Object lifecycle in Java - Mini Project: Build & Apply

Understanding Object Lifecycle in Java

📖 Scenario: Imagine you are creating a simple Java program to track a book in a library system. You will learn how to create an object, use it, and understand when it is created and destroyed.

🎯 Goal: You will build a Java class Book and write code to create an object of this class, use it, and observe its lifecycle through constructor and finalize method messages.

📋 What You'll Learn

Create a Book class with a constructor and a finalize method

Create a Book object in the main method

Set a reference to null to make the object eligible for garbage collection

Call the garbage collector to observe object destruction

Print messages in constructor and finalize to track lifecycle

💡 Why This Matters

🌍 Real World

Understanding object lifecycle helps manage memory in applications, preventing memory leaks and improving performance.

💼 Career

Java developers must know how objects are created and destroyed to write efficient and reliable software.

1

Create the Book class with a constructor

Create a public class called Book with a constructor that prints "Book object created" when called.

Java

public class Book {
    // Your code here
}

Need a hint?

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

2

Add a finalize method to the Book class

Add a protected void finalize() method inside the Book class that prints "Book object is being destroyed".

Java

public class Book {
    public Book() {
        System.out.println("Book object created");
    }
    // Your code here
}

Need a hint?

The finalize method is called by the garbage collector before destroying the object.

3

Create and nullify a Book object in main

In a public static void main(String[] args) method, create a Book object called myBook. Then set myBook to null to make it eligible for garbage collection.

Java

public class Book {
    public Book() {
        System.out.println("Book object created");
    }
    @Override
    protected void finalize() throws Throwable {
        System.out.println("Book object is being destroyed");
        super.finalize();
    }
    public static void main(String[] args) {
        // Your code here
    }
}

Need a hint?

Setting the object reference to null means no variable points to the object anymore.

4

Call garbage collector and observe output

In the main method, after setting myBook to null, call System.gc() to request garbage collection. Then print "End of main method".

Java

public class Book {
    public Book() {
        System.out.println("Book object created");
    }
    @Override
    protected void finalize() throws Throwable {
        System.out.println("Book object is being destroyed");
        super.finalize();
    }
    public static void main(String[] args) {
        Book myBook = new Book();
        myBook = null;
        // Call garbage collector and print message
        // Your code here
    }
}

Need a hint?

Calling System.gc() requests the JVM to run garbage collection, but it may not happen immediately.