Java Standard Library: Complete Overview

Java Standard Library (Java Standard Edition): Full Overview

The Java Standard Library (Java Standard Edition — JavaSE) is a fundamental set of packages, classes, and interfaces that comes with every JDK (Java Development Kit). Unlike third-party frameworks, the standard library does not require installing additional dependencies — it is available "out of the box." It is what makes Java such a powerful and portable language, providing the developer with ready-made solutions for most everyday tasks: from working with strings to multithreading and network interaction.

In this overview, we will break down the key packages of the standard library, show practical code examples, and explain how these tools are used in real-world development.

1. Package `java.lang` — The Absolute Foundation

The java.lang package is imported automatically in any Java program. It contains classes without which no application can be imagined.

Object — the root class for all objects. Methods equals(), hashCode(), toString().
String, StringBuilder, StringBuffer — working with immutable and mutable strings.
Math — mathematical functions (Math.sqrt(), Math.random()).
System — console input/output, managing system properties.
Thread and Runnable — basic multithreading tools.

public class LangExample {    public static void main(String[] args) {        // Working with strings        String text = "Hello, Java!";        System.out.println(text.length()); // 13

        // Math        double sqrt = Math.sqrt(144);        System.out.println(sqrt); // 12.0

        // Object class        Object obj = new Object();        System.out.println(obj.hashCode());    }}

2. Package `java.util` — Collections and Utilities

This is the most extensive package. It includes the Collections Framework, working with dates, random numbers, and much more.

Collections Framework

List (ArrayList, LinkedList) — ordered lists with duplicates.
Set (HashSet, TreeSet) — sets without repetitions.
Map (HashMap, TreeMap) — key-value dictionaries.
Queue (PriorityQueue) — queues.

import java.util.*;

public class UtilExample {    public static void main(String[] args) {        // ArrayList        List<String> list = new ArrayList<>();        list.add("Java");        list.add("Python");        list.add("Java");        System.out.println(list); // [Java, Python, Java]

        // HashSet (unique elements)        Set<String> set = new HashSet<>(list);        System.out.println(set); // [Java, Python]

        // HashMap        Map<String, Integer> map = new HashMap<>();        map.put("Apple", 10);        map.put("Pear", 5);        System.out.println(map.get("Apple")); // 10    }}