Java

Java is a statically typed, object-oriented language. You compile .java files to bytecode (.class files), and the JVM (Java Virtual Machine) runs that bytecode on any operating system. The JVM starts by interpreting it, then a JIT (just-in-time) compiler turns hot code into fast machine code while the program runs.

Why job posts ask for it

Banks, insurers, telecoms and large retailers run much of their backend on Java, usually with the Spring Boot framework, plus Kafka, SQL databases and microservices. It was the most-asked missing skill in the Canadian entry-level posts this wiki tracks (see lesson 1).

If you know C#, Python or TypeScript

Idea	Java
Types	Declared and checked at compile time; var infers local types (Java 10+)
Memory	Garbage collected, like C#, Python and JavaScript
Concurrency	Real threads that run in parallel; virtual threads since Java 21
Collections	List/ArrayList, Map/HashMap (a hash map), Set/HashSet
Web framework	Spring Boot (the role ASP.NET Core plays in C#, or FastAPI in Python)
Versions	Long-term support releases: 17, 21, 25

import java.util.HashMap;
import java.util.Map;
 
public class TwoSum {
    static int[] twoSum(int[] nums, int target) {
        Map<Integer, Integer> seen = new HashMap<>();
        for (int i = 0; i < nums.length; i++) {
            Integer j = seen.get(target - nums[i]);
            if (j != null) return new int[] { j, i };
            seen.put(nums[i], i);
        }
        return new int[0];
    }
}

The same Two Sum solution as in Python, with types spelled out.

Check yourself

What does the JVM run, and why does that make Java portable?

It runs bytecode. The same bytecode runs on any OS that has a JVM, so you compile once.

What is the Java equivalent of Python’s dict?

Map, most often the HashMap implementation.