Object lifecycle in Java

Introduction

Understanding the object lifecycle helps you know how objects are created, used, and removed in a program.

When you want to create a new object to store data or behavior.

When you need to know how long an object stays in memory.

When you want to clean up resources used by an object.

When debugging to see when objects are created or destroyed.

When managing memory efficiently in your program.

Syntax

Java

ClassName obj = new ClassName();
// use obj
obj = null; // object eligible for garbage collection

Objects are created using the new keyword.

Objects are removed automatically by Java's garbage collector when no longer used.

Examples

This creates a new Car object and stores it in myCar.

Java

Car myCar = new Car();

This removes the reference to the Car object, making it ready for cleanup.

Java

myCar = null;

This example shows creation and hints at destruction using finalize() and garbage collection.

Java

public class Person {
    public Person() {
        System.out.println("Person created");
    }
    @Override
    protected void finalize() throws Throwable {
        System.out.println("Person destroyed");
        super.finalize();
    }
}

Person p = new Person();
p = null;
System.gc();

Sample Program

This program creates an object, removes its reference, and calls garbage collection to show when the object is destroyed.

Java

public class ObjectLifecycleDemo {
    static class Demo {
        Demo() {
            System.out.println("Object created");
        }
        @Override
        protected void finalize() throws Throwable {
            System.out.println("Object destroyed");
            super.finalize();
        }
    }

    public static void main(String[] args) {
        Demo obj = new Demo();
        obj = null; // Remove reference
        System.gc(); // Suggest garbage collection
        try {
            Thread.sleep(1000); // Wait to see finalize output
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
}

Important Notes

Garbage collection runs automatically but calling System.gc() only suggests it; it may not run immediately.

The finalize() method is called before an object is removed but is deprecated in newer Java versions.

Setting object references to null helps the garbage collector know the object is no longer needed.

Summary

Objects are created with new and live while referenced.

When no references remain, objects become eligible for garbage collection.

Java automatically cleans up unused objects to free memory.

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