Java Deep Dive Test

Anyone can drive a car. A mechanic can also tell you why it pulls to the left when you brake, and what will break next if you ignore it. This test is for the Deep Dive track, and it asks the mechanic's questions. It is not about syntax. It asks how the JIT compiles your code, what the memory model promises between threads, what the bytecode looks like, and how HashMap and the garbage collectors work inside.

How the Test Works

• What's in it: 20 multiple-choice questions, then 3 puzzles where you work out what a short program prints or what can go wrong.
• Time: 30 minutes, without notes, an IDE or JShell.
• Pass mark: 16 out of 23. The quiz marks the 20 questions for you. Mark the puzzles yourself, one point each, and add them to your quiz score.
• What it covers: the JIT, the Java Memory Model, lock-free code, bytecode, collection internals, garbage collection and the newest JVM features. Start with JVM Architecture Overview if these topics are new to you.

Part 1: 20 Questions

Part 2: Puzzles

Work out each answer on paper before you open the solution. The solution shows a full program, its real output and a Try it button. Give yourself one point for each puzzle you get right.

Puzzle 1: A Flag Without Volatile

What can happen when these two threads run? How do you fix it?

static boolean ready = false;   // not volatile
static int value = 0;
 
// Thread 1
value = 42;
ready = true;
 
// Thread 2
while (!ready) { }
System.out.println(value);

The answer to this one is not a single output, so the solution shows the fixed program instead.

public class VisibilityFixed {
    static volatile boolean ready = false;
    static int value = 0;
 
    public static void main(String[] args) throws InterruptedException {
        Thread reader = new Thread(() -> {
            while (!ready) { }
            System.out.println(value);
        });
        reader.start();
 
        value = 42;
        ready = true;
        reader.join();
    }
}

42

Puzzle 2: Two Lists, One Class

What is printed?

List<String> a = new ArrayList<>();
List<Integer> b = new ArrayList<>();
System.out.println(a.getClass() == b.getClass());

import java.util.ArrayList;
import java.util.List;
 
public class ErasurePuzzle {
    public static void main(String[] args) {
        List<String> a = new ArrayList<>();
        List<Integer> b = new ArrayList<>();
        System.out.println(a.getClass() == b.getClass());
    }
}

true

Puzzle 3: The Key That Changed

What is printed?

record Key(List<String> parts) {}
 
var parts = new ArrayList<>(List.of("a"));
var map = new HashMap<Key, String>();
map.put(new Key(parts), "value");
parts.add("b");
System.out.println(map.get(new Key(List.of("a"))) + " "
        + map.get(new Key(parts)) + " " + map.size());

import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
 
public class MutableKeyPuzzle {
    record Key(List<String> parts) {}
 
    public static void main(String[] args) {
        var parts = new ArrayList<>(List.of("a"));
        var map = new HashMap<Key, String>();
        map.put(new Key(parts), "value");
        parts.add("b");
        System.out.println(map.get(new Key(List.of("a"))) + " "
                + map.get(new Key(parts)) + " " + map.size());
    }
}

null null 1

What Your Score Means