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Adrian Foster
Adrian Foster

Constructor [UPDATED]


A constructor in Java is a special method that is used to initialize objects. The constructor is called when an object of a class is created. It can be used to set initial values for object attributes:




constructor


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The following example adds an int y parameter to the constructor. Inside the constructor we set x to y (x=y). When we call the constructor, we pass a parameter to the constructor (5), which will set the value of x to 5:


Note: The difference between an explicit constructor like the one above and the default constructor is that the latter doesn't actually invoke the array iterator through argument spreading.


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.


Within the constructor body, you can access the object being created through this and access the class that is called with new through new.target. Note that methods (including getters and setters) and the prototype chain are already initialized on this before the constructor is executed, so you can even access methods of the subclass from the constructor of the superclass. However, if those methods use this, the this will not have been fully initialized yet. This means reading public fields of the derived class will result in undefined, while reading private fields will result in a TypeError.


If the parent class constructor returns an object, that object will be used as the this value on which class fields of the derived class will be defined. This trick is called "return overriding", which allows a derived class's fields (including private ones) to be defined on unrelated objects.


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 of the new prototype. Changing the prototype of the current class's prototype property doesn't affect which constructor super() calls.


\n The ValidationError class doesn't need an explicit constructor, because it doesn't need to do any custom initialization.\n The default constructor then takes care of initializing the parent Error from the argument it is given.\n


If the parent class constructor returns an object, that object will be used as the this value on which class fields of the derived class will be defined. This trick is called \"return overriding\", which allows a derived class's fields (including private ones) to be defined on unrelated objects.


The constructor data property of an Object instance returns a reference to the constructor function that created the instance object. Note that the value of this property is a reference to the function itself, not a string containing the function's name.


But even when Object.getPrototypeOf(a).constructor is re-assigned, it won't change other behaviors of the object. For example, the behavior of instanceof is controlled by Symbol.hasInstance, not constructor:


There is nothing protecting the constructor property from being re-assigned or shadowed, so using it to detect the type of a variable should usually be avoided in favor of less fragile ways like instanceof and Symbol.toStringTag for objects, or typeof for primitives.


Every constructor has a prototype property, which will become the instance's [[Prototype]] when called via the new operator. ConstructorFunction.prototype.constructor will therefore become a property on the instance's [[Prototype]], as previously demonstrated.


However, ensuring that Child.prototype.constructor always points to Child itself is crucial when some caller is using constructor to access the original class from an instance. Take the following case: the object has the create() method to create itself.


Note: Manually updating or setting the constructor can lead to different and sometimes confusing consequences. To prevent this, just define the role of constructor in each specific case. In most cases, constructor is not used and reassigning it is not necessary.


Whenever an instance of a class or a struct is created, its constructor is called. A class or struct may have multiple constructors that take different arguments. Constructors enable the programmer to set default values, limit instantiation, and write code that is flexible and easy to read. For more information and examples, see Instance constructors and Using constructors.


A constructor is a method whose name is the same as the name of its type. Its method signature includes only an optional access modifier, the method name and its parameter list; it does not include a return type. The following example shows the constructor for a class named Person.


If a constructor can be implemented as a single statement, you can use an expression body definition. The following example defines a Location class whose constructor has a single string parameter named name. The expression body definition assigns the argument to the locationName field.


The previous examples have all shown instance constructors, which create a new object. A class or struct can also have a static constructor, which initializes static members of the type. Static constructors are parameterless. If you don't provide a static constructor to initialize static fields, the C# compiler initializes static fields to their default value as listed in the Default values of C# types article.


In class-based, object-oriented programming, a constructor (abbreviation: ctor) is a special type of subroutine called to create an object. It prepares the new object for use, often accepting arguments that the constructor uses to set required member variables.


A constructor resembles an instance method, but it differs from a method in that it has no explicit return type, it is not implicitly inherited and it usually has different rules for scope modifiers. Constructors often have the same name as the declaring class. They have the task of initializing the object's data members and of establishing the invariant of the class, failing if the invariant is invalid. A properly written constructor leaves the resulting object in a valid state. Immutable objects must be initialized in a constructor.


Most languages allow overloading the constructor in that there can be more than one constructor for a class, with differing parameters. Some languages take consideration of some special types of constructors. Constructors, which concretely use a single class to create objects and return a new instance of the class, are abstracted by factories, which also create objects but can do so in various ways, using multiple classes or different allocation schemes such as an object pool.


Constructors that can take at least one argument are termed as parameterized constructors. When an object is declared in a parameterized constructor, the initial values have to be passed as arguments to the constructor function. The normal way of object declaration may not work. The constructors can be called explicitly or implicitly. The method of calling the constructor implicitly is also called the shorthand method.


If the programmer does not supply a constructor for an instantiable class, Java compiler inserts a default constructor into your code on your behalf. This constructor is known as default constructor. You would not find it in your source code (the java file) as it would be inserted into the code during compilation and exists in .class file. The behavior of the default constructor is language dependent. It may initialize data members to zero or other same values, or it may do nothing at all. In Java, a "default constructor" refer to a nullary constructor that is automatically generated by the compiler if no constructors have been defined for the class or in the absence of any programmer-defined constructors (e.g. in Java, the default constructor implicitly calls the superclass's nullary constructor, then executes an empty body). All fields are left at their initial value of 0 (integer types), 0.0 (floating-point types), false (boolean type), or null (reference types). 041b061a72


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