Android Code Execution Step by Step Explanation with simple Hello World Example

Hay friend lets dig step by step Android Code execution with simple  Hello World App code:



Before going to dig a code  first lets understand some UI parts in android as bellow:

User Interface (UI) Group:

UI group is used to specify how to arrange the UI components.

Following are the UI groups used in android:

1)      Linear Layout :-Component arranged in  One after another manner.

2)      Table Layout :-Components arranged in row and columns manner.

3)      Frame Layout

4)      Relative Layout- By using this layout it is possible to move,resize and place UI components wherever on the screen.

5)      Grid Layout

UI Components:

1)      Text View

2)      Edit Text

3)      Button [Custom, Image, Toggle, Radio, Switch, Checkbox etc.]

4)      Image View

5)      Autocomplete Text View

6)      Spinner

7)      List View

8)      Gallery

9)      Grid View

10)   Web view

11)   Fragment

12)   View Switcher or View Flipper.

At this point of time just remember these things detailed explanation about all UI groups and Components we will discuss in upcoming posts....

So lets begin with our Hello World code Execution.......

Fig 1: Create new Project give name "Hello_World"

Fig 2: This will be your Project Structure.

Steps to create an Activity file:

 (**These steps are applied to any new activity we create)

         1)    Create class with sub-type of ”android.app.Activity”    

         2)    Same like main method in c/c++ /Java  in android activity OnCreate()  will invoke first so                    provide the implementation for OnCreate( )

         3)    Design the UI in XML, by using the following method we can set XML file context (UI) to the          java file as shown bellow....

                                                     “SetContentView (R.layout.xml-file)”

Example of: “Hello World” app:

 res->layout->activity_main.xml

         <?xml version="1.0" encoding="utf-8"?>  '

           Here you can see UTF-8 as default it is nothing but a Unicode format use in it

        

           In Android Studio default you will get as Relative Layout change it to Linear Layout,  Here xmlns means "XML namespace "   specifies package

where as namespace is nothing but the collection of Classes ,Interfaces,and collections etc.

  

                < LinearLayout xmlns: android=http://schemas.android.com/apk/res/android
                           android:layout_width="match_parent"

                           android:layout_height="match_parent"

                          android:orientation="vertical"

                                                                 >

             <TextView
                     android:layout_width="wrap_content"
                    android:layout_height="wrap_content"
                   android:text="Hello World!" />

           </LinearLayout>

  Here we have specified the attributes of  layout i.e. width and height we using and here "Parent" is the screen in which its going to be displayed.

And we can have orientation of components we are placing in Layout as Horizontal or Vertical as required

 

   

Main_activity.java file

package com.example.ygutte.Hello_World;



import android.app.Activity;

import android.support.v7.app.AppCompatActivity;

import android.os.Bundle; 



public class MainActivity extends Activity {

//must extends the Activity class


    @Override

    protected void onCreate (Bundle savedInstanceState) {

//Bundle is a class used to get state of an activity

        super.onCreate (savedInstanceState);

        SetContentView (R.layout.activity_main);   

   // to maintain the status of activity of supper class e.g. Function Overloading

/* Here we set the layout of screen as we designed in above xml file so that we use

                                  SetContentView () –converts xml to java

                                                                    -As abstract between xml and java file */

    }

}

Hope you understood the code now you can start yours...... 

Happy Coding...........