./curric/oop/constructor

Constructors

In the previous examples, we dealt with existing objects. However, we never touched on the creation of new objects. jaransCar did not magically appear into the world as a ‘92 Jeep Wrangler. A factory had to construct it first. In Java, classes have a special method caled the constructor, invoked by the new keyword. A constructor, just like any method, can take arguments, and it can be overloaded.

public class Car {
    public int year;
    public String make;
    public String model;
    
    private boolean hasMuffler;
    
    // constructor syntax:
    public Car(int year, String make, String model) {
        // we have to use this to avoid ambiguity
        this.year = year;
        this.make = make;
        this.model = model;
        hasMuffler = true;
    }
    
    public void goVrooom() {
        if (hasMuffler) {
            // make a muffled vrooom
        } else { 
            // make a loud vrooom
        }  
     
    
    public void detachMuffler() {
        hasMuffler = false;
    }
}

Note that this always refers to the current object, an intuitive and consistent behavior (@javascript). We use it to distinguish between the year, make, and model provided as arguments and the fields of the class. This assumes that all cars are created with a muffler. But what if we want to leave the possibility to special order a muffler-less car? We can simply add an overloaded constructor.

public class Car {
    public int year;
    public String make;
    public String model;
    
    private boolean hasMuffler;
    
    public Car(int year, String make, String model) {
        this.year = year;
        this.make = make;
        this.model = model;
        hasMuffler = true;
    }
    
    public Car(int year, String make, String model, boolean hasMuffler) {
        this.year = year;
        this.make = make;
        this.model = model;
        this.hasMuffler = hasMuffler;
    }
    
    public void goVrooom() {
        if (hasMuffler) {
            // make a muffled vrooom
        } else { 
            // make a loud vrooom
        }  
     
    
    public void detachMuffler() {
        hasMuffler = false;
    }
}