Constructors Used in Inheritance Hierarchies

Syntax and Signature

A java constructor has the same name as the name of the class to which it belongs. Constructor’s syntax does not include a return type, since constructors never return a value.

Constructors may include parameters of various types. When the constructor is invoked using the new operator, the types must match those that are specified in the constructor definition.

Calling

When you create an object of a class you automatically call the constructor. The one called depends upon the signature provided.

Overloading

Like methods, constructors can also be overloaded. Since the constructors in a class all have the same name as the class, their signatures are differentiated by their parameter lists.

super Keyword

Every constructor calls its superclass constructor. An implied super() is therefore included in each constructor which does not include an explicit super() call as its first statement. The super() statement invokes a constructor of the super class.

The implicit super() can be replaced by an explicit super(). The super statement must be the first statement of the constructor.

The explicit super allows parameter values to be passed to the constructor of its superclass and must have matching parameter types A super() call in the constructor of a subclass will result in the call of the relevant constructor from the superclass, based on the signature of the call.

Default Constructor

Java provides a default constructor which takes no arguments and performs no special actions or initializations, when no explicit constructors are provided.

The only action taken by the implicit default constructor is to call the superclass constructor using the super() call. Constructor arguments provide you with a way to provide parameters for the initialization of an object.

You don't have to provide any constructors for your class, but you must be careful when doing this. The compiler automatically provides a no-argument, default constructor for any class without constructors.

This default constructor will call the no-argument constructor of the superclass. In this situation, the compiler will complain if the superclass doesn't have a no-argument constructor so you must verify that it does.

If your class has no explicit superclass, then it has an implicit superclass of Object, which does have a no-argument constructor. The constructor of the Object class performs no actions.