When is a constructor called a default constructor? [duplicate]

Question

Which of the below classes will use a default constructor, when we initialize an object from them?

class X {} class Y { Y () {} } class Z { Z(int i ) {} }

class Z will not use a default constructor. class X will use a default constructor.

But what about Y? is a user defined empty constructor called a default constructor? Like they say on wikipedia (Java section) http://en.wikipedia.org/wiki/Default_constructor

Or should there be no constructor defined in the class when we can speak of a default constructor when initializing an object?

Bohemian · Accepted Answer · 2014-02-21 08:46:41Z

The "default" constructor is the no-args (no arguments) constructor.

If you don't declare any constructors, an implicit no-args constructor will be defined.

If you declare any constructors, the implicit no-args constructor is not defined.

A constructor is always called when constructing a new object, and a constructor of every superclass of the class is also called. If no constructor is explicitly called, the default constructor is called (which may or may not be declared).

Abimaran Kugathasan · Accepted Answer · 2014-02-21 08:51:33Z

If you have define a empty argument constructor to a Class, then user defined empty argument constructor will replace default constructor which generated by compiler.

The default constructor is the no-argument constructor automatically generated unless you define another constructor with no argument. Check JLS 8.8.9 for more details

is a user defined empty constructor called a default constructor?

No, It's called used defined constructor.

Or should there be no constructor defined in the class when we can speak of a default constructor when initializing an object?

Default constructor mean, Compiler generated, no argument constructor. Every other Constructors are called used defined constructors.

A default constructor is a constructor that either has no parameters, or if it has parameters, all the parameters have default values. If no user-defined constructor exists for a class A and one is needed, the compiler implicitly declares a default parameterless constructor.

Nambi · Accepted Answer · 2014-02-21 08:46:08Z

DefaultConstructor JLS

The default constructor is the one Java provides by default. Anything you write explicitly is not a default.

Suresh Atta · Accepted Answer · 2014-02-21 08:47:17Z

Default Constructor comes in to picture then and then only,

When You haven't provide any constructors for your class

Prefer to read :Providing Constructors for Your Classes

You don't have to provide any constructors for your class, but you must be careful when doing this. The compiler automatically provides a no-argument, default constructor for any class without constructors. This default constructor will call the no-argument constructor of the superclass. In this situation, the compiler will complain if the superclass doesn't have a no-argument constructor so you must verify that it does. If your class has no explicit superclass, then it has an implicit superclass of Object, which does have a no-argument constructor.

Ruchira Gayan Ranaweera · Accepted Answer · 2014-02-21 08:48:09Z

If you don't define any constructor default constructor will come in to the scene. every other case you have to go with what ever the define constructors.

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