What is Activity in Android?

 What is the activity?

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It is a predefined class of Java that is used to provide a code layer and design layer to create an android application screen.

The activity uses a .java file to manage code and a .xml file to manage design.

The activity has two different classes

1) Activity  --->  It is the parent class that is used to provide functionality for the current version only.

2) AppCompatActivity ---> It is a child class of activity that is used to provide functionality for the current version and the previous version of the android device.

class A extends Activity

{

 

}

class B extends AppCompatActivity

{

}

here class A and B both are activity class

What is onCreate() in Android Activity?

It is a predefined method of Activity class that will be called automatically when Activity class will be initialized. This is the initial method of activity.

What is a Bundle?

It is used to pass data from one activity to another, Bundle can contain any type of data using key=>value pair.

What is super.onCreate()?

it is used to call AppcompatActivity class (parent class) onCreate()

what is setContentView()?

It is used to call the design file (.xml) of android activity.

setContentView(R.layout.activity_main);    // R.java ---> 

Minimum Configuration for Android:-

6GB to 8GB Ram

Processor i3 or i5 

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Question Create an activity to implement an addition program of two numbers in android?

1) Open android studio 

2) File ---> New Project ---> Empty Activity

3)  go into the design file and design layout for the addition program

4) go into the java file and write code

5)  create instance of Widget elements

    EditText txtnum1,txtnum2;

    Button btn;

    TextView txtres;

6)  get data of widget element from xml file to java file.

    

 Android provide findViewById() to get the element of xml file to java code file.

 WidgetClass ref = findViewById(R.id.<idname>);

 TextView txtres = findViewById(R.id.txtres);

7)  Create event for the button:-

Event means if we perform any action on the particular widget on the android application then the event will be raised.

for example, if we click on the Button then we use OnClick event method that exists under OnClickListner anonymous class.

this class will be bind by Button instance using setOnclickLitsner.

Event use three different component

1) Listener

2) Handler

3) Performer

Complete code of design file for Addition Program?

Code for Java File:-

package com.example.additionapplication;

import androidx.appcompat.app.AppCompatActivity;

import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.TextView;

public class MainActivity extends AppCompatActivity {

    EditText t1,t2;
    Button btn;
    TextView txtres;
    int a,b,c;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        t1 = findViewById(R.id.txtnum1);
        t2 = findViewById(R.id.txtnum2);
        btn = findViewById(R.id.btnadd);
        txtres = findViewById(R.id.txtres);
        btn.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                a = Integer.parseInt(t1.getText().toString());  // to get from xml file to java varibale
                b = Integer.parseInt(t2.getText().toString());
                c = a+b;
                txtres.setText("Result is "+c);

            }
        });

    }
}

Code for XML File:-

<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:background="#FFC107"
    android:orientation="vertical"
    tools:context=".MainActivity">

    <EditText
        android:layout_width="300dp"
        android:layout_height="60dp"
        android:id="@+id/txtnum1"
        android:hint="Enter first number"
        >

    </EditText>
    <EditText
        android:layout_width="300dp"
        android:layout_height="60dp"
        android:id="@+id/txtnum2"
        android:hint="Enter second number"
        >

    </EditText>

    <Button
        android:layout_width="300dp"
        android:layout_height="35dp"
        android:id="@+id/btnadd"
        android:text="Submit"
        >

    </Button>


    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:id="@+id/txtres">

    </TextView>
</LinearLayout>