The java.lang package is a fundamental part of the Java programming language, providing classes that are essential for performing basic operations. It is automatically imported by default in every Java program, eliminating the need for explicit import statements. The java.lang package includes classes that handle basic data types, string manipulation, system operations, exception handling, and threading. It is the backbone of Java and serves as a crucial foundation for building Java applications.
Core Functionality:
- Object Class: Object is the superclass of all Java classes, defining universal methods like toString(), equals(), hashCode(), and clone() that every object inherits.
- String Handling: Classes like String, StringBuilder, and StringBuffer support efficient text manipulation, with String being immutable and the others mutable for performance in dynamic string operations.
- Wrapper Classes: Classes such as Integer, Long, Double, Float, Boolean, and Character wrap primitive types, enabling their use in collections and providing utility methods for conversions and operations.
- Enum Support: The Enum class provides the foundation for enumerated types, allowing type-safe constants with built-in methods for iteration and comparison.
System and Runtime Access:
- System Class: Facilitates interaction with the environment through standard I/O streams (System.out, System.in, System.err), system properties, and methods like currentTimeMillis() for time tracking.
- Runtime Class: Enables low-level JVM interactions, such as memory management (freeMemory(), totalMemory()), executing native processes (exec()), and triggering garbage collection (gc()).
Multithreading and Concurrency:
- Threading: The Thread class and Runnable interface allow creation and management of threads for concurrent programming, with methods to control thread lifecycle (start(), sleep(), join()).
- Synchronization: Object methods like wait(), notify(), and notifyAll() support thread coordination and synchronization.
Exception Handling:
- Throwable Hierarchy: Throwable is the root of all errors and exceptions, with Error (for severe issues like OutOfMemoryError) and Exception (for recoverable issues like IOException).
- Common Exceptions: Includes frequently encountered exceptions like NullPointerException, ClassCastException, ArrayIndexOutOfBoundsException, and IllegalStateException.
Mathematical and Utility Support:
- Math Classes: Math provides static methods for trigonometric, exponential, and rounding operations, while StrictMath ensures platform-independent results.
- Utility Classes: Character, Boolean, and others offer helper methods for type-specific operations, such as checking character properties or parsing values.
Reflection and Metadata:
- Reflection API: Classes like Class, Method, Field, and Constructor enable runtime inspection and manipulation of class structures, supporting dynamic programming and frameworks like Spring or Hibernate.
- Annotation Support: java.lang includes annotation-related classes like Override and Deprecated for metadata and compiler hints.
Security and Class Loading:
- ClassLoader: Defines how classes are loaded into the JVM, supporting custom class loading for applications and frameworks.
- SecurityManager: Provides a mechanism (though largely deprecated) for enforcing security policies in Java applications.
Immutability and Immutables:
- Many classes, like String and wrapper classes, are immutable, promoting thread safety and predictable behavior in concurrent environments.
Lightweight and Universal:
- The package is designed to be compact yet comprehensive, ensuring that even minimal Java applications can leverage its functionality without external dependencies.