Constructors in AP CSA

A constructor is a special method that runs automatically when an object is created with new. Its job is to initialize the object’s instance variables to valid starting values.

Constructor Rules

• Same name as the class
• No return type (not even void)
• Called automatically when new ClassName() is used
• A class can have multiple constructors (overloading)

public class Counter {
    private int count;

    // Constructor -- no return type, same name as class
    public Counter() {
        count = 0;    // initialize to safe default
    }

    public Counter(int startValue) {
        count = startValue;   // overloaded constructor
    }
}

Default vs No-Arg Constructor

If you write no constructor, Java provides a default one that sets numeric fields to 0, booleans to false, and objects to null. The moment you write any constructor, the default disappears.

public class Box {
    private int size;
        public Box() {
        size = 5;
    }
        public Box(int s) {
        size = s * 2;
    }
        public int getSize() {
        return size;
    }
}
Box b = new Box(3);
System.out.println(b.getSize());