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User Input and Output

Ayesha
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User Input and Output

Video Explanation​

Output: Printing to Console​

println() — Print with Newline​

println("Hello, World!")
println(42)
println(3.14)
println(true)

print() — Print without Newline​

print("Hello, ")
print("World!")
// Output: Hello, World!

System.out.printf() — Formatted Output​

System.out.printf("Name: %s, Age: %d, Score: %.2f%n", "Alice", 25, 98.5)
// Output: Name: Alice, Age: 25, Score: 98.50

String Templates​

val name = "Bob"
val score = 95
println("Student: $name scored $score marks")
println("Next year score: ${score + 5}")

Input: Reading from Console​

readLine() — Read a Line of Text​

fun main() {
    print("Enter your name: ")
    val name = readLine()
    println("Hello, $name!")
}

Note: readLine() returns String? (nullable String).

Reading and Converting Input​

fun main() {
    print("Enter your age: ")
    val age = readLine()?.toInt() ?: 0
    println("You are $age years old")
}

Reading Different Data Types​

Integer Input​

fun main() {
    print("Enter a number: ")
    val num = readLine()!!.toInt()
    println("Square: ${num * num}")
}

Double Input​

fun main() {
    print("Enter price: ")
    val price = readLine()!!.toDouble()
    println("With tax: ${price * 1.18}")
}

Boolean Input​

fun main() {
    print("Are you a student? (true/false): ")
    val isStudent = readLine()!!.toBoolean()
    println("Student status: $isStudent")
}

Safe Input Handling​

Always handle potential errors when reading user input:

fun main() {
    print("Enter a number: ")
    val input = readLine()
    val num = input?.toIntOrNull()

    if (num != null) {
        println("Double: ${num * 2}")
    } else {
        println("Invalid number entered!")
    }
}

Multiple Inputs​

fun main() {
    print("Enter first number: ")
    val a = readLine()!!.toInt()

    print("Enter second number: ")
    val b = readLine()!!.toInt()

    println("Sum: ${a + b}")
    println("Difference: ${a - b}")
    println("Product: ${a * b}")
}

Formatted Output with trimIndent​

fun main() {
    val name = "Alice"
    val age = 22
    val gpa = 3.85

    val report = """
        ====== Student Report ======
        Name : $name
        Age  : $age
        GPA  : ${"%.2f".format(gpa)}
        ============================
    """.trimIndent()

    println(report)
}

Output:

====== Student Report ======
Name : Alice
Age  : 22
GPA  : 3.85
============================

Using Scanner (Java Interop)​

import java.util.Scanner

fun main() {
    val scanner = Scanner(System.`in`)

    print("Enter your name: ")
    val name = scanner.nextLine()

    print("Enter your age: ")
    val age = scanner.nextInt()

    println("Hello $name, you are $age years old!")
    scanner.close()
}

Common Conversion Functions​

FunctionConverts ToSafe Version
toInt()InttoIntOrNull()
toDouble()DoubletoDoubleOrNull()
toLong()LongtoLongOrNull()
toFloat()FloattoFloatOrNull()
toBoolean()BooleanN/A

Summary​

  • Use println() for output with newline, print() without.
  • Use readLine() to read console input — it returns String?.
  • Convert input using .toInt(), .toDouble(), etc.
  • Use safe variants like .toIntOrNull() to avoid crashes.
  • String templates with $ and ${} make output formatting easy.
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