Constructors In Java
Constructor is a block of code which creates the
instance of the class. Constructor name should be the class name. It is same as
a method but without return type. Constructor will be invoked when we try to
create new object to a class with new
keyword. It returns object of the class, it implicitly done by the java
run-time, we should not add any return type to it. If we try to add return type
to a constructor it will be treated as a method but it will show a warning
saying “This
method has a constructor name”.
Constructors are three
types
Ø Default
Constructor
Ø No-Args
constructor
Ø Parameterized
constructor
Default
Constructor: as we see in the Test.java class we don’t have any constructor
then the JVM will implicitly add the
Default constructor which having the super() which calling the super class
constructor.
In the
above class Test.java we didn’t write any constructor but JVM will add the Default constructor. The JVM will add Default constructor only if the class don’t have any
other constructor in the class. The above class will be like as follows with a Default constructor.
No-Args constructor: A
constructor without any arguments is called No-args Constructor. It is used to
do some pre-initialization.
Out Put:
In the above class when we called new Test() then the No-argument
constructor is called .
Parameterized
constructor:
Constructor
with parameters called Parameterized constructor, which will create the object of
the class with parameters specified..
Out Put:
In the above class when the
line number 9 executed then the parameterized constructor will be called, and initialize
the instance variables and create an object with given parameters.
Note:
Constructor can be public, protected, default
and private, but it cannot be final and static.
Private Constructors are used
in singleton design patterns.