What type of method is called when an object is created?

As you know, a class provides the blueprint for objects; you create an object from a class. Each of the following statements taken from the CreateObjectDemo program creates an object and assigns it to a variable:

Índice Show

Declaring a Variable to Refer to an Object
Instantiating a Class
Initializing an Object
Method examples
Method shorthand
“this” in methods
“this” is not bound
Arrow functions have no “this”

Point originOne = new Point(23, 94); Rectangle rectOne = new Rectangle(originOne, 100, 200); Rectangle rectTwo = new Rectangle(50, 100);

The first line creates an object of the Point class, and the second and third lines each create an object of the Rectangle class.

Each of these statements has three parts (discussed in detail below):

Declaration: The code set in bold are all variable declarations that associate a variable name with an object type.
Instantiation: The new keyword is a Java operator that creates the object.
Initialization: The new operator is followed by a call to a constructor, which initializes the new object.

Declaring a Variable to Refer to an Object

Previously, you learned that to declare a variable, you write:

This notifies the compiler that you will use name to refer to data whose type is type. With a primitive variable, this declaration also reserves the proper amount of memory for the variable.

You can also declare a reference variable on its own line. For example:

If you declare originOne like this, its value will be undetermined until an object is actually created and assigned to it. Simply declaring a reference variable does not create an object. For that, you need to use the new operator, as described in the next section. You must assign an object to originOne before you use it in your code. Otherwise, you will get a compiler error.

A variable in this state, which currently references no object, can be illustrated as follows (the variable name, originOne, plus a reference pointing to nothing):

Instantiating a Class

The new operator instantiates a class by allocating memory for a new object and returning a reference to that memory. The new operator also invokes the object constructor.

Note: The phrase "instantiating a class" means the same thing as "creating an object." When you create an object, you are creating an "instance" of a class, therefore "instantiating" a class.

The new operator requires a single, postfix argument: a call to a constructor. The name of the constructor provides the name of the class to instantiate.

The new operator returns a reference to the object it created. This reference is usually assigned to a variable of the appropriate type, like:

Point originOne = new Point(23, 94);

The reference returned by the new operator does not have to be assigned to a variable. It can also be used directly in an expression. For example:

int height = new Rectangle().height;

This statement will be discussed in the next section.

Initializing an Object

Here's the code for the Point class:

public class Point { public int x = 0; public int y = 0; //constructor public Point(int a, int b) { x = a; y = b; } }

This class contains a single constructor. You can recognize a constructor because its declaration uses the same name as the class and it has no return type. The constructor in the Point class takes two integer arguments, as declared by the code (int a, int b). The following statement provides 23 and 94 as values for those arguments:

Point originOne = new Point(23, 94);

The result of executing this statement can be illustrated in the next figure:

Here's the code for the Rectangle class, which contains four constructors:

public class Rectangle { public int width = 0; public int height = 0; public Point origin; // four constructors public Rectangle() { origin = new Point(0, 0); } public Rectangle(Point p) { origin = p; } public Rectangle(int w, int h) { origin = new Point(0, 0); width = w; height = h; } public Rectangle(Point p, int w, int h) { origin = p; width = w; height = h; } // a method for moving the rectangle public void move(int x, int y) { origin.x = x; origin.y = y; } // a method for computing the area of the rectangle public int getArea() { return width * height; } }

Each constructor lets you provide initial values for the rectangle's origin, width, and height, using both primitive and reference types. If a class has multiple constructors, they must have different signatures. The Java compiler differentiates the constructors based on the number and the type of the arguments. When the Java compiler encounters the following code, it knows to call the constructor in the Rectangle class that requires a Point argument followed by two integer arguments:

Rectangle rectOne = new Rectangle(originOne, 100, 200);

This calls one of Rectangle's constructors that initializes origin to originOne. Also, the constructor sets width to 100 and height to 200. Now there are two references to the same Point object—an object can have multiple references to it, as shown in the next figure:

The following line of code calls the Rectangle constructor that requires two integer arguments, which provide the initial values for width and height. If you inspect the code within the constructor, you will see that it creates a new Point object whose x and y values are initialized to 0:

Rectangle rectTwo = new Rectangle(50, 100);

The Rectangle constructor used in the following statement doesn't take any arguments, so it's called a no-argument constructor:

Rectangle rect = new Rectangle();

All classes have at least one constructor. If a class does not explicitly declare any, the Java compiler automatically provides a no-argument constructor, called the default constructor. This default constructor calls the class parent's no-argument constructor, or the Object constructor if the class has no other parent. If the parent has no constructor (Object does have one), the compiler will reject the program.

Objects are usually created to represent entities of the real world, like users, orders and so on:

let user = { name: "John", age: 30 };

And, in the real world, a user can act: select something from the shopping cart, login, logout etc.

Actions are represented in JavaScript by functions in properties.

