Java comes with a bunch of ready-to-use tools called the Java API. These tools help you do useful stuff in Java without having to write all the code yourself.

The Java API, short for Application Programming Interface, is like a toolbox filled with prewritten code. It includes classes, packages, interfaces, and methods that give Java developers a lot of functionality to work with. Think of it as a set of building blocks that you can use to create your Java programs.

The Java API is organized into different groups called packages. Each package focuses on a specific type of task or functionality. This makes it easier for developers to find the tools they need for their projects.

• Core Java API
  • This API includes fundamental classes and interfaces for basic programming tasks, such as data manipulation, input/output operations, networking, concurrency, and exception handling. The Core Java API includes the next importante packages:
    • java.lang: Contains fundamental classes and interfaces that are automatically imported into every Java program. It includes classes like Object, String, Integer, Boolean, and essential interfaces like Comparable and Runnable.
    • java.util: Provides utility classes and data structures, such as lists, sets, maps, iterators, and date/time handling classes. Common classes in this package include ArrayList, HashMap, HashSet, Calendar, and Date.
    • java.io: Contains classes for input and output operations, including reading from and writing to files, streams, and other data sources. Classes in this package include File, InputStream, OutputStream, Reader, and Writer.
    • java.net: Provides classes for networking operations, such as connecting to remote servers, sending and receiving data over the network, and working with URLs. Classes in this package include URL, URLConnection, Socket, and ServerSocket.
    • java.util.concurrent: Offers support for concurrent programming, including classes and interfaces for working with threads, executors, locks, and concurrent data structures. Classes in this package include Executor, ThreadPoolExecutor, Lock, ConcurrentHashMap, and CountDownLatch.
    • java.math: Contains classes for arbitrary-precision arithmetic and numerical operations on large numbers. Classes in this package include BigInteger, and BigDecimal.
• Java SE API
  • This API extends the core Java API, this means that includes all the packages of the Core Java API and provides additional functionality for developing desktop, web, and mobile applications. It includes packages for graphical user interfaces (GUIs), database connectivity (JDBC), XML processing, security, and more. This are two packages that includes Java SE API.
    • java.awt and javax.swing: Contain classes and interfaces for creating graphical user interfaces (GUIs) in Java. These packages include classes for components like buttons, text fields, panels, frames, and dialog boxes.
    • java.security: Provides classes and interfaces for implementing security features in Java applications, including cryptography, secure communication, permissions, and access control.
• Java EE API
  • This API builds upon the Java SE API and is specifically tailored for developing enterprise-level applications. It includes packages for building web applications, enterprise integration, messaging, transaction management, and distributed computing. Below are a few packages included in the Java EE API:
    • javax.servlet and javax.servlet.http: These packages provide classes and interfaces for building web applications in Java, including handling HTTP requests and responses, managing sessions, and working with servlet filters.
    • javax.ws.rs: Contains classes and interfaces for building RESTful web services using the JAX-RS (Java API for RESTful Web Services) framework.
    • javax.jms: Provides classes and interfaces for working with Java Message Service (JMS), allowing Java applications to send and receive messages asynchronously.
    • javax.ejb: Contains classes and interfaces for building enterprise JavaBeans (EJBs), which are server-side components used for implementing business logic in Java EE applications.
    • javax.persistence: Provides classes and interfaces for working with the Java Persistence API (JPA), allowing Java applications to interact with relational databases using object-relational mapping (ORM) techniques.
    • javax.enterprise.concurrent: Offers support for concurrent programming in Java EE applications, including asynchronous execution, concurrency utilities, and managed executor services.
    • javax.faces and javax.faces.component: Contains classes and interfaces for building JavaServer Faces (JSF) applications, which are web application frameworks for building user interfaces in Java EE applications.
    • javax.websocket: Provides classes and interfaces for building WebSocket-based communication in Java EE applications, allowing for full-duplex, bidirectional communication between client and server.
    • javax.transaction: Contains classes and interfaces for working with distributed transactions in Java EE applications, enabling multiple database operations to be treated as a single transaction.
• Java ME API
  • This API includes several packages tailored for developing applications on resource-constrained devices such as mobile phones, embedded systems, and IoT devices. Some of the key packages in the Java ME API include:
    • javax.microedition.io: Provides classes and interfaces for networking operations, including connecting to remote servers, sending and receiving data over various protocols, and working with connections on constrained devices.
    • javax.microedition.midlet: Contains classes and interfaces for developing MIDlets, which are applications designed to run on mobile devices with limited resources and capabilities. MIDlets typically consist of a single class that extends the MIDlet class.
    • javax.microedition.lcdui: Provides classes and interfaces for creating user interfaces in Java ME applications. It includes classes for building simple user interfaces with basic UI components such as forms, text fields, lists, and buttons.
    • javax.microedition.rms: Contains classes and interfaces for working with the Record Management System (RMS), which provides a simple database-like storage mechanism for persistent data on Java ME devices.
    • javax.microedition.media: Offers classes and interfaces for multimedia playback and recording on Java ME devices. It includes support for playing audio and video files, capturing images and video from device cameras, and controlling media playback.
    • javax.microedition.pim: Provides classes and interfaces for working with Personal Information Management (PIM) data, such as contacts, calendar events, tasks, and notes, on Java ME devices.

