Java Standard Library: Complete Overview

The Java Standard Library (Java Standard Edition API) is a powerful and extensive set of classes and interfaces that provides developers with ready-made solutions for most common tasks. It is an integral part of the Java platform and covers everything from basic data types and working with collections to multithreading, input-output, and network communication. In this overview, we will look at the key packages and their capabilities.

Package `java.lang` — The Foundation of the Language

The java.lang package is imported automatically. It contains fundamental classes without which it is impossible to write any Java program.

Object — the root class of the hierarchy. All classes inherit its methods (equals(), hashCode(), toString()).
String, StringBuilder, StringBuffer — for working with immutable and mutable strings.
Wrapper Types — Integer, Double, Boolean, and others — allow using primitives in collections.
System — a utility class for working with the console (System.out), system properties, and arrays (System.arraycopy()).
Math — provides mathematical constants and functions (Math.sin(), Math.sqrt()).

// Example of using java.langpublic class LangExample {    public static void main(String[] args) {        String text = "Hello, Java!";        int length = text.length(); // 12        double root = Math.sqrt(25); // 5.0        System.out.println(text.toUpperCase()); // HELLO, JAVA!    }}

Package `java.util` — Collections and Utilities

This is one of the most popular packages. It includes the Collections Framework — a unified architecture for storing and processing data.

Collections

List — an ordered list (allows duplicates). Implementations: ArrayList, LinkedList.
Set — a set of unique elements. Implementations: HashSet, TreeSet.
Map — a dictionary (key-value). Implementations: HashMap, TreeMap.
Queue/Deque — queues for data processing (e.g., PriorityQueue).

import java.util.*;

public class CollectionsExample {    public static void main(String[] args) {        List<String> list = new ArrayList<>();        list.add("Java");        list.add("Python");        list.add("Java"); // Duplicate is allowed

        Set<Integer> set = new HashSet<>();        set.add(10);        set.add(20);        set.add(10); // Duplicate will be ignored

        Map<String, Integer> map = new HashMap<>();        map.put("Alice", 30);        map.put("Bob", 25);

        System.out.println("List: " + list);        System.out.println("Set: " + set);        System.out.println("Map: " + map);    }}

Utilities

Collections — static methods for sorting, searching, and synchronizing collections.
Arrays — convenient work with arrays (Arrays.sort(), Arrays.asList()).
Date, Calendar — for working with dates and times (deprecated in favor of java.time).
Random — generating pseudo-random numbers.

Package `java.io` — Input-Output

This package provides classes for working with data streams: files, console, network connections. It is based on the concept of InputStream/OutputStream for bytes and Reader/Writer for characters.