Objects, References, and Constructors

A variable of a class type holds a reference, not the object

A class is a blueprint; an object is one thing built from that blueprint. The keyword new does three jobs in one step: it reserves memory for a new object, runs a constructor to initialise that object's instance variables, and returns a reference to the object.

Rectangle box = new Rectangle(3, 5);

Read that line as three separate ideas: Rectangle box declares a variable whose type is a class, new Rectangle(3, 5) builds the object, and = stores the reference in the variable. The variable does not contain the rectangle; it contains an arrow that points at the rectangle.

Three consequences appear on the exam every year. First, b = a; between two object variables copies the arrow, so both names then refer to the same object and a change made through one name is visible through the other. Second, a class-type variable that has not been given an object holds null, and calling any method on it throws a NullPointerException at run time, not at compile time. Third, a class may declare several constructors with different parameter lists; the compiler picks the one whose parameter types match the arguments you supply.