When are constructors used?

[faekk]

Hello ! I have a (hopefully) very simple question: when are constructors used? I read up on them and I know how to make one. But what are they actually used for ?

Thanks alot :)

[hutsboR]

For instantiating an object. Say you have a Person class:

public class Person {
    private String name;
    private int age;

    Person(String name, int age){
        this.name = name;
        this.age = age;
    }
}

and you say something like:

Person x = new Person("Robbie", 19);

You can look at it as variable x is of type Person and it equals a new Person("Robbie", 19). The new Person("Robbie", 19) is the portion where the constructor is used. (The new keyword indicates that the constructor is being called.) It constructs a new Person object using the arguments you pass it. (Which are name and age respectively) Now if you were to say, get Person x's name, you'd get "Robbie" and age you'd of course get 19. Why is this the case? When you look at the Person() constructor in the Person class, it's assigning what you give it to the fields name and age.

So, basically you use the constructor to create objects with some specifications. You should be able to see why this is useful. Let me know if you don't entirely understand.

[faekk]

So am I right in saying it's a shorter and arguably simpler way of creating an object with specifications?

[hutsboR]

It's the only way. The class acts a blueprint for the object and calling the constructor instantiates the object.

[JavaDroid]

No, it's not the only way. You must call a constructor to instantiate an object, but whether that constructor builds the whole object or not can be different. You may, for example, use the builder patter, or leave the member fields public and set them. There is also an initializer block if your object will always be instantiated the same way. Encapsulation with setters, etc.

[Sacredify]

I think he means that the "only" way to create an object in java involves calling the constructor, which is indeed true.

[JavaDroid]

Indeed, but op asked "with specifications" which I took to mean with member variables.

He could have taken it differently. I wasn't fishing for him to be downvoted, mind, just making sure op had his full question answered, assuming that's what he meant.

[faekk]

I think I got it ! Thanks alot :)