Core Java Fundamentals: JVM, Compilation, Memory, and Types

Core Java Fundamentals

Java applications run on the JVM, which provides portability, memory management, and runtime safety.

JVM, JDK, JRE

• JDK includes the compiler, standard libraries, and tools used to build Java applications.
• JRE includes the runtime environment to execute bytecode.
• JVM executes bytecode and isolates the application from platform-specific details.

Compilation Process

• javac compiles .java source files into .class bytecode files.
• Bytecode is portable until a JVM implementation loads it.
• The compiler performs syntax checking, type checking, and bytecode generation.

Bytecode

• Bytecode is the intermediate representation executed by the JVM.
• It is stack-based and independent of processor architecture.
• The JVM verifies bytecode before execution for security and correctness.

JVM Memory Regions

• The JVM divides memory into logical regions such as Heap, Stack, and Metaspace.
• Heap stores objects and arrays, is shared across threads, and is managed by GC.
• Stack stores local variables, method frames, and references; it is thread-local and short-lived.
• Metaspace stores class metadata and native memory required by class loaders.

Primitive vs Reference Types

• Primitive types include byte, short, int, long, float, double, char, and boolean.
• Reference types include objects, arrays, and interfaces.
• Use primitives for performance and avoid unnecessary boxing in hot paths.
• Wrapper classes like Integer, Long, and Boolean provide object semantics but can introduce allocations.