AndroidSketchpad

Welcome to kotlin Series

In this post, we are going to discuss a very important topic of Kotlin. The topic is kotlin null safety.
I think you already heard this keyword, null safety, Kotlin is a null safety language. right?
what is the null safety? In kotlin by default, you can not assign a value of a variable as null. All the variable must have a value.

but if you want an assign a variable as null, you have to use?
code-

val nullValue:String? = null

if you want to create a variable but want to assign a value later then use

var name:String?
name = "shudipto"

now print the value

println("Name: $name")

output:

Name: shudipto

now assign this value to null

name = null

print this new value

println(name)

Output:

null

Note: if you want to set this variable must have value then use below code-

print(name!!)

now we create a function that returns null

//null return function
fun returnNull():String? {
    return null
}

print this value:

 println("Function: ${returnNull()}")

Output:

 println("Function: ${returnNull()}")

now we use the force operator to assign the return value of this methods

val returnNull1:String ?= returnNull()

now if we want to learn the length of this variable we can use length method

//val returnNull2 = returnNull1.length

but this line shows an error because the value of the variable is null. This is the advantage of kotlin. If you are in java the here this line program will be crashed. but in kotlin, we can do by this way without any crash

val returnNull2 = returnNull1?.length

Now, this program will not crash. but you can do another thing that is if the value is null then you can save a string or default value I am using a string

val returnNull2 = returnNull1?.length ?: "value is null"

print the value

println("ReturnNull2: $returnNull2")

output:

ReturnNull2: value is null

see the variable value is null so the default value is assigned. you can also use this method for the specific data type variable:

val returnNull4:String = returnNull() ?: "No name"

print the value:

println(returnNull4)

Output:

No name

That's it. Hope you understand the concept. This is very important in Android App development. It prevent some unexpected crash.

Thank you very much.
Happy coding

welcome to kotlin series.

Kotlin Series

In this series, we are basically going through some basic and advanced tutorial about kotlin language.
we try to serve you best practices on those tutorials.

Here is the collection of all post

1. String -  kotlin string tutorial
2. Array and loop - array and loop tutorial
3. File reader and write - kotlin IO tutorial. 
4. ArrayList - kotlin ArrayList
5. How to control loop in kotlin - kotlin loop control
6. Collection - kotlin collection such as hash map, array, list
7.Null Safety - kotlin null safety

Series tutorial:
Kotlin OOP series:

1. OOP Basic
2. Constructor
3. Inheritance
4. Method Overriding

Advance:
1.Bookstore a simple program with function and input-output from the console with conditions. 

Welcome to kotlin Series

In the tutorial, we are going to discuss Kotlin collection.
we touch on below topics
1. Hashmap
2. Array
3. List
       3.a. mutable list
       3.b. immutable list
4. Set
       4.a. mutable set
       4.b. immutable set

Let's start-

Hashmap: In kotlin, we can use java hashmap easily. But kotlin has an own hashmap. First, we go through Java hashmap create an instance of Hashmap class and parameter will be string and string after input some data and print data to check.
see the code-

val hasMap = HashMap<String,String>()

hasMap.put("s","Shudipto")
hasMap["d"] = "trafder"
hasMap["f"] = "Himu"
hasMap["g"] = "Rupa"

now print a value

println(hasMap["s"])

Output:

Shudipto

I am using a for loop to print all the value that stored in hashmap with their key. code-

for ((v,k) in hasMap.entries){
        println("$v -> $k")
    }

Output:

s -> Shudipto
d -> trafder
f -> Himu
g -> Rupa

that's it in this way we use java hashmap. But kotlin has already a default hashmap. now use kotlin hashmap.
See the code

val hasmap = hashMapOf(1 to "shudipto")
hasmap.put(2,"trafder")
hasmap.put(3,"himu")

print a value

println(hasmap[1])

Output:

shudipto

use a for loop to see all the value with the key that stored in the hashmap.

