Kotlin - Booleans


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Many times we come across a situation where we need to take decision in Yes or No, or may be we can say True or False. To handle such situation Kotlin has a Boolean data type, which can take the values either true or false.

Kotlin also has a nullable counterpart Boolean? that can have the null value.

Create Boolean Variables

A boolean variable can be created using Boolean keyword and this variable can only take the values either true or false:

Example

fun main(args: Array<String>) {
   val isSummer: Boolean = true
   val isCold: Boolean = false
  
   println(isSummer)
   println(isCold)
   
}

When you run the above Kotlin program, it will generate the following output:

true
false

In fact, we can create Kotlin boolean variables without using Boolean keyword and Kotlin will understand the variable type based on the assigned values either true or false

Kotlin Boolean Operators

Kotlin provides following built-in operators for boolean variables. These operators are also called Logical Operators:

Operator Name Description Example
&& Logical and Returns true if both operands are true x && y
|| Logical or Returns true if either of the operands is true x || y
! Logical not Reverse the result, returns false if the operand is true !x

Example

Following example shows different calculations using Kotlin Logical Operators:

fun main(args: Array<String>) {
   var x: Boolean = true
   var y:Boolean = false

   println("x && y = " +  (x && y))
   println("x || y = " +  (x || y))
   println("!y = " +  (!y))
}

When you run the above Kotlin program, it will generate the following output:

x && y = false
x || y = true
!y = true

Kotlin Boolean Expression

A Boolean expression returns either true or false value and majorly used in checking the condition with if...else expressions. A boolean expression makes use of relational operators, for example >, <, >= etc.

fun main(args: Array<String>) {
   val x: Int = 40
   val y: Int = 20

   println("x > y = " +  (x > y))
   println("x < y = " +  (x < y))
   println("x >= y = " +  (x >= y))
   println("x <= y = " +  (x <= y))
   println("x == y = " +  (x == y))
   println("x != y = " +  (x != y))
}

When you run the above Kotlin program, it will generate the following output:

x > y = true
x < y = false
x >= y = true
x <= y = false
x == y = false
x != y = true

Kotlin and()and or() Functions

Kotlin provides and() and or() functions to perform logical AND and logical OR operations between two boolean operands.

These functions are different from && and || operators because these functions do not perform short-circuit evaluation but they always evaluate both the operands.

fun main(args: Array<String>) {
   val x: Boolean = true
   val y: Boolean = false
   val z: Boolean = true

   println("x.and(y) = " +  x.and(y))
   println("x.or(y) = " +  x.or(y))
   println("x.and(z) = " +  x.and(z))
}

When you run the above Kotlin program, it will generate the following output:

x.and(y) = false
x.or(y) = true
x.and(z) = true

Kotlin also provides not() and xor() functions to perform Logical NOT and XOR operations respectively.

Boolean to String

You can use toString() function to convert a Boolean object into its equivalent string representation.

You will need this conversion when assigning a true or false value in a String variable.

fun main(args: Array<String>) {
   val x: Boolean = true
   var z: String
   
   z = x.toString()

   println("x.toString() = " +  x.toString())
   println("z = " +  z)
}

When you run the above Kotlin program, it will generate the following output:

x.toString() = true
z = true

Quiz Time (Interview & Exams Preparation)

Answer : A

Explanation

Kotlin Boolean variable can have only two values either true or false

Q 2 - What will be the output of the following program:

fun main(args: Array<String>) {
   val x: Boolean = true
   var y: String
   
   y = x
}

A - This will compile successfully without error and warning

B - This will raise just a warning

C - Compilation will stop with error

D - None of the above

Answer : C

Explanation

Compilation will stop with type mismatch error because we are trying to store a boolean value into a String variable. We should use toString() to convert Boolean value into string value before we assign it into string variable.

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