Object lifetime in Java - Mini Project: Build & Apply

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Object lifetime

📖 Scenario: Imagine you are managing a small library system. Each book is represented as an object. You want to understand when these book objects are created and when they are no longer needed (destroyed).

🎯 Goal: You will create a simple Java program that shows the creation and destruction of book objects by printing messages when objects are created and when they are about to be destroyed by the garbage collector.

📋 What You'll Learn

Create a Book class with a constructor that prints a creation message

Override the finalize() method in Book to print a destruction message

Create multiple Book objects in the main method

Remove references to some Book objects and call the garbage collector

Print messages to observe object lifetime

💡 Why This Matters

🌍 Real World

Understanding object lifetime helps manage memory in programs and avoid memory leaks.

💼 Career

Java developers must know how objects are created and cleaned up to write efficient and reliable applications.

1

Create the Book class with constructor

Create a class called Book with a constructor that prints "Book created" when a new object is made.

Java

class Book {
    // Your code here
}

💡 Need a hint?

The constructor has the same name as the class and no return type.

2

Add finalize method to Book class

Inside the Book class, add a protected void finalize() method that prints "Book destroyed" when the object is about to be removed by the garbage collector.

Java

class Book {
    public Book() {
        System.out.println("Book created");
    }

    // Your code here
}

💡 Need a hint?

Use @Override annotation and call super.finalize() inside the method.

3

Create Book objects and remove references

In the main method, create three Book objects named b1, b2, and b3. Then set b2 and b3 to null to remove their references.

Java

class Book {
    public Book() {
        System.out.println("Book created");
    }

    @Override
    protected void finalize() throws Throwable {
        System.out.println("Book destroyed");
        super.finalize();
    }
}

public class Main {
    public static void main(String[] args) {
        // Create three Book objects
        // Remove references to b2 and b3
        // Your code here
    }
}

💡 Need a hint?

Remember to declare variables with the type Book and assign new objects using new Book().

4

Call garbage collector and print message

After setting b2 and b3 to null, call System.gc() to suggest garbage collection. Then print "Garbage collector called" to observe the output.

Java

class Book {
    public Book() {
        System.out.println("Book created");
    }

    @Override
    protected void finalize() throws Throwable {
        System.out.println("Book destroyed");
        super.finalize();
    }
}

public class Main {
    public static void main(String[] args) {
        Book b1 = new Book();
        Book b2 = new Book();
        Book b3 = new Book();

        b2 = null;
        b3 = null;
        // Call garbage collector
        // Print message
        // Your code here
    }
}

💡 Need a hint?

Use System.gc() to suggest garbage collection and System.out.println() to print the message.