//print all value with index
for ((position,value) in hasmap.entries){
    println("Index:$position value:$value")
}

Output:

//print all value with index
for ((position,value) in hasmap.entries){
    println("Index:$position value:$value")
}

That's all on hashmap. next topics Array

Array: kotlin has a special array class. let's see the example. But before jump into code, you can check our two tutorial about ArrayList and Array and loop. This will help you to understand the array concept easily.
see the code-

println("\nKotlin Array")
val array = arrayOf("Shudipto",1,2,3.5,"Trafder")
array[4] = "himu"

now print all the value with for loop-

for ((p,v) in array.withIndex()){
    println("Index:$p value:$v")
}

Output:

Index:0 value:Shudipto
Index:1 value:1
Index:2 value:2
Index:3 value:3.5
Index:4 value:himu

That's it on Array. Next topics is  list

List:  In kotlin, there are two types of list available. Mutable and immutable list.
see the code-
mutable list-

println("\n\nMutable list of array")
val list = mutableListOf(1,2,34,5,6,7,8,"Shudipto")

list[0] = "trafder"

immutable list-

val list2 = listOf(1,2,34,5,6,7,8,"Shudipto")

Mutable vs immutable list: you can not update immutable list after assign.

now print all the value by using for loop
mutable list-

for ((p,v) in list.withIndex()){
    println("Index:$p value:$v")
}

Output:

Mutable list of array
Index:0 value:trafder
Index:1 value:2
Index:2 value:34
Index:3 value:5
Index:4 value:6
Index:5 value:7
Index:6 value:8
Index:7 value:Shudipto

Immutable List

println("\n\nimmutable list of array")
for ((p,v) in list2.withIndex()){
    println("Index:$p value:$v")
}

Output:

immutable list of array
Index:0 value:1
Index:1 value:2
Index:2 value:34
Index:3 value:5
Index:4 value:6
Index:5 value:7
Index:6 value:8
Index:7 value:Shudipto

That's it on the list. Nest on set.

Set: As list, kotlin has also two types of set. one is mutable and second is immutable.
so see the code
mutable set-

val mSet = mutableSetOf(1,2,4,5,6,6,8,1)
mSet.add(5)

immutable set

val set = setOf(15,2,3,10,5,6,15,10)

mutable vs immutable set: you can update mutable set anytime. But immutable set can not update after assign.
print all the value
mutable set-

for ((p,v) in mSet.withIndex()) println("Index:$p value:$v")

output:

Index:0 value:1
Index:1 value:2
Index:2 value:4
Index:3 value:5
Index:4 value:6
Index:5 value:8

Immutable set-

for ((p,v) in set.withIndex()) println("Index:$p value:$v")

Output:

Index:0 value:15
Index:1 value:2
Index:2 value:3
Index:3 value:10
Index:4 value:5
Index:5 value:6

That's it on set.

oh! we finish all the topics. Before finish this post I will give you a gift topics.
List vs set:
you probably see that list and set is working basically same process. List and Set both save a collection of data. They also have the mutable and immutable option. you can save any time of data type and you can also mix data type. but the question is, what is the difference between list and set?
the answer is, the set is not stored duplicate value but list save. If you are working with duplicate value then you must use list. or, If you are want to ignore duplicate value then use set.

Thank you for reading.
Hope you enjoy this tutorial.
Happy coding.

Welcome to kotlin Series

In this small post, we are going to discuss how to control loop in Kotlin.
Basically, we use 2 keywords for the control loop.
1. continue
2. break
if we want to skip one process, then we use to continue the keyword

for (a in 1..5){
        if (a==4){
            continue
        }
        println("A: $a")
    }

Output:

A: 1
A: 2
A: 3
A: 5

or if we stop the loop then we use break keyword

for (a in 1..5){
    if (a==4){
        break
     }
    println("A: $a")
}

Output:

A: 1
A: 2
A: 3

But in the nested loop if we want to stop the second loop after a selected value.
But in this case, if we use break keyword in the second loop-

for (a in 1..5){
        println("A: $a")

        for (b in 1..5){
            if (a == 4){
                break
            }
            println("B: $b")
        }
    }

and output:

A: 1
B: 1
B: 2
B: 3
B: 4
B: 5
A: 2
B: 1
B: 2
B: 3
B: 4
B: 5
A: 3
B: 1
B: 2
B: 3
B: 4
B: 5
A: 4
A: 5
B: 1
B: 2
B: 3
B: 4
B: 5

so we see loop is not stopped. It only stops inner loop after reaching the value of a is 4. no value of this loop is not printed and it repeated every time with increasing value with a
But our target on the basis of above value, we want to stop whole loop not only second loop after the value of a is 4.
For this case, we use loop keyword.
See the code-

loop@ for (a in 1..5){
        
        println("value a:$a")

        for (b in 1..4){
            if (a == 4){
                break@loop
            }

            println(" b:$b")
        }
    }

    println("Loop done")

Output:

value a:1
 b:1
 b:2
 b:3
 b:4
value a:2
 b:1
 b:2
 b:3
 b:4
value a:3
 b:1
 b:2
 b:3
 b:4
value a:4
Loop done

Oh! now our loop is stopped when the value of a is 4
sometimes we need to control the inner loop with the main loop condition. So loop keyword is saved a lot of time, we don't need to implement the complex logic.