Method examples

For a start, let’s teach the user to say hello:

let user = { name: "John", age: 30 }; user.sayHi = function() { alert("Hello!"); }; user.sayHi(); // Hello!

Here we’ve just used a Function Expression to create a function and assign it to the property user.sayHi of the object.

Then we can call it as user.sayHi(). The user can now speak!

A function that is a property of an object is called its method.

So, here we’ve got a method sayHi of the object user.

Of course, we could use a pre-declared function as a method, like this:

let user = { // ... }; // first, declare function sayHi() { alert("Hello!"); } // then add as a method user.sayHi = sayHi; user.sayHi(); // Hello!

When we write our code using objects to represent entities, that’s called object-oriented programming, in short: “OOP”.

OOP is a big thing, an interesting science of its own. How to choose the right entities? How to organize the interaction between them? That’s architecture, and there are great books on that topic, like “Design Patterns: Elements of Reusable Object-Oriented Software” by E. Gamma, R. Helm, R. Johnson, J. Vissides or “Object-Oriented Analysis and Design with Applications” by G. Booch, and more.

Method shorthand

There exists a shorter syntax for methods in an object literal:

// these objects do the same user = { sayHi: function() { alert("Hello"); } }; // method shorthand looks better, right? user = { sayHi() { // same as "sayHi: function(){...}" alert("Hello"); } };

As demonstrated, we can omit "function" and just write sayHi().

To tell the truth, the notations are not fully identical. There are subtle differences related to object inheritance (to be covered later), but for now they do not matter. In almost all cases, the shorter syntax is preferred.

“this” in methods

It’s common that an object method needs to access the information stored in the object to do its job.

For instance, the code inside user.sayHi() may need the name of the user.

To access the object, a method can use the this keyword.

The value of this is the object “before dot”, the one used to call the method.

For instance:

let user = { name: "John", age: 30, sayHi() { // "this" is the "current object" alert(this.name); } }; user.sayHi(); // John

Here during the execution of user.sayHi(), the value of this will be user.

Technically, it’s also possible to access the object without this, by referencing it via the outer variable:

let user = { name: "John", age: 30, sayHi() { alert(user.name); // "user" instead of "this" } };

…But such code is unreliable. If we decide to copy user to another variable, e.g. admin = user and overwrite user with something else, then it will access the wrong object.

That’s demonstrated below:

let user = { name: "John", age: 30, sayHi() { alert( user.name ); // leads to an error } }; let admin = user; user = null; // overwrite to make things obvious admin.sayHi(); // TypeError: Cannot read property 'name' of null

If we used this.name instead of user.name inside the alert, then the code would work.

“this” is not bound

In JavaScript, keyword this behaves unlike most other programming languages. It can be used in any function, even if it’s not a method of an object.

There’s no syntax error in the following example:

function sayHi() { alert( this.name ); }

The value of this is evaluated during the run-time, depending on the context.

For instance, here the same function is assigned to two different objects and has different “this” in the calls:

The rule is simple: if obj.f() is called, then this is obj during the call of f. So it’s either user or admin in the example above.

We can even call the function without an object at all:

function sayHi() { alert(this); } sayHi(); // undefined

In this case this is undefined in strict mode. If we try to access this.name, there will be an error.

In non-strict mode the value of this in such case will be the global object (window in a browser, we’ll get to it later in the chapter Global object). This is a historical behavior that "use strict" fixes.

Usually such call is a programming error. If there’s this inside a function, it expects to be called in an object context.

If you come from another programming language, then you are probably used to the idea of a "bound this", where methods defined in an object always have this referencing that object.

In JavaScript this is “free”, its value is evaluated at call-time and does not depend on where the method was declared, but rather on what object is “before the dot”.

The concept of run-time evaluated this has both pluses and minuses. On the one hand, a function can be reused for different objects. On the other hand, the greater flexibility creates more possibilities for mistakes.

Here our position is not to judge whether this language design decision is good or bad. We’ll understand how to work with it, how to get benefits and avoid problems.

Arrow functions have no “this”

Arrow functions are special: they don’t have their “own” this. If we reference this from such a function, it’s taken from the outer “normal” function.

For instance, here arrow() uses this from the outer user.sayHi() method:

let user = { firstName: "Ilya", sayHi() { let arrow = () => alert(this.firstName); arrow(); } }; user.sayHi(); // Ilya

That’s a special feature of arrow functions, it’s useful when we actually do not want to have a separate this, but rather to take it from the outer context. Later in the chapter Arrow functions revisited we’ll go more deeply into arrow functions.

Summary

Functions that are stored in object properties are called “methods”.
Methods allow objects to “act” like object.doSomething().
Methods can reference the object as this.

The value of this is defined at run-time.

When a function is declared, it may use this, but that this has no value until the function is called.
A function can be copied between objects.
When a function is called in the “method” syntax: object.method(), the value of this during the call is object.

Please note that arrow functions are special: they have no this. When this is accessed inside an arrow function, it is taken from outside.