Constructors in JAVA

A class contains constructors that are invoked to create objects from the class blueprint.

Constructor declarations look
like method declarations—except that they use the name of the class and have no return type.
For example, Bicycle has one constructor:

public Bicycle(int startCadence, int startSpeed, int startGear) {

	gear = startGear;

	cadence = startCadence;

	speed = startSpeed;

}

To create a new Bicycle object called myBike, a constructor is called by the new operator:

Bicycle myBike = new Bicycle(30, 0, 8);

new Bicycle(30, 0, 8) creates space in memory for the object and initializes
its fields.


Although Bicycle only has one constructor, it
could have others, including a no-argument constructor:

public Bicycle() {

	gear = 1;

	cadence = 10;

	speed = 0;

}

Bicycle yourBike = new Bicycle(); invokes the no-argument constructor to create a new Bicycle object called yourBike.


Both constructors could have been declared in Bicycle
because they have different argument lists. As with methods, the Java
platform differentiates constructors on the basis of the number
of arguments in the list and their types. You cannot write two
constructors that have the same number and type of arguments for
the same class, because the platform would not be able to tell
them apart. Doing so causes a compile-time error.


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.


You can use a superclass constructor yourself. The MountainBike class at the beginning of this
lesson
did just that. This will be discussed later, in the
lesson
on interfaces and inheritance.



You can use access modifiers in a
constructor's declaration to control which
other classes can call the constructor.