Constructor chaining in Java - Step-by-Step Execution

Concept Flow - Constructor chaining

Call Constructor A

↓

Constructor A calls Constructor B

↓

Constructor B executes

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Return to Constructor A

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Constructor A completes

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Object fully constructed

Constructor chaining means one constructor calls another constructor in the same class or parent class to reuse code and initialize the object step-by-step.

Execution Sample

Java

class Box {
  int width, height;
  Box() { this(10, 20); }
  Box(int w, int h) { width = w; height = h; }
}

This code shows a no-argument constructor calling another constructor with two arguments to set width and height.

Execution Table

Step	Constructor Called	Parameters	Action	State Change	Output
1	Box()	none	Calls Box(int w, int h) with (10, 20)	width=0, height=0 (initial)
2	Box(int w, int h)	10, 20	Sets width=10, height=20	width=10, height=20
3	Box()	none	Returns after Box(int, int) finishes	width=10, height=20
4	Box()	none	Constructor completes	width=10, height=20

💡 No more constructors to call; object construction finished with width=10 and height=20

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
width	0	0	10	10	10
height	0	0	20	20	20

Key Moments - 3 Insights

Why does the no-argument constructor call the two-argument constructor?

To avoid repeating code for setting width and height, the no-argument constructor reuses the two-argument constructor by calling it with default values (see execution_table step 1).

What happens if the constructor chaining is not done properly?

If a constructor does not call another properly, variables may remain uninitialized or default, causing incorrect object state (see variable_tracker showing initial zeros before step 2).

Can constructor chaining call constructors in parent classes?

Yes, using super() calls the parent class constructor. This example shows chaining within the same class using this().

Visual Quiz - 3 Questions

Look at the execution_table at step 2, what are the values of width and height?

A width=20, height=10

B width=0, height=0

C width=10, height=20

D width and height are uninitialized

💡 Hint

Check the 'State Change' column at step 2 in execution_table

At which step does the Box() constructor finish execution?

A Step 2

B Step 4

C Step 3

D Step 1

💡 Hint

Look for 'Constructor completes' in the Action column of execution_table

If the Box() constructor did not call Box(int w, int h), what would be the final values of width and height?

A width=0, height=0

B width=10, height=20

C width and height would be random

D width=20, height=10

💡 Hint

Refer to variable_tracker start values before any constructor runs

Concept Snapshot

Constructor chaining in Java:
- One constructor calls another using this() or super()
- Helps reuse code and set default values
- Called constructors run first, then return
- Ensures object fields are properly initialized
- Avoids code duplication in constructors

Full Transcript

Constructor chaining is when one constructor calls another constructor in the same class or parent class. This helps reuse code and initialize object fields step-by-step. For example, a no-argument constructor can call a two-argument constructor with default values. The called constructor runs first, sets the fields, then returns control to the original constructor. This ensures the object is fully initialized without repeating code. If chaining is missing, fields may remain at default values. Constructor chaining uses this() for same-class calls and super() for parent-class calls.