Constructors in Java | Constructor Chaining, Overloading & this | Java Full Course #13

Summary

The lesson explains how Java constructors initialize objects, including default and parameterized constructors, constructor overloading, the this keyword, and constructor chaining. Examples use a Student class with fields such as name, age, roll number, and college.

Instance Fields and Default Values

Fields declared in a class are instance variables associated with objects. If they are not explicitly initialized, Java assigns default values:

• Numeric primitives such as int receive 0.
• Floating-point primitives receive 0.0.
• boolean fields receive false.
• Reference fields such as String receive null.

Local Variables

Unlike instance fields, local variables do not receive default values. They must be assigned before use, or the compiler reports an error.

Constructor Basics

A constructor initializes an object. It has the same name as its class and has no return type—not even void. Constructors are invoked when an object is created with new.

Default Constructors

If a class declares no constructors, Java provides a no-argument default constructor. It leaves fields at their default values unless they are initialized elsewhere.

Once a class declares any constructor, Java no longer supplies the default no-argument constructor automatically.

Parameterized Constructors

Parameterized constructors accept values so fields can be initialized when an object is created, rather than assigned one by one afterward. This can also make certain values mandatory at creation time.

Constructor Overloading

A class can define multiple constructors with different parameter lists—for example, constructors that accept no values, a name, a name and age, or all of the student’s details. Java selects the matching constructor based on the arguments supplied.

The this Keyword

this refers to the current object. It helps distinguish fields from parameters with the same name:

this.name = name;

Constructor Chaining

A constructor can delegate to another constructor in the same class using this(...). This allows constructors to share initialization logic and reduces repeated code. The call to this(...) must be the first statement in the constructor.

Constructor Calls and Memory

Constructors cannot be called like ordinary methods. They are invoked during object creation or through constructor delegation.

Object creation requires heap memory. If allocation cannot be satisfied, Java can fail at runtime—typically with an OutOfMemoryError.

Tutorial and Review Points

The instructor demonstrates constructor syntax and behavior with Student class examples, including printing default field values, creating overloaded constructors, and tracing the order of chained constructor calls.

Interview-oriented topics include when Java generates a default constructor, why local variables must be initialized, restrictions on calling constructors, and what can happen if heap allocation fails.

Main speaker/source: A single instructor presenting the Java lesson on the Coder Army channel.