How is null value compatibility ensured between Java and Kotlin?

Null value compatibility between Java and Kotlin is ensured by Kotlin's type system, which explicitly distinguishes between nullable and non-nullable types.

• In Kotlin, types are non-nullable by default, for example, String cannot accept null.
• For variables that can be null, a nullable type with ? is used, for example, String?.

When interacting with Java code, Kotlin uses annotations (such as @Nullable, @NotNull) to determine nullability. If annotations are absent, Kotlin considers the types platform types, and the developer must handle potential nulls manually.

Example:

// Java method
public String getName() { return null; }

// Kotlin
val name: String? = javaObject.name // nullable, since Java can return null

// To avoid errors, Kotlin requires null checks before use
if (name != null) {
    println(name.length)
}

Thus, Kotlin helps safely work with nulls from Java, reducing the risk of NullPointerException.