Thank you for reading this post.
Happy coding

Welcome to kotlin Series

In this post, we are going to learn about ArrayList. In android the use of ArrayList is huge.
So let's move on today task list.
1. we create an ArrayList
2. insert some value
3. update some value
4. search in ArrayList
5. delete some value

Let's start:
Note: we discuss array and loop in this post Kotlin: Array and Loop. you can take a look at this post of array section it will help you to understand today's topics easily.
Create an instance of ArrayList class and make this variable as immutable by using val

val list = ArrayList<String>()

Now insert some value by calling add method

list.add("Shudipto")
list.add("Trafder")
list.add("Is")
list.add("a")
list.add("good")
list.add("student")

Now print the first element of this array list

println("First Name:${list[0]}")

Result:

First Name:Shudipto

Now print all the element by using a for loop. see the code-

println("All element")
println("\nOnly value")
for (a in list){
     println("value:$a")
}

Output:

All element

Only value
value:Shudipto
value:Trafder
value:Is
value:a
value:good
value:student

Now print all the element with the index number. see the code-

println("\nIndex with value")
for ((position,a) in list.withIndex()){
    println("Index:$position value:$a")
}

Output:

Index with value
Index:0 value:Shudipto
Index:1 value:Trafder
Index:2 value:Is
Index:3 value:a
Index:4 value:good
Index:5 value:student

Now update an element-
we update the value of index number 5.
The new value will be developer.

println("\n\nUpdate an element")
list[5] = "developer"

Now print all the value with index

println("\nIndex with value")
for ((position,a) in list.withIndex()){
     println("Index:$position value:$a")
}

Output:

Index with value
Index:0 value:Shudipto
Index:1 value:Trafder
Index:2 value:Is
Index:3 value:a
Index:4 value:good
Index:5 value:developer

Search in ArrayList: we make a search in our ArrayList.
If we found the data and we print name found and if not we print name not found.
see the code below

println("\n\nSearch in array list")
if (list.contains("Shudipto")){
    println("Name found")
} else println("Not found")

Output:

Search in array list
Name found

Now delete some data: we can remove by two ways.
1. by value
2. by index number
I am showing you those two ways by the value, we remove "a" by index number, we remove index number 5
see the code below

println("Delete some data")
//with value
list.remove("a")
//with position
list.removeAt(4)

Note: in code, we use index number 4 because we just remove one element so the last index number is now 4.
Now print all the value with index number

println("\nIndex with value")
for ((position,a) in list.withIndex()){
     println("Index:$position value:$a")
}

Output:

Index with value
Index:0 value:Shudipto
Index:1 value:Trafder
Index:2 value:Is
Index:3 value:good

Today's full code:

fun main(args: Array<String>) {

    val list = ArrayList<String>()

    list.add("Shudipto")
    list.add("Trafder")
    list.add("Is")
    list.add("a")
    list.add("good")
    list.add("student")

    println("First Name:${list[0]}") //list.get(0)

    println("All element")
    println("\nOnly value")
    for (a in list){
        println("value:$a")
    }

    println("\nIndex with value")
    for ((position,a) in list.withIndex()){
        println("Index:$position value:$a")
    }

    println("\n\nUpdate an element")
    //list.set(5,"developer")
    list[5] = "developer"

    println("All element")
    println("\nOnly value")
    for (a in list){
        println("value:$a")
    }

    println("\nIndex with value")
    for ((position,a) in list.withIndex()){
        println("Index:$position value:$a")
    }

    println("\n\nSearch in array list")
    if (list.contains("Shudipto")){
        println("Name found")
    } else println("Not found")

    println("Delete some data")
    //with value
    list.remove("a")
    //with position
    list.removeAt(4)

    println("\nIndex with value")
    for ((position,a) in list.withIndex()){
        println("Index:$position value:$a")
    }
}

Full output:

First Name:Shudipto
All element

Only value
value:Shudipto
value:Trafder
value:Is
value:a
value:good
value:student

Index with value
Index:0 value:Shudipto
Index:1 value:Trafder
Index:2 value:Is
Index:3 value:a
Index:4 value:good
Index:5 value:student


