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.