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Answered 2010-08-10 08:18:39

Any time you don't write any constructor for your class, the compiler supplies the default constructor.

In other words, if you wrote a constructor the compiler won't.

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When the programmer has not coded a constructor for any java class.


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.


if we write a code in java then we don't mansion a default constructor in it then compiler by default provide a default constructor and the main impotent role of default constructor in our program is that it initialize the member of the class then it is reason compiler provide a default constructor in our java program code .


A default constructor is generated for a class if we did not specify any constructor for it. A default constructor is a public constructor that takes no arguments. For example: public class Test { } The compiler will turn this into: public class Test { public Test() { ... } }


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)Determine Whether a Default Constructor Will Be CreatedThe following example shows a Lamborghini class with two constructors:class Lamborghini {Lamborghini() { }Lamborghini(String name) { }}Will the compiler put in a default constructor for the class above? "No!" What about for the following variation of the class?class Lamborghini {Lamborghini(String name) { }}Now will the compiler insert a default constructor? "No Again!" What about this class?class Lamborghini { }Now we're talking. The compiler will generate a default constructor for the preceding class, because the class doesn't have any constructors defined. OK, what about this one below?class Lamborghini {void Lamborghini() { }}You might be tempted to say that, the compiler won't create one, since there already is a constructor in the Lamborghini class? Take another look at the Lamborghini class.What's wrong with the Lamborghini() constructor? It isn't a constructor at all! It's simply a method that happens to have the same name as the class. Remember, the return type is a dead straight way to tell us that we're looking at a method, and not a constructor. So, here again the compiler will put the default no-arg constructor in the class.


The compiler places a default no-arg constructor in any java class that does not have an explicit constructor coded into it. for ex: public class Car { ... ... //lots of code but no constructor } In the above case, the compiler will place the below constructor into the code: public Car() { super(); } But, if you have a constructor in your class that takes arguments then the compiler will not put the default constructor. Ex: public class Car { public Car(String name){ ... } ... //lots of code } Above, we have a Car constructor that takes a string name as argument. so, the compiler wont put the default constructor in the code. now, if you try to do: Car obj = new Car(); you will get an error because this constructor is not defined.


When any constructor is deffined in your class, the java compiler create a default no argument constructor for you. This constructor only have an invocation to the super class constructor (" super( ) ").


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(); } }


Determine Whether a Default Constructor Will Be CreatedThe following example shows a Lamborghini class with two constructors:class Lamborghini {Lamborghini() { }Lamborghini(String name) { }}Will the compiler put in a default constructor for the class above? "No!" What about for the following variation of the class?class Lamborghini {Lamborghini(String name) { }}Now will the compiler insert a default constructor? "No Again!" What about this class?class Lamborghini { }Now we're talking. The compiler will generate a default constructor for the preceding class, because the class doesn't have any constructors defined. OK, what about this one below?class Lamborghini {void Lamborghini() { }}You might be tempted to say that, the compiler won't create one, since there already is a constructor in the Lamborghini class? Take another look at the Lamborghini class.What's wrong with the Lamborghini() constructor? It isn't a constructor at all! It's simply a method that happens to have the same name as the class. Remember, the return type is a dead straight way to tell us that we're looking at a method, and not a constructor. So, here again the compiler will put the default no-arg constructor in the class.


The following example shows a Lamborghini class with two constructors: class Lamborghini { Lamborghini() { } Lamborghini(String name) { } } Will the compiler put in a default constructor for the class above? "No!" What about for the following variation of the class? class Lamborghini { Lamborghini(String name) { } } Now will the compiler insert a default constructor? "No Again!" What about this class? class Lamborghini { } Now we're talking. The compiler will generate a default constructor for the preceding class, because the class doesn't have any constructors defined. OK, what about this one below? class Lamborghini { void Lamborghini() { } } You might be tempted to say that, the compiler won't create one, since there already is a constructor in the Lamborghini class? Take another look at the Lamborghini class. What's wrong with the Lamborghini() constructor? It isn't a constructor at all! It's simply a method that happens to have the same name as the class. Remember, the return type is a dead straight way to tell us that we're looking at a method, and not a constructor. So, here again the compiler will put the default no-arg constructor in the class.


That depends on how you define body of class. If you do not define constructors then compiler will provide default constructor and it is not overloaded. If you create your own constructor you automatically overload default constructor.


It is also called the default constructor. It is the constructor that the java compiler places in every class where the programmer has not explicitly coded a constructor. Ex: public class Car { ... //lots of code but no constructor } If a person writes a class as above, then the compiler places this code in the compiled version of the file. public Car() { super(); } This is called the non-parameterized or default 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.


