Constructors

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Constructors

Constructors in Java can be seen as Methods in a Class. But there is a big difference between Constructors and Methods. These differences can be defined in terms of purpose, syntax and Invocation.

Purpose of Constructor (Vs Method)
Constructors have only one purpose in life and that is to create an Instance of a Class. This instantiation includes memory allocation and member initialization(Optional).
In contrast, methods cannot be used to create an Instance of a Class.

Syntax of Constructor(Vs Method)

Syntax of constructor is different then Method in below aspects

  • Constructors cannot have Non Access Modifier while methods can have.
  • Constructors cannot have a return type(Not even void) while methods must have one.
  • Constructor name must be the same as Class name while methods can have different names.
  • As per Java naming convention, method names should be camelcase while constructor names should start with caps letter.

Note*: A method can have the same name as that of Class name.

Invocation of Constructor(Vs Method)
There is a difference between how constructors and methods are called. Constructors cannot be called explicitly, constructor will be invoked implicitly when the instance of the class is getting created(Using new Keyword)

Constructor Invocation Example

Method Invocation Example

A constructor in a class has the same name as that of the given class. 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. 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.

Constructor Rules

  • A constructor cannot have a return type.
  • Constructor must have the same name as that of the class.
  • Constructors cannot be marked static
  • Constructor cannot be marked abstract
  • Constructor cannot be overridden.
  • Constructor can be Final.

If a class defines an explicit constructor, it no longer has a default constructor to set the state of the objects. If such a class requires a default constructor, its implementation must be provided. Any attempt to call the default constructor will be a compile time error if an explicit default constructor is not provided in such case.

Constructor 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.

It is possible to use this() construct, to implement local chaining of constructors in a class. The this() call in a constructor invokes the other constructor with the corresponding parameter list within the same class. Java requires that any this() call must occur as the first statement in a constructor.

Constructor Chaining:
Every constructor calls its superclass constructor. An implied super() is therefore included in each constructor which does not include either this() or 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. This is called constructor chaining.

The super() construct as with this() construct: if used, must occur as the first statement in a constructor, and it can only be used in a constructor declaration. This implies that this() and super() calls cannot both occur in the same constructor. Just as the this() construct leads to chaining of constructors in the same class, the super() construct leads to chaining of subclass constructors to superclass constructors. if a constructor has neither a this() nor a super() construct as its first statement, then a super() call to the default constructor in the superclass is inserted.

If a class only defines non-default constructors, then its subclasses will not include an implicit super() call. This will be flagged as a compile-time error. The subclasses must then explicitly call a superclass constructor, using the super() construct with the right arguments to match the appropriate constructor of the superclass.

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