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
}

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
}

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
    }
}

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
    }
}

Hint

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

Practice

1. Which statement best describes the lifecycle of an object in Java?

easy

A. Objects must be manually deleted to free memory.

B. An object is created automatically without new and never gets removed.

C. Objects live forever once created.

D. An object is created with new and exists as long as it has references.

Solution

1. Step 1: Understand object creation

   In Java, objects are created using the new keyword which allocates memory.

2. Step 2: Understand object lifetime

   An object remains alive as long as there is at least one reference pointing to it. When no references remain, it becomes eligible for garbage collection.

3. Final Answer:

   An object is created with new and exists as long as it has references. -> Option D

4. Quick Check:

   Object lifecycle = created with new and referenced [OK]

Hint: Objects live only while referenced, created with new [OK]

Common Mistakes:

• Thinking objects live forever
• Believing manual deletion is needed
• Assuming objects are created without new

2. Which of the following is the correct way to create a new object of class Car in Java?

easy

A. Car myCar = Car();

B. Car myCar = new Car();

C. new Car myCar();

D. Car myCar = create Car();

Solution

1. Step 1: Recall Java object creation syntax

   In Java, to create an object, use the syntax: ClassName variable = new ClassName();

2. Step 2: Match options with correct syntax

   Car myCar = new Car(); matches the correct syntax. Other options have syntax errors or invalid keywords.

3. Final Answer:

   Car myCar = new Car(); -> Option B

4. Quick Check:

   Use new keyword to create objects [OK]

Hint: Use 'new' keyword with class name and parentheses [OK]

Common Mistakes:

• Omitting 'new' keyword
• Incorrect order of keywords
• Using invalid method-like syntax

3. What will be the output of the following Java code?

class Demo {
    public static void main(String[] args) {
        Demo obj1 = new Demo();
        Demo obj2 = obj1;
        obj1 = null;
        if (obj2 != null) {
            System.out.println("Object is alive");
        } else {
            System.out.println("Object is gone");
        }
    }
}

medium

A. Object is alive

B. Object is gone

C. Compilation error

D. Runtime exception

Solution

1. Step 1: Analyze object references

   Initially, obj1 points to a new Demo object. Then obj2 is assigned the same reference as obj1.

2. Step 2: Check null assignment and condition

   obj1 is set to null, but obj2 still references the object. The if condition checks obj2 != null, which is true.

3. Final Answer:

   Object is alive -> Option A

4. Quick Check:

   Object lives while referenced = true [OK]

Hint: Object lives if any reference points to it [OK]

Common Mistakes:

• Assuming object is gone when one reference is null
• Confusing reference variables with objects
• Expecting compilation or runtime errors

4. Identify the error in the following code related to object lifecycle:

public class Test {
    public static void main(String[] args) {
        String s = new String("hello");
        s = null;
        System.out.println(s.length());
    }
}

medium

A. Compilation error due to null assignment

B. No error, prints length of string

C. NullPointerException at runtime

D. String object is not created

Solution

1. Step 1: Understand object reference and null assignment

   The variable s initially references a String object. Then it is set to null, so it no longer points to any object.

2. Step 2: Analyze method call on null reference

   Calling s.length() when s is null causes a NullPointerException at runtime.

3. Final Answer:

   NullPointerException at runtime -> Option C

4. Quick Check:

   Calling method on null reference causes exception [OK]

Hint: Don't call methods on null references [OK]

Common Mistakes:

• Thinking null assignment causes compile error
• Expecting output instead of exception
• Ignoring null pointer risks

5. Consider the following code snippet:

class Node {
    Node next;
    int value;
    Node(int val) { value = val; }
}

public class Test {
    public static void main(String[] args) {
        Node a = new Node(1);
        Node b = new Node(2);
        a.next = b;
        b = null;
        // Which nodes are eligible for garbage collection here?
    }
}

Which nodes are eligible for garbage collection after b = null;?

hard

A. Neither a nor b nodes are eligible

B. Only the node originally referenced by b is eligible

C. Both nodes are eligible

D. Only the node referenced by a is eligible

Solution

1. Step 1: Analyze references after assignment

   Variable a references a Node with value 1. This node's next points to the Node with value 2.

2. Step 2: Check if nodes are still reachable

   Even though b is set to null, the Node with value 2 is still referenced by a.next. So both nodes are still reachable and not eligible for garbage collection.

3. Final Answer:

   Neither a nor b nodes are eligible -> Option A

4. Quick Check:

   Objects reachable via references are not collected [OK]

Hint: Objects reachable from any reference stay alive [OK]

Common Mistakes:

• Assuming null variable means object is collected
• Ignoring references inside objects
• Confusing variable null with object eligibility