This question already has answers here:
Can an abstract class have a constructor?
(22 answers)
Closed 9 years ago.
Why does an abstract class in Java have a constructor?
What is it constructing, as we can't instantiate an abstract class?
Any thoughts?
A constructor in Java doesn't actually "build" the object, it is used to initialize fields.
Imagine that your abstract class has fields x and y, and that you always want them to be initialized in a certain way, no matter what actual concrete subclass is eventually created. So you create a constructor and initialize these fields.
Now, if you have two different subclasses of your abstract class, when you instantiate them their constructors will be called, and then the parent constructor will be called and the fields will be initialized.
If you don't do anything, the default constructor of the parent will be called. However, you can use the super keyword to invoke specific constructor on the parent class.
Two reasons for this:
1) Abstract classes have constructors and those constructors are always invoked when a concrete subclass is instantiated. We know that when we are going to instantiate a class, we always use constructor of that class. Now every constructor invokes the constructor of its super class with an implicit call to super().
2) We know constructor are also used to initialize fields of a class. We also know that abstract classes may contain fields and sometimes they need to be initialized somehow by using constructor.
All the classes including the abstract classes can have constructors.Abstract class constructors will be called when its concrete subclass will be instantiated
Because another class could extend it, and the child class needs to invoke a superclass constructor.
I guess root of this question is that people believe that a call to a constructor creates the object. That is not the case. Java nowhere claims that a constructor call creates an object. It just does what we want constructor to do, like initialising some fields..that's all. So an abstract class's constructor being called doesn't mean that its object is created.
Because abstract classes have state (fields) and somethimes they need to be initialized somehow.
Implementation wise you will often see inside super() statement in subclasses constructors, something like:
public class A extends AbstractB{
public A(...){
super(String constructorArgForB, ...);
...
}
}
Related
This question already has answers here:
Why call super() in a constructor?
(8 answers)
Closed 5 years ago.
I'm a Java beginner learning about Java compiler rules as below:
If the class has no super class, extend it to Object class
If the class has no constructor, add a default no-parameter constructor
If the first line of the constructor is not "super()" or "this()", add "super()" to call the default constructor of the super class.
I understand that all objects we create are derived from the super class Object.
My question is what does the constructor in the Object class do when called?
Edit: My question is about what the constructor does in the class Object? I'm aware that subclasses call superclass's constructor by default.
For example, I have the following code (where I explicitly extend to Object and call super() to illustrate what the compiler does). My question is, what does the call to super() do?
public class Person extends Object
{
private String name;
public Person(String n)
{
super();
this.name = n;
}
}
My question is, what does the call to super() do?
It calls the default constructor for java.lang.Object. And to answer what you seem to be really asking, from the Java Language Specification, #8.8.9
8.8.9. Default Constructor
If a class contains no constructor declarations, then a default constructor is implicitly declared. The form of the default constructor for a top level class, member class, or local class is as follows:
The default constructor has the same accessibility as the class (§6.6).
The default constructor has no formal parameters, except in a non-private inner member class, where the default constructor implicitly declares one formal parameter representing the immediately enclosing instance of the class (§8.8.1, §15.9.2, §15.9.3).
The default constructor has no throws clauses.
If the class being declared is the primordial class Object, then the default constructor has an empty body. Otherwise, the default constructor simply invokes the superclass constructor with no arguments.
Note the final paragraph.
All Java classes that don't have any explicit superclass, extend from Object, except for Object itself. There's no need to make it explicit in your code, the compiler will take care of.
Also, the super() call in the constructor will invoke the constructor of Object. If you look at the implementation, you'll see that Object doesn't have any constructor at all, so it'll use an implicit empty constructor which does nothing. Details such as constructing a vtable to make sure that the inherited methods are there to use or initialising the object's monitor are not part of the constructor but are performed by the JVM implementation when the new operator is called.
public Object() {
}
The Object class has few methods that come handy in most of the classes. For example, the infamous toString() is implemented in the Object class. Also, the hashCode() is implemented there.
I'd recommend you to have a deep look at the Object.class file to understand what methods are inherited in every single Java class.
Regarding your 3 rules at the top, are "good" for academic purposes but never used in reality. You'll never see an explicit extends Object, and only call the super() or this() when you need to overwrite the default behaviour.
Also, avoid abusing of inheritance and use more composition (implement an interface), although, it depends on the every use case.
For example, I have the following code (where I explicitly extend to
Object and call super()). My question is, what does the call do
super() do?
Actually, Object() does nothing.
Now you should not reduce the constraint to invoke super() (with args or not) for any subclass constructor to the case where you have a class that doesn't extend a class explicitly.
This case is the only one where invoking super() may seem not really useful.
But as soon a class extends a class distinct from Object, this constraint makes really sense as the child construction has to apply first its parent construction.
But the language specifies this point in a general way. It has no exception for classes that extend directly Object.
Probably because, it makes things more simple and that is not a real constraint as the compiler adds for you the call to the super default constructor : super().
class child
{
child()
{
super();
System.out.println("Hello");
}
public static void main(String arg[])
{
child obj=new child();
}
}
In this code when I a create an object of class child , the child constructor is called. But why is it not giving error as there is no parent class. What does the super() do here?
Whose constructor is the super() keyword calling?
In Java every object implicitly extends Object. Calling super here will just call Object's constructor. On another note you should really abide by naming conventions such as capitalizing class names.
It is calling the constructor of the Object class as all object in java extends by default to the Object class.
