Skip to main content

Calling Java from Kotlin

Platform types​

Java without nullability annotations may return a platform type, shown conceptually as String!. Kotlin lets you treat it as nullable or non-null, so responsibility moves to you.

public String findName() { return null; }
val unsafe: String = javaApi.findName()  // may throw at runtime
val safe: String? = javaApi.findName()

Add recognized Java nullability annotations and keep platform types from spreading beyond the interop boundary.

Java getters become properties​

user.getName()/user.setName(value) may be used as user.name when they follow JavaBean conventions.

SAM conversion​

For a Java interface with one abstract method:

interface OnCompleteListener { void onComplete(boolean success); }

Kotlin can pass a lambda:

api.setListener { success -> println(success) }

Checked exceptions​

Kotlin does not force callers to catch Java checked exceptions. They still occur at runtime.

val text = try {
    javaReader.read()
} catch (error: IOException) {
    "Fallback"
}

✕ Trust an unannotated Java return as non-null

val length = legacyApi.name.length

✓ Normalize at the boundary

val name = legacyApi.name?.takeIf { it.isNotBlank() }
    ?: return InvalidLegacyUser
Interview-ready answer

Java APIs without nullability annotations create platform types. Kotlin relaxes compile-time checks for them, so validate immediately and convert to explicit nullable/non-null Kotlin types. Java SAM interfaces accept lambdas, and checked exceptions remain runtime concerns even though Kotlin does not enforce catches.