Java Interop: Mix Kotlin into Existing Projects

Calling Java from Kotlin

Call Java directly; getters become properties: user.name calls getName(). Java lists map to Kotlin (Mutable)List.

Calling Kotlin from Java

• @JvmStatic - expose companion methods as real Java statics.
• @JvmOverloads - generate Java overloads for default args.
• @JvmField - expose a property as a public field.
• Top-level functions land in a FileNameKt class; rename with @file:JvmName.

Nullability Annotations That Matter

// Java side - helps Kotlin trust your APIs:
public @NotNull String slug(@NotNull String title)

Annotate Java boundaries and Kotlin treats them as non-null - fewer surprises, fewer !!.

Checked Exceptions and SAM

• Kotlin has no checked exceptions - Java ones become runtime callables.
• Java functional interfaces convert with lambdas: executor.submit { work() }.
• Use fun interface in Kotlin for the same SAM trick.

Migration Strategy That Works

1. New files in Kotlin, old files untouched.
2. Convert high-value POJOs to data classes first.
3. Convert utils to extension functions.
4. Convert one module per sprint with tests green throughout.

Key Points

• Interop is bidirectional and reliable.
• Annotations at boundaries buy null safety.
• Migrate module by module, never big-bang.