Object lifecycle in Java - Step-by-Step Execution

Concept Flow - Object lifecycle

Object Declaration

↓

Memory Allocation

↓

Constructor Called

↓

Object Ready to Use

↓

Object Usage

↓

Object No Longer Referenced

↓

Garbage Collector Cleans Object

↓

Memory Freed

This flow shows how a Java object is created, used, and then cleaned up by the system when no longer needed.

Execution Sample

Java

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

Box b = new Box();

This code creates a Box object, calling its constructor which prints a message.

Execution Table

Step	Action	Evaluation	Result
1	Declare variable b	b is null (no object yet)	b = null
2	Allocate memory for new Box	Memory allocated for Box object	Memory reserved
3	Call Box constructor	Constructor runs	Prints 'Box created'
4	Assign object reference to b	b points to new Box object	b = reference to Box
5	Use object b	Object can be used	Operations on b possible
6	b set to null or goes out of scope	No references to Box object	Object eligible for garbage collection
7	Garbage collector runs	Object memory freed	Memory reclaimed

💡 Execution stops when object is no longer referenced and garbage collector frees memory.

Variable Tracker

Variable	Start	After Step 1	After Step 4	After Step 6	Final
b	undefined	null	reference to Box object	null	null

Key Moments - 3 Insights

Why is the object not immediately deleted after setting b to null?

Because Java uses garbage collection which runs later; setting b to null only removes the reference, making the object eligible for cleanup (see step 6 and 7 in execution_table).

What happens during the constructor call?

The constructor initializes the object and can run code like printing messages (see step 3 in execution_table).

Can the object be used before the constructor finishes?

No, the object is only ready to use after the constructor completes (see step 4 in execution_table).

Visual Quiz - 3 Questions

Look at the execution table, what is the value of variable b after step 1?

A undefined

B reference to Box object

C null

D memory allocated

💡 Hint

Check the 'Evaluation' column at step 1 in the execution_table.

At which step does the constructor print 'Box created'?

A Step 3

B Step 4

C Step 2

D Step 5

💡 Hint

Look at the 'Result' column in the execution_table for the printing action.

If variable b never goes out of scope or is set to null, what happens to the object?

A It is immediately garbage collected

B It remains in memory and usable

C Constructor runs again

D Memory is freed manually

💡 Hint

Refer to step 6 and 7 in execution_table about object eligibility for garbage collection.

Concept Snapshot

Object lifecycle in Java:
1. Declare variable (reference)
2. Allocate memory for object
3. Call constructor to initialize
4. Use object via reference
5. Remove references (null or out of scope)
6. Garbage collector frees memory later

Full Transcript

This visual trace shows the lifecycle of a Java object from declaration to garbage collection. First, a variable is declared and set to null. Then memory is allocated for the object, and the constructor runs, printing a message. The variable is assigned the object reference, making it ready to use. When the variable is set to null or goes out of scope, the object has no references and becomes eligible for garbage collection. Finally, the garbage collector frees the memory. Key points include understanding that the constructor runs before the object is usable, and that garbage collection happens later, not immediately after losing references.

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