Java Practice Drills: Tricky Code Tracing & Memory Dry Runs

Mastering Java requires more than memorizing syntax; it demands the ability to execute code mentally just like the JVM. Below are battle-tested dry run challenges frequently asked in technical coding interviews and university practical exams.

Drill 1: The Post-Increment Assignment Trap

public class Drill1 {
    public static void main(String[] args) {
        int i = 0;
        i = i++ + ++i * 2 + i++;
        System.out.println("i = " + i);
    }
}

Drill 2: String Constant Pool vs Heap Object Trap

public class Drill2 {
    public static void main(String[] args) {
        String s1 = "Veewom";
        String s2 = "Vee" + "wom";
        String part = "wom";
        String s3 = "Vee" + part;
        String s4 = new String("Veewom").intern();

        System.out.println(s1 == s2); // Check 1
        System.out.println(s1 == s3); // Check 2
        System.out.println(s1 == s4); // Check 3
    }
}

Drill 3: Return Value in Finally Block Override

public class Drill3 {
    public static int testFlow() {
        try {
            int x = 10 / 2;
            return 100;
        } catch (Exception e) {
            return 200;
        } finally {
            return 300; // Warning: finally block does not complete normally
        }
    }

    public static void main(String[] args) {
        System.out.println("Result: " + testFlow());
    }
}

Drill 4: Static Initialization & Inheritance Order

class Parent {
    static { System.out.print("1 "); }
    { System.out.print("2 "); }
    Parent() { System.out.print("3 "); }
}

class Child extends Parent {
    static { System.out.print("4 "); }
    { System.out.print("5 "); }
    Child() { System.out.print("6 "); }
}

public class Drill4 {
    public static void main(String[] args) {
        System.out.print("Start: ");
        new Child();
        System.out.println();
    }
}