what's the point of java constructor?

Question

So I\'m learning java. I\'m one month in and I just learned about constructors. But I don\'t see the whole purpose of creating one. Why and when would I ever want to use one

Answer 1

It might seem as if you're having trouble understanding the basic concepts of objects and object-oriented programming. An explanation by example; This class represents a type of thing, namely a car:

public class Car{

    // Licence plate number. This is private, so it can
    // not be accessed directly from outside the class.
    private String hiddenRegNr = ""; 
    private static String description = "This is a car". 

    // The constructor, which sets the value of hiddenRegNr
    public Car(String regNr){
        hiddenRegNr = regNr;
    }

    // Method for reading hiddenRegNr, the only 
    // way to access it after instantiation. 
    public String getRegNr(){
        return hiddenRegNr;
    }

    // A static method. Can be used withouth instantiation.
    public static String getDesc(){
        return description;
    }
}

From some other class, you could call this class, and make instances of it; actual representations of different cars. They represent different cars, but are based on the same "model", i.e., the class Car.

Car myFirstCar = new Car("SR12345");
Car myOtherCar = new Car("XZ54321");

Now you have two different cars, with two different registration numbers. These are independent instances of the type car. They exist in memory, and may contain different values (in this case, different registration numbers).

myFirstCar.getRegNr(); // Will return SR12345
mySecondCar.getRegNr(); // will return xz54321

The first thing to notice here, is that you can only specify the registration number once per car, namely upon creation. That is the point of the constructor: It sets values, and does other stuff that needs to be done when objects (instances) are created.

Now, note the difference between getRegNr() and getDesc(): The keyword "Static" means that the second method is related directly to the class, and not to each instance. This means that:

• Calls to getDesc() is made on the class, not an instance:

  Car.getDesc();

• Calls to getDesc() will return the same value for all instances of the class car

• The variable description (which is also static) will be the same for all instances of Car

• The static method getDesc() CAN NOT access the variable hiddenRegNr, since that variable is related to a specific instance. Trying to refer to a variable in an instance from a static context (such as getDesc()) will cause an exception to be thrown. You might say that hiddenRegNr will not have been set when you call

  Car.getDesc();

because the constructor was never called, so it was never given any value.