From documentation:
Class Object is the root of the class hierarchy. Every class has Object as a superclass.
All objects, including arrays, implement the methods of this class.
You're invoking the Object method, from which all other classes descend eventually.
First up, clarifying the class hierarchy in this situation, the inheritance section in the Java tutorial states:
Excepting Object, which has no superclass, every class has one and only one direct superclass (single inheritance). In the absence of any other explicit superclass, every class is implicitly a subclass of Object.
Then, for the tutorial stuff on using super:
Note: If a constructor does not explicitly invoke a superclass constructor, the Java compiler automatically inserts a call to the no-argument constructor of the superclass.
If the super class does not have a no-argument constructor, you will get a compile-time error. Object does have such a constructor, so if Object is the only superclass, there is no problem.
Every class we create in Java is an implicit descendant of "Object" object (in other words, subclass of Object). Hence when ever you are making a call to super() it implicitly calls the constructor of Object class. The basic reason to have this feature is to generalize the common features like:
synchnronizaton - like wati()
object identity - like hashcode(), equals()
and many more.
Thanks,
JK
The Object class, in the java.lang package, sits at the top of the class hierarchy tree. Every class is a descendant, direct or indirect, of the Object class.
All objects, including arrays, implement the methods of this class. Thats why super() in your case is actually calling the constructor of Object class.
This question already has answers here:
Can an abstract class have a constructor?
(22 answers)
Closed 9 years ago.
we know that new Instance() subclass in java , the first recursive call will be the parent class constructor,If the parent class is an abstract class,will be Calling its constructor? if this true ,Means that the parent class will be instantiated？ But the abstract class can not be instantiated,Who can explain this question. Thank You.
You must understand that constructor is not responsible for creating instance, but only for initializing it. Creating instance is role of new keyword (this even returns reference for new created instance which constructor can't since it doesn't have return type).
So even when you are calling constructor of superclass (which can be abstract) you are not creating instance of that superclass, but you are executing code that will initialize fields inherited from it.
Parent class constructor is always called irrespective of if its a concrete class or an abstract class. Instantiating the class is different from carrying it's definition to subclass. In case you instantiate the subclass , though you carried the structure and methods of the parent class in sub class object, it never means instantiation of parent class.
Abstract class constructor is actually used to be inherited by subclasses, because the constructor is equivalent to initialize method is called when the subclass constructor must call the parent class constructor, so you can generate when an object in a subclass abstract class abstract class to initialize the fields on demand and performs some initialization code. In fact, not necessarily to create an instance of a class before calling constructor, the subclass also need to call the parent class constructor. And it does not necessarily generate an instance constructor is called, in some implementations under special or exceptional circumstances, create an instance constructor is not called. The constructor calls does not necessarily generate an instance, but it must be an instance of the call, just like an ordinary instance method.
Is it mandatory to call base class constructor in Java?
In C++ it was optional, so I am asking this.
When I extend ArrayAdapter, I get this error: "Implicit super constructor ArrayAdapter<String>() is undefined. Must explicitly invoke another constructor"
So, what is the purpose of calling base constructor? When I create object base class constructor will call & then it comes to derived right.
The no-args constructor is called implicitly if you don't call one yourself, which is invalid if that constructor doesn't exist. The reason it is required to call a super constructor is that the superclass usually has some state it expects to be in after being constructed, which may include private variables that can't be set in a sub-class. If you don't call the constructor, it would leave the object in a probably invalid state, which can cause all kinds of problems.
It's not necessary to call the no-args constructor of super class. If you want to call a constructor with args, user super keyword like show below:
super(arg1, ...);
I saw this question in one of my question papers:
Why should the derived class constructor always access base class constructor?
I'm wondering whether the question is valid?
So that you may have a valid object of type "Base" before you start messing with the inherited functionality in your derived object!
It's not valid. There's no 'should' about it: it must, and the compiler enforces it, by calling the base class's default constructor if it exists, and giving you a compile error if it doesn't, which forces you to call one of the existing constructors.
There is one exception to always, a default constructor in a superclass isn't usually called explicit.
If a constructor does not explicitly invoke a superclass constructor, the Java compiler automatically inserts a call to the no-argument constructor of the superclass. If the super class does not have a no-argument constructor, you will get a compile-time error. Object does have such a constructor, so if Object is the only superclass, there is no problem.
Because the base class may do work you are not aware of.
Because the base class may have members that require initialization.
Since the derived class constructor wants to inherits from the base class constructor, it is necessary to call the base class constructor. Otherwise, if you don't, you will not inherit values initialised in base class constructor.
Calling a superclass constructor is not a must but a should paired with a strong advise - as long as the superclass has a default constructor. Otherwise the compiler forces you to call at least one of the superclasses constructors.
If the default constructor is present, it's called anyway, even without an explicit super() statement in the subclasses construtor.
A visible part of class construction is the initialization of fields. But there's more under the hood (memory allocation, registration, etc). All this has to be done for all superclasses when a derived class is created.
You do not need to call constructor when there is a default constructor (i.e a no-argument constructor) present in the base class. When there is a constructor with arguments present and no default constructor, then you don't need to declare a constructor in the child class.
You only need to initialize the child class constructor when there is no default constructor present in the parent class.
Example :
class A {
public int aInstanceVariable;
public A(int aInstanceVariable) {
this.aInstanceVariable = aInstanceVariable;
}
}
class B extends A {
// In this case we have to call super in base class constructor
}
However, when public A() {} is present in class A, there is no need to initialize the child class constructor.