Java Constructors

Context: FIT2099_MOC · how an object is built and initialised · runs on new · sets the fields Problem it solves: create an object and initialise its fields, optionally offering several ways to construct it.

Quick Revision

• 🎯 Trigger: need to build/initialise an object ➔ define a constructor (same name as the class, no return type).
• ⚡ Key Constraint: a constructor runs every time you new an object; if you write none, Java gives a default zero-argument one that does nothing.

🔧 Minimal Working Example

class CoffeeMachine {
    String brand;
 
    CoffeeMachine() {}                    // 1. empty (zero-arg) constructor
    CoffeeMachine(String newBrand) {      // 2. with an argument (overload)
        this.brand = newBrand;            // this.brand = the current object's field
    }
}
// usage
CoffeeMachine defaultCM = new CoffeeMachine();          // calls #1
CoffeeMachine nespresso = new CoffeeMachine("Nespresso"); // calls #2

Expected output: two objects; nespresso.brand = "Nespresso", defaultCM.brand = null.

• Name & return ➔ a constructor is a special method named exactly like the class, with no return type (not even void).
• Default ➔ every class has a zero-arg constructor; Java supplies one if you don’t — but adding a parameterised one removes the free default (add it back explicitly).
• this ➔ refers to the current object; this.brand = newBrand disambiguates the field from the parameter.
• Overloading ➔ multiple constructors with different parameter lists — pick the one whose arguments match.

🔀 Variations

• The three D’s ➔ Declaration (Integer x; — name + type), Instantiation (new Integer(42) — build the object), Initialisation (assign it: x = new Integer(42)).
• All in one line ➔ Integer theAnswer = new Integer(42);.

✍️ Practice

Practice 1: A Student has a name and an id. Provide a no-arg constructor and one that sets both fields.

Reference solution

class Student {
    String name;
    int id;
    Student() {}
    Student(String name, int id) {
        this.name = name;
        this.id = id;
    }
}

• Key move: overloaded constructors; this.field = param to store the arguments.

⚠️ Common Mistakes

• 💡 No return type on a constructor ➔ writing void CoffeeMachine() makes it a method, not a constructor — the object won’t initialise as expected.
• 💡 Declared but not initialised = crash/error ➔ Integer x; System.out.println(x); fails (“might not have been initialized”); declaration alone reserves the name, not a value.