• Collection API (Java Collections Framework)
  • The Collection API (Java Collections Framework) in Java refers to a set of interfaces, classes, and methods provided by the for working with collections of objects. Collections in Java are used to store, manipulate, and organize groups of elements, such as lists, sets, maps, stacks, and queues. With this API we can perform operations like: add, update, remove, search, and rearrange to manipulate objects.
    • java.util: Contains core interfaces and classes for collections, such as Collection, List, Set, Map, and various implementations like ArrayList, LinkedList, HashSet, TreeSet, HashMap, and TreeMap. Also offers the classes Collections and Arrays.
    • java.util.concurrent: Contains classes and interfaces for concurrent programming, like ConcurrentHashMap, ConcurrentLinkedQueue, CopyOnWriteArrayList, Semaphore, CountDownLatch, and CyclicBarrier.

• Stream API
  • This API, introduced in Java 8, enables the processing of sequences of elements in a functional and declarative manner. It empowers developers to execute aggregate operations on collections of data, including filtering, mapping, and reducing large streams of data. Java achieves these tasks through lambda expressions, embodying a form of functional programming in Java. The Stream API works closely with the Collections and I/O APIs.

• I/O API
  • Java provides a set of classes and interfaces for reading and writing data to and from various sources, such as files, streams, and network connections. This API allows Java programs to interact with external resources and devices, facilitating input and output operations. Some of the key packages include:
    • java.io: This package contains classes and interfaces for handling input and output operations, including streams for reading and writing binary and character data, file I/O operations, and object serialization.
    • java.nio: The New I/O (NIO) package provides support for non-blocking I/O operations and memory-mapped file I/O. It includes classes and interfaces for working with buffers, channels, selectors, and file system operations.
    • java.nio.channels: This package contains classes and interfaces for performing I/O operations through channels, which represent connections to entities that can perform I/O operations, such as files, sockets, and pipes.
    • java.nio.file: Provides classes and interfaces for working with files and directories in the file system. It allows Java programs to perform file I/O operations, such as creating, deleting, copying, moving, and reading/writing files.
    • java.net: Contains classes and interfaces for performing network I/O operations, such as connecting to remote servers, sending and receiving data over network sockets, and working with URLs and URIs.
    • java.util.zip: Provides classes and interfaces for working with compressed files and ZIP archives, including classes for reading and writing ZIP files and streams.

• Java Concurrency API
  • This API take advantage of multicore CPU architectures. multithreaded programs in Java, allowing developers to create applications that efficiently utilize multiple threads of execution. It includes classes and interfaces for coordinating concurrent activities, managing thread execution, and synchronizing access to shared resources. The key package of this API are:
    • java.util.concurrent: This package contains classes and interfaces for creating concurrent data structures, such as ConcurrentHashMap and ConcurrentLinkedQueue, which are thread-safe alternatives to their non-concurrent counterparts. It also provides utilities for executing tasks asynchronously, scheduling tasks for execution at a later time, and managing thread pools.
• JDBC API
  • This API allows to connect to vendor database and execute SQL statements. This API defines a standard way to interact with relational databases and performs operations such as connecting to a database, executing SQL queries, processing query results, and managing transactions. The key package and interfaces of this API are in the java.sql package:
    • java.sql.Connection: Interface representing a connection to a database, providing methods for creating statements and managing transactions.
    • java.sql.Statement: Interface representing a SQL statement that can be executed against a database, allowing developers to execute queries, updates, and other SQL commands.
    • java.sql.PreparedStatement: Interface extending Statement that represents a precompiled SQL statement, providing support for parameterized queries and improving performance for repeated executions.
    • java.sql.CallableStatement: Interface extending PreparedStatement that represents a SQL stored procedure call, allowing developers to execute stored procedures in the database.
    • java.sql.ResultSet: Interface representing a set of rows returned by a database query, providing methods for iterating over the rows and accessing column values.
    • java.sql.ResultSetMetaData: Interface representing metadata about the columns in a ResultSet, allowing developers to retrieve information about column names, types, and properties.
    • java.sql.DriverManager: Class providing methods for managing JDBC drivers and establishing database connections.

• JPA API (Java Persistence API)
  • This API is a specification for accessing, managing, and persisting data between Java objects and a relational database. JPA provides a set of standard interfaces and annotations that allow developers to define object-relational mappings (ORM) and perform database operations using object-oriented paradigms. This API is part of the Java EE platform and is defined by the javax.persistence package.

These are some of the Java API that exist, but there are numerous third-party Java APIs developed by various organizations and individuals to extends the Java functionality.