如果对象是超类且无参数构造函数具有空体，那么默认构造函数如何用于实例化对象？

Question

default constructor invoke no_argument constructor in super class reference and this last constructor used to instantiate object from this class, How ? i mean what does superclass constructor body exactly do ?

example

public class Child extend Fragment {}

if i tried to instantiate this class Child

Child a = new Child() ;

while compiling compiler invoke no-argument constructor in Fragment class . in android.app package i found that no-argument constructor has no body

public Fragment() {}

Question how could this constructor be useful ? it do nothing !

PS i mean by

how could this constructor be useful?

how does it instantiate its class although it empty , where is the code that instantiate super class?

Answer 1

The syntax for calling a superclass constructor is

super();

or:

 super(parameter list);

Question how could this constructor be useful ? it do nothing !

The super constructor could be used to initialized properties common for all subclasses. If it's empty it still can be used by reflection API.

Fragment.class.getConstructor().newInstance();

or

Child a = new Child() ;
a.getClass().getSuperclass().getConstructor().newInstance();

the last line explicitly invokes a constructor of the superclass.

Answer 2

Question how could this constructor be useful ? it do nothing !

The purpose of a constructor, whether empty or not, is to provide an instance of the class. The constructor body, if it exists, may further do additional operations necessary when first creating the instance.

The constructor may not be doing anything itself but it returns an instance, on which you may further call public methods defined in the class.

For example, in your mentioned Fragment class, let's suppose there is an attach() method. If the attach() method is non-static, you will need to create an instance (using the empty constructor) to call the attach() method on it irrespective of whether the constructor does anything or not.

how does it instantiate its class although it empty

A constructor doesn't need any code in its body to be instantiated. Calling new ClassName() (with appropriate parameters, if any) is enough. It is the job of JVM to create the object at runtime. You don't need to write any specific code inside a constructor for that. That extra code, if you wish, can help the instance start off with a specific state by setting some fields or calling methods, etc.

where is the code that instantiate super class?

A constructor in a subclass (or any class, for that matter) only generates an instance of that class. It doesn't need to instantiate its super class. You may call the super class constructor (using super() ) if it has some important initialization code, but it's not compulsory. Also, subclasses don't inherit constructors from super classes. But they do inherit the public and protected fields and methods.

Regarding the Fragment class, you have have some confusion regarding its use which can be clarified here .

Answer 3

an empty constructor (aka default constructor) means only that an instance can be created without any further work. This can be due to framework requirements or the instantation is ok with the default values for the attributes.

If the empty constructor is the only constructor in the class, it is not required to write it, its there by default.

Although if there are other constructors (with parameters), one must explicitly state the default constructor.