Core Java Interview Questions

The JVM (Java Virtual Machine) consists of three main subsystems: 1. **Class Loader Subsystem**: Loads, links, and initializes class files. 2. **Runtime Data Areas**: - Method Area: Class structures, static variables - Heap Area: Objects and arrays - Stack Area: Method calls, local variables - PC Register: Current instruction address - Native Method Stack: Native method information 3. **Execution Engine**: - Interpreter: Executes bytecode line by line - JIT Compiler: Compiles hot code to native machine code - Garbage Collector: Manages memory

**JVM (Java Virtual Machine)**: The abstract machine that executes Java bytecode. It provides platform independence but is itself platform-dependent. **JRE (Java Runtime Environment)**: Implementation of JVM. Includes JVM + Core Libraries + Supporting Files. It allows you to run Java applications but not develop them. **JDK (Java Development Kit)**: Full development environment. Includes JRE + Development Tools (javac, debugger, javadoc). Required for writing and compiling Java code.

The `static` keyword means that a member belongs to the class itself rather than to any specific instance. - **Static Variable**: Shared among all instances. Only one copy exists. - **Static Method**: Can be called without creating an object. Cannot access instance variables directly. - **Static Block**: Executed once when the class is loaded. Used for static initialization. - **Static Class**: Only nested classes can be static.

1. **Security**: Strings are widely used for sensitive info (passwords, connections). Immutability prevents modification after creation. 2. **Thread Safety**: Immutable objects are inherently thread-safe. 3. **String Constant Pool**: Saves memory by sharing identical string literals. This is only safe because strings cannot change. 4. **Caching HashCode**: The hash code is cached at creation, making it great for Map keys.

- `==` operator checks for **reference equality**. It returns true if both references point to the exact same object in memory. - `.equals()` method checks for **logical equality** (content comparison). By default in Object class, it behaves like `==`, but classes like String override it to compare values.