An abstract class constructor can also be used to execute code that is relevant for every child class. This prevents code duplication. We cannot create an instance of an abstract class,But we can create instances of classes those are derived from the abstract class.
How do constructors work in Java?
Each time an object is created using a new () keyword, at least one constructor (it could be the default constructor) is invoked to assign initial values to the data members of the same class. Rules for writing constructors are as follows: The constructor (s) of a class must have the same name as the class name in which it resides.
Do nested classes have a default constructor?
In fact, ALL classes, including nested and anonymous classes, will get a default constructor if one is not specified (in the case of anonymous classes it is impossible to specify one, so you will always get the default constructor). A good example of an abstract class having a constructor is the Calendar class.
Can a class have no constructor?
That is usually true, but not always. It is possible for a class to have no constructor. (An important distinction to draw here is that the JVM does not require all class files to have a constructor; however, any class defined in Java does have a default constructor if a constructor is not explicitly declared.
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Syntax
There are some additional syntax restrictions:
•A class method called constructor cannot be a getter, setter, async, or generator.
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Description
A constructor enables you to provide any custom initialization that must be done before any other methods can be called on an instantiated object.
If you don't provide your own constructor, then a default constructor will be supplied for you. If your class is a base class, the default constructor is empty:
If your class is a derived class, the default constructor calls the parent constructor, passing along any arguments that were provided:
That enables code like this to work:
The ValidationError class doesn't need an explicit constructor, because it doesn't need to do any custom initialization. The default constructor then takes care of initializing the parent Error from the argument it is given.
However, if you provide your own constructor, and your class derives from some parent class, then you must explicitly call the parent class constructor using super(). For example:
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Examples
Using the constructor
This code snippet is taken from the classes sample (live demo).
Calling super in a constructor bound to a different prototype
super() calls the constructor that's the prototype of the current class. If you change the prototype of the current class itself, super() will call the constructor that's the new prototype. Changing the prototype of the current class's prototype property doesn't affect which constructor super() calls.
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The main rule of constructors is that they should have the same name as the class. A class can have more than one constructor.
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