Nothing happens. The compiler successfully compiles the class. When a class does not have a specific constructor, the compiler places a default no argument construtor in the class and allows you to compile and execute the class. public class Test { } and public class Test { public Test(){ } } are one and the same.


An object is an instance of a class. You create an object through instantiation. // Define the class... class Foo {/*...*/}; // Instantiate the class... Foo x; When you instantiate a class, a class constructor is invoked. If you do not declare any constructors in your class, the compiler generates a default constructor, a copy constructor and a move constructor, all of which have public access. Thus the above definition if Foo is equivalent to the following definition: // Define the class... class Foo { public: Foo (); // default constructor Foo (const Foo&); // copy constructor Foo (Foo&&); // move constructor }; In most cases, the compiler-generated constructors will suffice, so we only need to define constructors when we wish to override the default implementation of the compiler-generated constructors. However, if we define any constructor that accepts one or more arguments, the compiler-generated default constructor is no longer generated; we must explicitly define one if we require one. The compiler-generated copy and move constructors are always generated unless we explicitly delete them or override them with our own implementations. Note that constructors are used specifically to initialise the class invariants.


Overloading a constructor means typing in multiple versions of the constructor, each having a different argument list, like the following examples: class Car { Car() { } Car(String s) { } } The preceding Car class has two overloaded constructors, one that takes a string, and one with no arguments. Because there's no code in the no-arg version, it's actually identical to the default constructor the compiler supplies, but remember-since there's already a constructor in this class (the one that takes a string), the compiler won't supply a default constructor. If you want a no-arg constructor to overload the with-args version you already have, you're going to have to type it yourself, just as in the Car example. Overloading a constructor is typically used to provide alternate ways for clients to instantiate objects of your class. For example, if a client knows the Car name, they can pass that to a Car constructor that takes a string. But if they don't know the name, the client can call the no-arg constructor and that constructor can supply a default name.


In the case of Java, not always. The class does require a constructor, but the compiler will automatically include an "empty constructor" (constructor without parameters) under certain conditions.In the case of Java, not always. The class does require a constructor, but the compiler will automatically include an "empty constructor" (constructor without parameters) under certain conditions.In the case of Java, not always. The class does require a constructor, but the compiler will automatically include an "empty constructor" (constructor without parameters) under certain conditions.In the case of Java, not always. The class does require a constructor, but the compiler will automatically include an "empty constructor" (constructor without parameters) under certain conditions.


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).


Overloading a constructor means typing in multiple versions of the constructor, each having a different argument list, like the following examples: class Car { Car() { } Car(String s) { } } The preceding Car class has two overloaded constructors, one that takes a string, and one with no arguments. Because there's no code in the no-arg version, it's actually identical to the default constructor the compiler supplies, but remember-since there's already a constructor in this class (the one that takes a string), the compiler won't supply a default constructor. If you want a no-arg constructor to overload the with-args version you already have, you're going to have to type it yourself, just as in the Car example. Overloading a constructor is typically used to provide alternate ways for clients to instantiate objects of your class. For example, if a client knows the Car name, they can pass that to a Car constructor that takes a string. But if they don't know the name, the client can call the no-arg constructor and that constructor can supply a default name


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.


Default constructor in Java initialises all instance variables with default values. class program { int a; // default constructor initialises a to 0 }


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.


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();


Overloading a constructor means typing in multiple versions of the constructor, each having a different argument list, like the following examples: class Car { Car() { } Car(String s) { } } The preceding Car class has two overloaded constructors, one that takes a string, and one with no arguments. Because there's no code in the no-arg version, it's actually identical to the default constructor the compiler supplies, but remember-since there's already a constructor in this class (the one that takes a string), the compiler won't supply a default constructor. If you want a no-arg constructor to overload the with-args version you already have, you're going to have to type it yourself, just as in the Car example. Overloading a constructor is typically used to provide alternate ways for clients to instantiate objects of your class.


Default constructor: X()Copy constructor: X(const X&)Copy assignment operator: X& operator=(const X&)Move constructor: X(X&&)Move assignment operator: X& operator=(XX&)Destructor: ~X()By default, the compiler will generate each of these operations if a program uses it. However, if the programmer declares any constructor for a class, the default constructor for that class is not generated. If the programmer declares a copy or move operation, no copy, move or destructor is generated. If the programmer declares a destructor, no move operation is generated (a copy constructor is generated for backward compatibility).We can also suppress generation of specific operations with the =delete pseudo-initialiser:class X {public:X (const X&) =delete; // suppress the compiler-generated copy operationsX& operator=(const X&) =delete;// ...};