Data types in Kotlin or Java?

Overview

Kotlin is JetBrains' modern language for Android development (and JVM in general). Java is its predecessor. Both compile to JVM bytecode and interoperate.

Kotlin Data Types

Basic Types

kotlinCopy
// Integer types
val age: Int = 25         // 32-bit
val bigNum: Long = 9_000_000_000L  // 64-bit
val small: Short = 1000   // 16-bit
val byte: Byte = 127      // 8-bit

// Floating point
val price: Double = 9.99  // 64-bit (preferred)
val weight: Float = 70.5f // 32-bit (f suffix)

// Boolean
val isActive: Boolean = true

// Character
val letter: Char = 'A'

// String
val name: String = "Alice"
val multiLine = """
  Hello,
  World!
""".trimIndent()

Collections

kotlinCopy
// Immutable (read-only)
val fruits: List<String> = listOf("apple", "banana", "cherry")
val scores: Map<String, Int> = mapOf("Alice" to 95, "Bob" to 87)
val tags: Set<String> = setOf("kotlin", "android")

// Mutable
val mutableList: MutableList<String> = mutableListOf("a", "b")
val mutableMap: MutableMap<String, Int> = mutableMapOf("x" to 1)
val mutableSet: MutableSet<Int> = mutableSetOf(1, 2, 3)

mutableList.add("c")
mutableMap["y"] = 2

Nullable Types

kotlinCopy
var email: String? = null   // Nullable
var phone: String? = "+91-9876543210"

// Safe call
println(phone?.length)     // Prints length or null

// Elvis operator (?? equivalent)
val display = phone ?: "No phone"

// Smart cast
if (phone != null) {
  println(phone.length)    // Smart cast to String (non-null)
}

Java Data Types

javaCopy
// Primitive types (not objects -- lower case)
int age = 25;           // 32-bit integer
long bigNum = 9_000_000_000L;  // 64-bit
float price = 9.99f;    // 32-bit float
double height = 5.11;   // 64-bit float
boolean isActive = true;
char letter = 'A';
byte b = 127;
short s = 256;

// Wrapper classes (Objects -- for collections)
Integer ageObj = 25;    // Autoboxed from int
Double priceObj = 9.99;
Boolean flag = true;

// Collections (use generics -- wrapper types only)
List<Integer> numbers = new ArrayList<>();
Map<String, Integer> scores = new HashMap<>();
Set<String> tags = new HashSet<>();

Kotlin vs Java vs Dart Comparison

int

Integer

Int

int

long

Long

Long

int

float

Float

Float

double

Double

Double

double

boolean

Boolean

Boolean

bool

String

String

String

List<T>

List<T>

List<T>

Map<K,V>

Map<K,V>

Map<K,V>

T?

T?

Key Difference: Java has primitive types (

textCopy

int

,

textCopy

boolean

) AND wrapper classes (

textCopy

Integer

,

textCopy

Boolean

). Kotlin and Dart only have object types (Kotlin compiles primitives to JVM primitives automatically).