ACSA Exam Prep
Unit 1 · Using Objects and Methods · Topic 1.13

Object Creation and Storage (Instantiation)

Skill 2.B

Big idea: new plus a constructor call builds an object. Which constructor runs depends on the arguments you pass.

Learning objectives
  • Identify, using its signature, the correct constructor being called.
  • Develop code to declare variables of the correct types to hold object references.
  • Develop code to create an object by calling a constructor.

Constructors

A constructor has the same name as the class and no return type. It sets up a new object's initial state. A class can have several constructors (overloading), each with a different parameter list.

Code
Point origin = new Point();
Point p = new Point(3, 4);
System.out.println(origin.getX() + "," + origin.getY());
System.out.println(p.getX() + "," + p.getY());
Output
0,0
3,4

Declaring vs. creating

Point p; only declares a variable. No object exists until new runs. A variable set to null refers to no object.

Arguments to a constructor are passed by value and must match a constructor's parameters in number, order, and type, just like a method call.

Point a;              // declared, no object
Point b = null;       // explicitly no object
Point c = new Point(1, 2);   // object created
Key points
  • Constructors have the class's name and no return type.
  • A constructor signature is the class name plus the ordered parameter types; constructors can be overloaded.
  • Objects are created with new followed by a constructor call.
  • A reference variable holds an object reference or null.
Watch out
  • new Point(3) won't compile if no constructor takes one int.
  • Declaring a variable doesn't create an object.
Quick check
Given constructors Book(String title) and Book(String title, int pages), which one runs for new Book("Dune", 412)?
Show answer
Book(String, int), because the arguments are a String and an int.
Practice 1.13 (10 questions)