Simplest App

Introduction

• So far we have created a simple page and connected the whole xml with our java file.
• Let us make a simple two pager application, where
  • first page:
    • Will take an input name and show the input text with prefix "Hello" after user click on a button.
    • Pass the text to the next page when clicked on another button.

Complete Code

• First page

  • MainActivity.java & activity_main.xml

  • activity_main.xml

    <androidx.constraintlayout.widget.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
        xmlns:app="http://schemas.android.com/apk/res-auto"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        xmlns:tools="http://schemas.android.com/tools"
        android:padding="12dp">
    
        <TextView
            android:id="@+id/tvMessage"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            app:layout_constraintVertical_bias="0.1"
            android:gravity="center"
            android:padding="4dp"
            android:textSize="26sp"
            android:textStyle="bold"
            android:fontFamily="cursive"
            app:layout_constraintBottom_toTopOf="@+id/buttonPassData"
            app:layout_constraintEnd_toEndOf="parent"
            app:layout_constraintStart_toStartOf="parent"
            app:layout_constraintTop_toTopOf="parent"
            tools:text="This is dummy data" />
    
        <LinearLayout
            android:id="@+id/linearLayout"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:orientation="vertical"
            app:layout_constraintBottom_toTopOf="@+id/buttonPassData"
            app:layout_constraintEnd_toEndOf="parent"
            app:layout_constraintStart_toStartOf="parent"
            app:layout_constraintTop_toBottomOf="@+id/tvMessage">
    
            <com.google.android.material.textfield.TextInputLayout
                android:layout_width="match_parent"
                android:layout_height="wrap_content">
    
                <com.google.android.material.textfield.TextInputEditText
                    android:id="@+id/editTextName"
                    android:layout_width="match_parent"
                    android:layout_height="wrap_content"
                    android:hint="@string/enter_your_name" />
    
            </com.google.android.material.textfield.TextInputLayout>
    
            <android.widget.Button
                android:id="@+id/buttonAddPrefix"
                android:layout_width="match_parent"
                android:layout_height="wrap_content"
                android:layout_marginTop="12dp"
                android:background="@drawable/button_ripple"
                android:text="@string/add_prefix"
                android:textColor="@color/white"
                android:textSize="18sp" />
    
        </LinearLayout>
    
        <android.widget.Button
            android:id="@+id/buttonPassData"
            android:layout_width="0dp"
            android:layout_height="wrap_content"
            app:layout_constraintWidth_percent="0.7"
            app:layout_constraintVertical_bias="0.9"
            android:background="@drawable/button_ripple"
            android:backgroundTint="@color/dark_red"
            android:text="@string/pass_data"
            android:textColor="@color/white"
            android:textSize="18sp"
            app:layout_constraintBottom_toBottomOf="parent"
            app:layout_constraintEnd_toEndOf="parent"
            app:layout_constraintStart_toStartOf="parent"
            app:layout_constraintTop_toTopOf="parent" />
    
    </androidx.constraintlayout.widget.ConstraintLayout>
    

  • Preview:

    

  • Initial MainActivity.java

    package com.lazymind.myfirstapp;
    
    import android.os.Bundle;
    import androidx.appcompat.app.AppCompatActivity;
    
    public class MainActivity extends AppCompatActivity {
    
        @Override
        protected void onCreate(Bundle savedInstanceState) {
            super.onCreate(savedInstanceState);
            setContentView(R.layout.activity_main);
        }
    
    }
    

  • If we run our app, we will see an initial design. But what about the functionality we want?

  • Till now, we have connected the whole design. But we need to access each component individually to read text, set text etc.

  • We connect each component by using findViewById(int) which finds the corresponding object in XML using the id.

  • Complete code:

    package com.lazymind.myfirstapp;
    
    import android.content.Intent;
    import android.os.Bundle;
    import android.view.View;
    import android.widget.Button;
    import android.widget.EditText;
    import android.widget.TextView;
    
    import androidx.appcompat.app.AppCompatActivity;
    
    public class MainActivity extends AppCompatActivity {
    
        private TextView tvMessage;
        private EditText editTextName;
        private Button buttonAddPrefix;
        private Button buttonPassData;
    
        @Override
        protected void onCreate(Bundle savedInstanceState) {
            super.onCreate(savedInstanceState);
            setContentView(R.layout.activity_main);
    
            tvMessage = findViewById(R.id.tvMessage);
            editTextName = findViewById(R.id.editTextName);
            buttonAddPrefix = findViewById(R.id.buttonAddPrefix);
            buttonPassData = findViewById(R.id.buttonPassData);
    
            setClickListener();
        }
    
        private void setClickListener(){
            // shorter way
            buttonAddPrefix.setOnClickListener(new View.OnClickListener() {
                @Override
                public void onClick(View view) {
                    String text = editTextName.getText().toString();
                    tvMessage.setText( "Hello "+text );
                }
            });
    
            // Actually what happens internally
            ClickHolder clickHolder = new ClickHolder();
            buttonPassData.setOnClickListener(clickHolder);
        }
    
        class ClickHolder implements View.OnClickListener{
    
            @Override
            public void onClick(View view) {
                String text = tvMessage.getText().toString();
                // Code for passing the data
                Intent intent = new Intent(MainActivity.this, SecondActivity.class);
                intent.putExtra("data",text);
                startActivity(intent);
            }
        }
    }
    

• Create second activity

  

• Second page
  • activity_second.xml

    <?xml version="1.0" encoding="utf-8"?>
    <androidx.constraintlayout.widget.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:id="@+id/main"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        tools:context=".SecondActivity">
    
        <TextView
            android:id="@+id/tvMessage"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:fontFamily="serif"
            android:gravity="center"
            android:padding="4dp"
            android:textSize="26sp"
            android:textStyle="bold"
            app:layout_constraintBottom_toBottomOf="parent"
            app:layout_constraintEnd_toEndOf="parent"
            app:layout_constraintStart_toStartOf="parent"
            app:layout_constraintTop_toTopOf="parent"
            tools:text="This is dummy data" />
    
    </androidx.constraintlayout.widget.ConstraintLayout>
    

  • SecondActivity.java

    package com.lazymind.myfirstapp;
    
    import android.os.Bundle;
    import android.widget.TextView;
    import androidx.appcompat.app.AppCompatActivity;
    
    public class SecondActivity extends AppCompatActivity {
    
        private TextView tvMessage;
    
        @Override
        protected void onCreate(Bundle savedInstanceState) {
            super.onCreate(savedInstanceState);
            setContentView(R.layout.activity_second);
    
            tvMessage = findViewById(R.id.tvMessage);
            // Receiving the data. Remember "data" was the key while passing
            String data = getIntent().getStringExtra("data");
            tvMessage.setText(data);
        }
    }
    

Now

• AndroidManifest.xml contains two activity. Second one is added automatically bt ide.
• Run the app.