Update an element
All element

Only value
value:Shudipto
value:Trafder
value:Is
value:a
value:good
value:developer

Index with value
Index:0 value:Shudipto
Index:1 value:Trafder
Index:2 value:Is
Index:3 value:a
Index:4 value:good
Index:5 value:developer


Search in array list
Name found
Delete some data

Index with value
Index:0 value:Shudipto
Index:1 value:Trafder
Index:2 value:Is
Index:3 value:good

Process finished with exit code 0

Thank you for reading this post.
Happy coding

Welcome to kotlin Series



In this post, we are going to learn about kotlin IO (input or output)

we basically learn about file reader and writer.
In this post, we are going to write a program that takes text from the console and save it on computer hard disk as a text file.
Let start-

First, we are going to create a method for saving file, method name writeToFile
Follow those step:
1. take a String parameter
 2. Create an instance of FileWriter
3. in try-catch block write this string by using write method.
See the code-

fun writeToFile(str: String) {

    val writer = FileWriter("text.txt", true)

    try {
        writer.write(str+"\n")

    } catch (ex: Exception) {
        println(ex.message)

    } finally {
        writer.close()
    }
}

Now time to create the Main method-
in the main method-
1. first we show a message to user to write some text
2. take those text and save in on a variable
3. after the call the writeToFile method and parameter is this variable.
see the code-

fun main(args: Array<String>) {
    println("Write a message")
    val str: String = readLine().toString()

    writeToFile(str)

}

Now time to read this text file and print the text in the console-
Follow those step-
1.create a method name readFile()
2. in try-catch block create an instance of FileReader class
3. print text to the console by using readText() method
see the code-

fun readFile(){

    try {
        val read = FileReader("text.txt")

        println(read.readText())


    } catch (e:Exception){
        println(e.message)
    }

}

In the main method, call this method-

fun main(args: Array<String>) {
    println("Write a message")
    val str: String = readLine().toString()

    writeToFile(str)

    readFile()

}

Now run this program-
after running the programming this will show you like below picture
input some text I input: Hello I am shudipto trafder


Note: Green colored text is your input text and white color text is output text( that read from the file)
this is the text file that saved


Thanks for reading this post.
Happy coding.

Welcome to this post. In this post, we are going to discuss how to share code on Git from Android Studio.

(I don't want to describe what is GitHub or why you need to use git. I just want to show you the process only)
Let's jump in.

Before share, you need to create a GitHub account click here for GitHub account.
After creating a GitHub account, you can now share your code into GitHub.

Open android studio. Now open an exciting project that you want to share.
Click on VCS on the main toolbar.
then import into version
after Share project on GitHub
Like below picture

You will get an error. To fix this error you need to add an SSH key.

Open git-bash


To add this you need to check first if you have any key open git-bash.
Now enter the command

ls -al ~/.ssh

if the key already exists you can jump on the copy to clip bord option.
If the key does not exist then you need to create a new key.
Follow this command-

ssh-keygen -t rsa -b 4096 -C "your@email.com"

in your@email.com you need to write your email that you used in GitHub account.

it will show-

Generating public/private rsa key pair.

When you're prompted to "Enter a file in which to save the key," press Enter if you want to save those file in the default location.

Enter a file in which to save the key (/c/Users/you/.ssh/id_rsa):[Press enter]

now enter your password.
Note for the beginner: If you enter your password it will not showing anything (even no start).

Enter passphrase (empty for no passphrase): [Type a passphrase]
Enter same passphrase again: [Type passphrase again]

your password must be same. your SSH key is created.
Now you need to add your SSH key to the ssh-agent before adding, you need to ensure the ssh-agent is running: to run ssh-agent-

eval $(ssh-agent -s)

Now add your ssh private key to ssh-agent

ssh-add ~/.ssh/id_rsa

Now run this command this command will copy the key into you clip bord.

clip < ~/.ssh/id_rsa.pub

show the picture

Time to add this key into you Github account.
Login your GitHub and go to Settings.
Select SSH and GPG key Now click New SSH key.



write the title as you want and the key will be your copied key.
press ctrl+c button to paste the copied key.
and save this by pressing add SSH key.

That's it. It will be working fine

Now click on
VCS on the main toolbar.
then import into version
after Share project on GitHub

it will show like below picture


if you are not logged in then it will prompt to login. Log in through your Github user name, and pass.

After login, it will show like this-