How can you integrate Kotlin code with Java code?

Seamless integration of Kotlin with Java thanks to JVM compatibility:

1. Using Kotlin from Java:

   • Kotlin classes and functions are accessible in Java.
   • Kotlin getter and setter properties are automatically generated and accessible from Java.
   • Kotlin top-level functions compile into static methods of a class named after the file + "Kt" (by default).
   • @JvmStatic annotation for static methods in companion object.
   • @JvmOverloads to generate overloaded methods for functions with default parameters.

   // Java code calling Kotlin
   SomeKotlinClass kotlinObject = new SomeKotlinClass("hello");
   System.out.println(kotlinObject.getProperty()); // Access property via getter
   
   KotlinFileKt.someTopLevelFunction(); // Call top-level function
   

2. Using Java from Kotlin:

   • Java classes and interfaces are directly accessible.
   • Kotlin lambdas can be used instead of SAM conversions in Java 8+.
   • Nullability of Java types is handled as "platform types" in Kotlin, requiring explicit null checks.

   // Kotlin code calling Java
   import java.util.ArrayList
   
   fun useJavaClass() {
       val javaList = ArrayList<String>() // Using Java class
       javaList.add("item")
   
       val javaString: String? = javaMethodReturningNullable() // Platform type
       if (javaString != null) {
           println(javaString.length) // Null check required
       }
   }
   

3. Interaction within a project:

   • Kotlin and Java files can coexist in the same project and call each other.
   • IDEs (e.g., IntelliJ IDEA) provide convenient navigation and refactoring between languages.

4. Build tools:

   • Gradle and Maven support mixed Kotlin/Java projects.

5. Joint usage:

   • You can start with Kotlin, gradually rewriting parts of Java code.
   • You can start with a small amount of Kotlin code in an existing Java project.