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Why Java always provides a default constructor to class when you use another constructor default constructor remove?
Wiki User
∙ 13y ago
Classes in Java inherit constructors from their parent classes. If you don't explicitly define a parent class, then Object is used, which has only the default empty constructor.
That "default" constructor is only there when defined by the parent class, so classes which do not have a no-argument constructor will not allow subclasses to automatically use it.
This is implemented this way because of the special nature of constructors. Java could not always provide a default constructor because it could not guarantee that all class members would be properly created or initialized.
Wiki User
∙ 13y ago
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Default constructor in java?
If you don't type a constructor into your class code, a default constructor will be automatically generated by the compiler. The default constructor is ALWAYS a no-arg constructor. (Obviously the compiler has no clue what all arguments you might want for your class. So it takes the safe way out with a no argument constructor) A no-arg constructor is not necessarily the default (i.e., compiler-supplied) constructor, although the default constructor is always a no-arg constructor. The default constructor is the one the compiler provides! While the default constructor is always a no-arg constructor, you're free to put in your own no-arg constructor.
What is the difference between default constructor and parameterized constructor?
A default constructor is one that has no parameters (C++ also calls constructors with all default parameters a default constructor), while a parameterized constructor is one that has at least one parameter without a default value. Default constructors can be provided by the compiler if no other constructors are defined for that class or any class the class inherits from, while parameterized constructors must always be defined by the developer.
How do you get a default constructor?
The default constructor is an empty (only call the super constructor) with no parameters constructor inserted by the java compiler when you don't define a constructor in your class. If you write something like this: public class NoConstructorClass{ //no constructor goes here } Then you get something like this: public class NoConstructorClass{ public NoConstructorClass(){ // Default constructor that you didn't write super(); } }
What is a constructoris it mandatory to use constructor in a class?
You always should define default constructor for your class. You must also define a copy constructor for your class if there are any pointers in the class. While it is not mandatory, failure to provide a default constructor can result in bad behavior, and failure to provide a copy constructor when you have pointers in the class will result in bad behavior. For example, without a default constructor, the compiler will not fully initialize the attributes of the class. It will initialize the virtual function table, and call base class constructors, but that is all - the attributes could be random garbage. For another example, without a copy constructor, the compiler will generate one that simply makes a bit wise copy of the attributes. If these attributes contain pointers, then you have two pointers to the same object, not necessarily a good thing, especially if one of them get "deleted".
What is the difference between implicit and explicit call of constructor in c plus plus?
An implicit constructor call will always call the default constructor, whereas explicit constructor calls allow to chose the best constructor and passing of arguments into the constructor.
Default constructor in java?
If you don't type a constructor into your class code, a default constructor will be automatically generated by the compiler. The default constructor is ALWAYS a no-arg constructor. (Obviously the compiler has no clue what all arguments you might want for your class. So it takes the safe way out with a no argument constructor) A no-arg constructor is not necessarily the default (i.e., compiler-supplied) constructor, although the default constructor is always a no-arg constructor. The default constructor is the one the compiler provides! While the default constructor is always a no-arg constructor, you're free to put in your own no-arg constructor.
What is a default parameterized constructor?
There is no such thing as a default parameterized constructor. The default constructor is always the 'no-arg' constructor and does not take any parameters or arguments as input
What is the difference between default constructor and parameterized constructor?
A default constructor is one that has no parameters (C++ also calls constructors with all default parameters a default constructor), while a parameterized constructor is one that has at least one parameter without a default value. Default constructors can be provided by the compiler if no other constructors are defined for that class or any class the class inherits from, while parameterized constructors must always be defined by the developer.
How do you get a default constructor?
The default constructor is an empty (only call the super constructor) with no parameters constructor inserted by the java compiler when you don't define a constructor in your class. If you write something like this: public class NoConstructorClass{ //no constructor goes here } Then you get something like this: public class NoConstructorClass{ public NoConstructorClass(){ // Default constructor that you didn't write super(); } }
What is a constructoris it mandatory to use constructor in a class?
You always should define default constructor for your class. You must also define a copy constructor for your class if there are any pointers in the class. While it is not mandatory, failure to provide a default constructor can result in bad behavior, and failure to provide a copy constructor when you have pointers in the class will result in bad behavior. For example, without a default constructor, the compiler will not fully initialize the attributes of the class. It will initialize the virtual function table, and call base class constructors, but that is all - the attributes could be random garbage. For another example, without a copy constructor, the compiler will generate one that simply makes a bit wise copy of the attributes. If these attributes contain pointers, then you have two pointers to the same object, not necessarily a good thing, especially if one of them get "deleted".
What is the difference between implicit and explicit call of constructor in c plus plus?
An implicit constructor call will always call the default constructor, whereas explicit constructor calls allow to chose the best constructor and passing of arguments into the constructor.
Is Java always provides a default constructor to a class?
Correct. If you omit a constructor, Java will assume that an empty one exists. Given the following code for a class: class MyClass { } You can make a call to the default constructor: MyClass mc = new MyClass(); Just keep in mind that the default constructor "goes away" if you implement another constructor. class MyClass { public MyClass(String str){ } } This line will now result in a "cannot find symbol" compiler error: MyClass mc = new MyClass();
What is overloaded constructor in oop?
Overloading a function simply means providing the same function name with different argument types. Class constructors are no different. In fact, even if you declare no constructors in a class, there will be two compiler-generated constructor overloads provided for you: a default constructor; and a copy constructor. If you declare any other constructors, the compiler-generated default constructor will no longer be generated. You must declare your own default constructor if you require one. The copy constructor is always generated, however the default implementation only performs a member-wise copy of the class members. If your class contains a pointer to allocated memory you must provide your own copy constructor to perform a deep-copy of those pointers, so each instances "owns" its own copy of the memory.
