What is Java's Default Constructor and How Does it Differ from Explicitly Defined Constructors?

Java's Default Constructor: An In-Depth Explanation

A default constructor is a special type of constructor that is automatically generated by the compiler when a class does not have any explicitly defined constructors. It is a no-argument constructor that initializes all of the object's instance variables to their default values.

Identifying the Default Constructor

In the example provided, neither of the constructors is the default constructor. A default constructor would look like this:

public Module()
{
  super();
  this.name = null;
  this.credits = 0;
  this.hours = 0;
}

This constructor has no arguments and initializes all instance variables to their default values (null for String, 0 for int).

Difference from Other Constructors

The default constructor differs from other constructors in the following ways:

• It has no arguments.
• It initializes all instance variables to their default values.
• It is only generated if the class does not have any explicitly defined constructors.

Implications of Defining Other Constructors

If a class defines at least one explicitly defined constructor, the compiler will not generate a default constructor. This means that all objects of the class must be initialized using one of the explicitly defined constructors.

Java Language Specification

The Java Language Specification provides further details on the default constructor in Section 8.8.9.

Clarification

While the default constructor is responsible for initializing instance variables, it does not actually set them to specific default values. Instead, the Java Virtual Machine (JVM) sets them to the default values during object creation.

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