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You have a .kt file — Kotlin source code. Google made Kotlin the preferred language for Android development in 2019, and it has been steadily replacing Java in Android codebases ever since. Kotlin runs on the JVM, interoperates seamlessly with Java, and fixes most of Java's verbosity and null-safety problems.
JetBrains created Kotlin in 2011 (named after an island near St. Petersburg). It compiles to JVM bytecode, JavaScript, or native binaries. The language's killer feature is null safety — the type system distinguishes between nullable and non-nullable types at compile time, eliminating the NullPointerException that has caused billions of dollars in bugs across Java's history. Kotlin also offers data classes, coroutines for async programming, extension functions, and much less boilerplate than Java.
Android Studio (free, based on IntelliJ) is the primary Kotlin IDE. IntelliJ IDEA (Community Edition, free) handles Kotlin natively — JetBrains made both the language and the IDE. VS Code has Kotlin support via extensions. For Android development, Kotlin is the clear choice over Java for new projects.