Why you require constructor in cpp?
Constructors are not a requirement of CPP. A default constructor and copy constructor are automatically generated by the compiler for every class you create when no constructors are declared. The only time you need to declare a constructor is when you wish to override the default behaviour of the generated constructors, to ensure the class is correctly initialised. When any constructor is declared, the default constructor is no longer generated by the compiler -- you must define your own default constructor (one that has no parameters, or where all the parameters have default values). The copy constructor is always generated for you regardless of how many other constructors you declare. But if the class contains pointers to allocated memory that is "owned" by the class then you must override the generated copy constructor with your own copy constructor. This is to ensure the memory is deep copied (the generated copy constructor only performs a shallow, member-wise copy of the members). Otherwise two objects of the same class will end up pointing at the same memory, which would be disastrous if either one were to be deleted. The other instance would be automatically invalidated because it would point to memory that was released by the other instance's destructor.
What is Programmer supplied default constructor and a system supplied constructor in C plus plus?
A programmer-supplied default constructor is one that you, the programmer, explicitly implements in a class definition. A system-supplied constructor is one that is generated for you automatically by the compiler and are known as compiler-generated constructors for that reason.If you do not define any constructors, the compiler will automatically generate 3 constructors for you:a default constructora copy constructora move constructorThe default constructor is a constructor that requires no arguments. The compiler-generated default constructor invokes the default constructor for each data member but primitive data type members do not have constructors and are therefore left in an uninitialised state unless we use inline initialisation.The copy constructor accepts a constant l-value reference to an existing object of the same type. Data members are initialised using member-wise copy semantics. Uninitialised data members remain uninitialised in both objects.The move constructor accepts a non-constant r-value reference to an existing object of the same type. Individual data members are initialised using move semantics where available, otherwise copy semantics are used (as per the copy constructor).If you define any constructor, the compiler will not generate a default constructor unless you define your own. The compiler-generated copy and move constructors are always generated unless you provide your own implementation for one or both.It is considered good practice to explicitly declare all three constructors in order to make it clear to the class maintainer whether constructors are compiler-generated, user-defined, or deleted. For example:class A {public:A ()=default;A (const A&)=delete;A (A&&);// ...};A::A (A&&) {// ...}In the above example, the default constructor is compiler-generated (=default), the copy constructor is disabled (=delete) and the move constructor is user-defined.In most cases, the compiler-generated constructors will suffice, but if you need to provide a non-default constructor it's easy to forget that the default constructor will not be generated. Therefore get into the habit of being explicit. In the majority of cases, your class declarations will take the following form:class A { public:A ()=default;A (const A&)=default;A (A&&)=default;// ...};If you genuinely do not need a default constructor then declare it with the =delete postscript and provide your own non-default constructor (all classes must have at least one constructor that is neither a copy or a move constructor). Note that a user-defined constructor that takes one or more arguments where every argument has a default value is also a default constructor.In addition to copy and move constructors, the compiler also generates corresponding copy and move assignment operators as well as a destructor. Treat all compiler-generated code as a unified set; in most cases your classes will have the following form:class A {public:A ()=default;~A ()=default;A (const A&)=default;A& operator= (const A&)=default;A (A&&)=default;A& operator= (A&&)=default;// ...};There will be cases where the compiler-generated behaviour is undesirable. For instance, if your class acquires a resource, you will want to override all compiler-generated code with your own implementations in order to manage that resource. However, if you use smart pointers or resource handles rather than "naked" pointers, the compiler-generated behaviour provides exactly the semantic you want. The only time you really need to override that behaviour is when defining your own resource handles or smart pointers.Another reason to override the default construction behaviour is when you need to perform additional tasks that are not provided by the default semantics, such as when testing a class invariant. The copy/move constructors and assignment operators never need to test an invariant so the default semantic usually suffices, but all other constructors, assignment operators and other mutators (setters) must. Ideally, every class should have one and only one invariant at most. A string class, for instance, has an invariant in that it must always refer to a null-terminated string, even when the string is empty. By delegating that invariant to the string class itself, other classes can embed string objects without having to worry about the string invariant, they need only worry about their own invariant (if any).
What is the method of constructor overloading?
The first thing to note about constructor overloading is that Java creates a no argument constructor for you if and only if you have not typed a constructor yourself. Every class has a constructor even abstract ones (default no argument constructor). Abstract constructors are always executed. To overload a constructor you can do the following: class Test { String name; Test(String n) { name = n; System.out.println("Constructing Test Object named: " + name); } } In the case above we are overloading the default no argument constructor with a constructor that takes a String parameter. You can write you own no argument constructor as follows: class Test { Test() { System.out.println("Constructing Test Object"); } } To override our own no argument constructor we do this: class Test { Test() { // our no argument constructor System.out.println("Constructing Test Object"); } String name; Test(String n) { // overloading our no argument constructor with this // constructor that take a String parameter name = n; System.out.println("Constructing Test Object named: " + name); } }
Is it mandatory to use the construtors in a class in c plus plus?
No. If you do not provide a default constructor, the compiler will provide a default constructor that simply allocates memory for the class, but it will not initialize the members of the class. If you do not provide a copy constructor, then the compiler will provide a copy constructor that allocates memory for the class, and then copies the member's data from class to class. This is bad if the class contains pointers, because only the pointer will be copied - the objects to which the pointers point will not be copied - and you could wind up deleting an object and then using it after deletion, with potentially devastating consequences. So, yes, it is mandatory, from a good practices point of view, and just plain mandatory when the class has pointers, to always provide a default constructor and a copy constructor, along with the appropriate destructor.
