Object lifecycle in Java - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Java code involving object creation and garbage collection?

Consider the following Java code snippet. What will be printed when it runs?

Java

public class Test {
    public static void main(String[] args) {
        Test obj = new Test();
        obj = null;
        System.gc();
        System.out.println("End of main");
    }
    protected void finalize() throws Throwable {
        System.out.println("Finalize called");
    }
}

A End of main

B End of main\nFinalize called

C Finalize called\nEnd of main

D No output

💡 Hint

Garbage collection in Java is not guaranteed to run immediately after System.gc().

The finalize() method may not be called immediately or at all before the program ends. The output will always print "End of main". "Finalize called" is not guaranteed.

❓ Predict Output

intermediate

What is the output when creating multiple objects and nullifying references?

What will be printed by this Java program?

Java

public class Demo {
    static int count = 0;
    Demo() {
        count++;
    }
    public static void main(String[] args) {
        Demo a = new Demo();
        Demo b = new Demo();
        a = null;
        System.gc();
        System.out.println(count);
    }
}

A 2

B 1

C 0

D Compilation error

💡 Hint

Count increments on each object creation, not on garbage collection.

Two objects are created, so count is 2. Nullifying a and calling System.gc() does not reduce count.

🔧 Debug

advanced

Identify the error in this Java object lifecycle code

What error will this code produce when compiled or run?

Java

public class Sample {
    public void finalize() throws Throwable {
        System.out.println("Cleaning up");
    }
    public static void main(String[] args) {
        Sample s = new Sample();
        s = null;
        System.gc();
    }
}

A No error, prints "Cleaning up"

B Compile-time error: finalize() must throw Throwable

C No output, program ends silently

D Runtime error: finalize() not found

💡 Hint

Check the signature of the finalize() method in Java.

The finalize() method is deprecated but still valid with this signature. It may not be called before program ends, so no output is guaranteed.

📝 Syntax

advanced

Which option correctly overrides the finalize method in Java?

Choose the correct method signature to override finalize() in Java.

A private void finalize() throws Throwable

B public void finalize()

C void finalize() throws Exception

D protected void finalize() throws Throwable

💡 Hint

The finalize() method must have a specific signature to override properly.

The correct signature is protected void finalize() throws Throwable. Other options have wrong access modifiers or exception types.

🚀 Application

expert

How many objects are eligible for garbage collection after this code runs?

Given the following Java code, how many objects are eligible for garbage collection after the last line executes?

Java

public class Node {
    Node next;
    public static void main(String[] args) {
        Node a = new Node();
        Node b = new Node();
        Node c = new Node();
        a.next = b;
        b.next = c;
        c.next = a;
        a = null;
        b = null;
    }
}

A 3

B 0

C 1

D 2

💡 Hint

Consider the references and the circular links between objects.

Even though a and b are set to null, the three Node objects reference each other in a cycle, so none are eligible for garbage collection.