What's the Difference Between Default and User-Defined Constructors in Java?

Default Constructors in Java

In Java, a default constructor is automatically generated if no other constructors are defined. It is a no-argument constructor that initializes fields to their default values. The following code does not define a default constructor:

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

This is a user-defined constructor that takes arguments. A default constructor would look like this:

public Module() {}

Or, if field types are primitive:

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

Key Differences

The primary difference between a default constructor and a user-defined constructor is:

• Default constructors do not require arguments, while user-defined constructors can have arguments.
• Default constructors automatically initialize fields to their default values (e.g., null for objects, 0 for numeric primitives), while user-defined constructors allow for custom initialization.

Default Constructor vs. No Constructor

Having no constructor at all is equivalent to having a default constructor. When there are no constructors, the Java compiler automatically generates a default one with no arguments and no field initialization.

Note:

• If any constructors are defined in a class, the default constructor is not generated.
• Default constructors can only be created implicitly, not explicitly.

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