Copy constructors and defensive copying

Question

What is a copy constructor?

Can someone share a small example that can be helpful to understand along with defensive copying p

Answer 1

Copy constructors one often sees in C++ where they are needed for partly hidden, automatically invoked operations.

java java.awt.Point and Rectangle come to mind; also very old, mutable objects.

By using immutable objects, like String, or BigDecimal, simply assigning the object reference will do. In fact, due to the early phase of Java after C++, there still is a silly copy constructor in String:

public class Recipe {
    List ingredients;

    public Recipe() {
        ingredients = new ArrayList();
    }

    /** Copy constructor */
    public Recipe(Recipe other) {
        // Not sharing: ingredients = other.ingredients;
        ingredients = new ArrayList();
        ingredients.addAll(other.ingredients);
    }

    public List getIngredients() {
        // Defensive copy, so others cannot change this instance.
        return new ArrayList(ingredients);
        // Often could do:
        // return Collections.immutableList(ingredients);
    }
}