trim().length() == 0){
                    Toast.makeText(MainActivity.this , "Enter Text" , Toast.LENGTH_SHORT).show();
                    return;
                }


                if(inputEditText != null){
                    if(titleTextView !=null){
                        titleTextView.setText(inputEditText.getText().toString());
                    }
                }

            }
        });

        clearButton.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {

                if(inputEditText.getText().toString().trim().length() == 0){

                    if(titleTextView.getText().toString().length() != 0){
                        titleTextView.setText("");
                    }

                    Toast.makeText(MainActivity.this , "No Text To Clear" , Toast.LENGTH_SHORT).show();
                    return;
                }

                if(titleTextView !=null){
                    titleTextView.setText("");
                }

                if(inputEditText != null){
                    inputEditText.setText("");
                }
            }
        });

    }//end of onCreate

    public void addbutton(View view){

    }



}