Class is something that describes the objects that will be created in JVM in runtime one object, many objects - it depends on your actual program - what does it do. Its important to understand that this alone is not really an object, only a definition of class, its like saying: "In my program persons that I'll want to create will look like this read will have name and age ".
What is real person? Jack, John, Alice - they're all persons and might be created in your program. Every Object should have a reference somewhere in the project so that you could be able to work with it:. So for now, we have 1 class definition class Person and we can create 3 objects instances of this class. So far so good. Now how Java should know that instance john actually has the name "John" and he is, say, 40?
In other words how exactly do you instantiate the data fields of the object during the object creation? The truth is that Java doesn't know but it allows You to define this by means of declaring constructors - a special way to instruct Java how to create an instance of the class, so we alter the class definition:. If we won't supply correct parameters to the constructor the first one should be string, the second one is an integer , then the code won't compile at all.
So typically we have one class definition and we can create many objects out of it. Now its true is that in Java there are ways to define a class that in a way that won't allow the creation of more than one instance this is called singleton - but I think it will confuse you even more at this point, so for now if you haven't heard about these singletons - don't bother. In a very simple term I am giving my view just to understand object, but in depth you have to go and open java docs.
So when you do new Employee ; first jvm will look for new keyword and memory will be allocated based upon the number of attributes of class. Now you have memory with some allocated space bytes in it, you need to fill this memory bytes with some concrete value like xyz,10,78 "charles",0,0 can be anything as you have only three attributes in class that's where constructor comes in place. So through constructor if you don't supply any value then all you class level member property will be initialized to default value.
Or if you want to supply any custom value for which we have defined another parameterized constructor in your class you can use. So basically in layman term we can say new is used to create the object and constructor is used to supply initial value to it later on with help of method you can always change the value of your object.
One important point as many times you use new keyword that many times you will create the new object. A class is only a blueprint from which individual objects are created. Describes the properties of the objects that are going to create using that class.
The first line creates an object of the Point class, and the second and third lines each create an object of the Rectangle class. This notifies the compiler that you will use name to refer to data whose type is type. With a primitive variable, this declaration also reserves the proper amount of memory for the variable. If you declare originOne like this, its value will be undetermined until an object is actually created and assigned to it. Simply declaring a reference variable does not create an object.
For that, you need to use the new operator, as described in the next section. You must assign an object to originOne before you use it in your code. Otherwise, you will get a compiler error. A variable in this state, which currently references no object, can be illustrated as follows the variable name, originOne , plus a reference pointing to nothing :.
The new operator instantiates a class by allocating memory for a new object and returning a reference to that memory. The new operator also invokes the object constructor. The new operator requires a single, postfix argument: a call to a constructor. The name of the constructor provides the name of the class to instantiate.
The new operator returns a reference to the object it created. This reference is usually assigned to a variable of the appropriate type, like:. The Class. This method also uses the newInstance function which is provided by the constructor and used when creating an object.
This time the constructor is parameterized. A new separate object is created when we serialize and then deserialize an object. To create an object, this method does not need any constructor. We shall use the Serializable interface in Java for serializing and deserializing new objects. Serialization involves converting an object state into a byte stream. We use the writeObject method to serialize an Object. Deserialization is the process of using the byte stream to recreate an object in Java.
We shall use the readObject method to deserialize an object. Whenever the clone method is called, it creates a new object, and then gets all the content in the old object copied to it.
A constructor does not get invoked when we use the clone method to create an object.
0コメント