📓1.13: Constructors

Constructors – Creating & Initializing Objects
Summary

📖 This page is a condensed version of CSAwesome Topic 1.13

Constructors – Creating & Initializing Objects

A Java class defines what objects of the class know (attributes) and what they can do (behaviors). Each class has constructors which are special methods used to initialize the attributes for a newly created object.

Constructors always have the same name as the class!

A new object instance is created with the new keyword followed by the class name, which is a call to the constructor (new ClassName()).

🏗️ Syntax pattern to construct a new object instance:

ClassName variableName = new ClassName(parameters);

Right side of =: declaring a variable of type ClassName

Left side of =: assigning values by calling a constructor

Examples:

World habitat = new World();    // create a new World object
Turtle t = new Turtle(habitat); // create a new Turtle object

The `World` Class Constructors

There can be more than one constructor defined in a class. This is called overloading the constructor.

Constructors are considered OVERLOADED when there are multiple constructors, but the constructors have different signatures. They can differ in the number, type, and/or order of parameters (input).

Think of overloading constructors as providing different ways of “setting up” an object, depending on how much information is known about the object upon creation.

The World class has two constructors:

World() — no arguments, creates a default-sized world (640×480 px)
World(int width, int height) — takes two integers to create a world of specific size.

World world1 = new World(); // default size 640x480
World world2 = new World(300, 400); // custom size 300x400

The `Turtle` Class Constructors

The Turtle class also has multiple constructors. It always requires a World argument for where the turtle will be drawn.

🗺️ The Turtle world does not use the Cartesian coordinate system with (0,0) in in the middle the screen. Instead: (0,0) is at the top left corner of the screen, x increases to the right, and y increases towards the bottom of the screen.

The default location is the middle of the world. Another constructor allows you to set a specific (x, y) location:

Turtle t1 = new Turtle(world1);
Turtle t2 = new Turtle(50, 100, world1);

Note: The order of arguments matters — Turtle(int x, int y, World w) is not the same as swapping them.

CSAwesome Activities: Turtle Class

🐢 To explore the concepts from this lesson in code, we’ll play around with the Turtle class on the CSAwesome website instead of taking notes.

👉 GO TO: , SIGN IN to your account, and complete all the turtle-related coding activities/challenges with a partner.

Object Variables & References

New objects are saved in reference type variables, which hold a reference to the object in memory.

A special reference value null means the variable doesn’t refer to any object yet:

World world1 = new World();
Turtle t1 = null; // no object yet
t1 = new Turtle(world1);

Video: Understanding `null`

Constructor Signatures

When using a class from a library (e.g., Turtle), you can look up the constructor headers/signatures in the documentation just like you would for methods.

Constructors are OVERLOADED if there are multiple with different signatures. For example, here are the two constructors for the Turtle class that take different parameters:

Constructors can differ in the number, type, and/or order of parameters.

Arguments, Parameters, and Call by Value

When a constructor like new Date(2005, 9, 1) is called:

Parameters = variables in the constructor’s signature (e.g., year, month, day)
Arguments = values passed in (e.g., 2005, 9, 1)
- Java passes arguments by value (copies).

The type of the values being passed in as arguments have to match the type of the parameter variables. We cannot give a constructor a String object when it is expecting an int.

The order of the arguments also matters. If you mix up the month and the day in the Date constructor, you will get a completely different date, for example January 9th (1/9) instead of Sept. 1st (9/1).

Summary

(AP 1.13.A.1) A class contains constructors that are called with the keyword new to create objects and initialize its attributes.
(AP 1.13.A.1) Constructors have the same name as the class.
(AP 1.13.C.1) An object is typically created using the keyword new followed by a call to one of the class’s constructors. new ClassName() creates a new object of the specified class and calls a constructor.
(AP 1.13.B.1) The new object is saved in a variable of a reference type which holds an object reference or null if there is no object.
(AP 1.13.A.2) A constructor signature consists of the constructor’s name, which is the same as the class name, and the ordered list of parameter types.
(AP 1.13.A.2) The parameter list, in the header of a constructor, lists the types of the values that are passed and their variable names.
(AP 1.13.A.3) Constructors are said to be overloaded when there are multiple constructors with different signatures. They must differ in the number, type, or order of parameters.
A no-argument constructor is a constructor that doesn’t take any passed in values (arguments).
(AP 1.13.C.2) Parameters allow constructors to accept values to establish the initial values of the attributes of the object.
(AP 1.13.C.3) A constructor argument is a value that is passed into a constructor when the constructor is called. The arguments passed to a constructor must be compatible in order and number with the types identified in the parameter list in the constructor signature.
(AP 1.13.C.3) When calling constructors, arguments are passed using call by value. Call by value initializes the parameters with copies of the arguments.
(AP 1.13.C.4) A constructor call interrupts the sequential execution of statements, causing the program to first execute the statements in the constructor before continuing. Once the last statement in the constructor has been executed, the flow of control is returned to the point immediately following where the constructor was called.

Acknowledgement

Content on this page is adapted from Runestone Academy - Barb Ericson, Beryl Hoffman, Peter Seibel.