java.lang

About

The java.lang package is one of the core packages in Java and is automatically imported into every Java program. It contains classes that are fundamental to the Java programming language, including basic types, system utilities, and fundamental classes that are essential for Java programming.

Key Classes and Interfaces in java.lang

Object

The root class of the Java class hierarchy. Every class has Object as a superclass. Provides basic methods such as equals(), hashCode(), toString(), clone(), wait(), notify(), and notifyAll().

Class

Instances of the Class class represent classes and interfaces in a running Java application. Provides methods to get metadata about a class, such as its name, fields, methods, constructors, and more.

System

Provides several useful class fields and methods. It cannot be instantiated. Key functionalities include system properties, standard input/output streams, array copying, garbage collection, and current time retrieval.

String

Represents a sequence of characters. Strings are immutable, meaning their values cannot be changed once created. Provides various methods for manipulating strings, such as concatenation, substring, and pattern matching.

StringBuilder and StringBuffer

Mutable sequences of characters. StringBuilder is not synchronized, making it faster, but not thread-safe. StringBuffer is synchronized, making it thread-safe.

Math

Provides methods for performing basic numeric operations such as exponentiation, logarithms, square roots, and trigonometric functions.

Integer, Long, Float, Double, Byte, Short, Character, Boolean

Wrapper classes for primitive types. These classes provide methods for converting between types, parsing strings, and performing various utility functions.

Thread

Represents a thread of execution in a program. Provides methods to manage and control thread behavior.

Runnable

The Runnable interface should be implemented by any class whose instances are intended to be executed by a thread. The class must define a method of no arguments called run.

Exception and RuntimeException

• The Exception class and its subclasses are a form of Throwable that indicates conditions that a reasonable application might want to catch.

• RuntimeException is the superclass of those exceptions that can be thrown during the normal operation of the Java Virtual Machine.

Error

A subclass of Throwable that indicates serious problems that a reasonable application should not try to catch. Most such errors are abnormal conditions.

Key Interfaces in java.lang

CharSequence

A readable sequence of char values. This interface is implemented by String, StringBuilder, StringBuffer, etc.

Comparable

This interface imposes a total ordering on the objects of each class that implements it. This ordering is referred to as the class's natural ordering, and the class's compareTo method is referred to as its natural comparison method.

Cloneable

A marker interface indicating that a class allows its instances to be cloned. The Object class provides a clone method if a class implements this interface.

Iterable

Implementing this interface allows an object to be the target of the "foreach" statement. The only method in this interface is the iterator() method, which returns an Iterator.