Java Interop

import java.util.ArrayList

fun main() {
    val javaList = ArrayList<String>()
    javaList.add("Kotlin")
    javaList.add("Java")
    println(javaList)
}

fun getJavaStyleValue(): String? {
    // Simulating a Java method with unknown nullability
    val value: String? = if (System.currentTimeMillis() % 2 == 0L) "value" else null
    return value
}

fun main() {
    val result = getJavaStyleValue()
    println("Result: ${result ?: "was null"}")
}

import java.util.Calendar

fun main() {
    val calendar = Calendar.getInstance()
    val year = calendar.get(Calendar.YEAR)
    println("Current year (via Java API): $year")
}

fun greet(name: String): String = "Hello, $name!"

fun main() {
    // Java code would call this as UtilsKt.greet("Kotlin")
    println(greet("Kotlin"))
}

1. Forgetting that a Java method returning a plain (non-annotated) reference type appears to Kotlin as a 'platform type', which skips Kotlin's null-safety checks until you add your own.
2. Assuming every Java naming convention carries over unchanged; Kotlin has some special call syntax for Java getters/setters (property syntax) that Java methods don't have natively.
3. Not realizing a Kotlin top-level function compiles to a static method on an automatically generated class (like FileNameKt) when called from Java.

• Kotlin code can call Java classes and methods directly, and Java code can call Kotlin classes and functions directly, since both compile to JVM bytecode.
• A Java method's return type with no nullability annotation becomes a 'platform type' in Kotlin, which bypasses compile-time null checks.
• Kotlin can access Java getter/setter pairs (getX()/setX()) using property-style syntax (obj.x).
• Kotlin top-level functions become static methods on a generated class named after the file when called from Java.