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Using classes and objects

From: Foundations of Programming: Fundamentals

Video: Using classes and objects

The best way to start getting familiar with classes and objects is to begin by using classes that are already defined in your language and just creating objects from those built-in classes. We can later define our own classes and create objects from those classes. Now this sounds complex. It really isn't. In fact, we've already done it. So in JavaScript, when you make an array variable, you have made an object. When you make a regular expression variable, you've made an object.

Using classes and objects

The best way to start getting familiar with classes and objects is to begin by using classes that are already defined in your language and just creating objects from those built-in classes. We can later define our own classes and create objects from those classes. Now this sounds complex. It really isn't. In fact, we've already done it. So in JavaScript, when you make an array variable, you have made an object. When you make a regular expression variable, you've made an object.

These are both built-in objects in JavaScript. Now on the other hand, when you make a variable that's a number or a variable that's a Boolean, these are not objects. These are what can be referred to as primitives. Primitives really just store a value. That's it. But as we've seen, when you have got an object, this is not just a dumping ground for data. When you have an array, you can ask things of it. They have properties like length and they have methods. We can ask them to sort themselves or reverse themselves.

With regular expressions, we can call the test method on a regular expression variable to see if the pattern matches another string. They're smart and they have methods and they have properties. And that's what we mean by an object. One of the great things is they are independent of each other. Objects might be based on the same class, but I can have a thousand array objects and work with each of them separately. Now there are a few other objects in JavaScript. One of them is the Date object. This would be the way we would create a new Date object variable.

Let's say var today = new Date. The variable today would now contain today's date, not as a string but as a Date object. We can create new Date objects with a particular data. And actually because this object definition stores time, we could even create a new Date object with a year, month, day, hours, minutes, seconds in it. Now you might think, but so what? Well, of course the great thing is these are objects. They have properties, and they have methods. We can start using the name of one of these variables like today or y2k and calling parts of it.

We can say today.getMonth and it would return 0-11 for the month. today.getFullYear returns the normal year. .getDate, .getDay, .getHours, and so on. So there is a lot of behavior that the Date object can give us to start pulling apart dates. And not only that, but they have the flip side of the get method, which are our set methods. We can create a new Date object and then use it to say .setMonth or .setFullYear or .setDay.

And usually it makes it much easier to work with dates which can be a little complex if you just decide to store them as individual variables representing a year and a month and a day. There is also a Math object built into JavaScript. This one is a little different. While objects like arrays and dates, we can think of them as mainly holding data and it's just nice that they have a little behavior that comes with that, the Math object on the other hand is there mainly to contain behavior. you don't really store data in it.

What you might do is first say create a variable called x and I am setting this to 200.6. And I would like to round this number. This is the way I do it. I would create a new variable called y and create it by calling Math.round, passing in that variable x. That would give me 201. It would round it up. So we often just use the Math object directly. Say we create several variables. One is 200, one is 10000, one is 4, but at some point in my program, I don't know which one is which and which one is higher.

I can create a new variable called biggest, which will be whatever gets returned from calling Math.max and passing in those three numbers. In fact, Math.max will take as many numbers as you want to pass into it and always return the biggest one. If we have a .max, it would be nice if we had a .min. And not surprisingly, we do. So the Math object is there and has many built-in methods and properties for giving us Pi, calculating random numbers, rounding things up or down, calculating logarithms, sines and cosines and so on.

So JavaScript has an Array class, it has a RegExp class, a Date class, and a Math class, and several others. Now to find out, you'd go to a JavaScript reference guide. But JavaScript really doesn't have very many. Other languages have hundreds or even thousands of predefined classes available grouped into different categories. Now what's very common in object- oriented languages is using the name of the class when you create the object. That's what we're doing here. We are creating a new variable. It's called today and it will be a Date object based on the Date class.

Now when you use the name of the class, by convention in JavaScript it has an uppercase first letter. There is nothing magical about that. It just helps us recognize it. That's how it was defined in the language. And if we make our own, we will probably use an uppercase first letter too. But you might think "Well, when I made an array, I wasn't using the word Array." Well, no, that's because JavaScript has a lot of shortcuts, but you can. So the easy way to make an array is this one, just using the square brackets.

But you can also create an array this way. Using the word Array, which is the name of the class, and the word new. In the same way when we are creating a regular expression, I can use the shortcut form in JavaScript to create a new regular expression object or I can use the name of the class, which is RegExp. And this word, new, is used in many object-oriented languages as an explicit indicator that what we're doing is creating a new object based on a class. Now some of you might be wondering about strings.

Well, strings are objects in most object-oriented programming languages. In JavaScript they do behave like objects. They have properties and methods we can access, although technically they're not. Technically they're primitives. That's really because JavaScript is not the purest of object-oriented languages. So let's talk about some languages that are pretty pure and what the difference is.

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This video is part of

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Foundations of Programming: Fundamentals

61 video lessons · 89809 viewers

Simon Allardice
Author

 
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  1. 4m 15s
    1. Welcome
      1m 17s
    2. Making the most of this course
      2m 8s
    3. Using the exercise files
      50s
  2. 22m 11s
    1. What is programming?
      5m 45s
    2. What is a programming language?
      4m 48s
    3. Writing source code
      5m 34s
    4. Compiled and interpreted languages
      6m 4s
  3. 16m 29s
    1. Why JavaScript?
      4m 45s
    2. Creating your first program in JavaScript
      6m 54s
    3. Requesting input
      4m 50s
  4. 31m 38s
    1. Introduction to variables and data types
      5m 16s
    2. Understanding strong, weak, and duck-typed languages
      3m 51s
    3. Working with numbers
      5m 4s
    4. Using characters and strings
      4m 5s
    5. Working with operators
      4m 47s
    6. Properly using white space
      6m 46s
    7. Adding comments to code for human understanding
      1m 49s
  5. 24m 48s
    1. Building with the if statement
      7m 35s
    2. Working with complex conditions
      4m 9s
    3. Setting comparison operators
      6m 59s
    4. Using the switch statement
      6m 5s
  6. 17m 54s
    1. Breaking your code apart
      4m 1s
    2. Creating and calling functions
      2m 56s
    3. Setting parameters and arguments
      6m 7s
    4. Understanding variable scope
      2m 23s
    5. Splitting code into different files
      2m 27s
  7. 13m 31s
    1. Introduction to iteration
      4m 28s
    2. Writing a while statement
      5m 24s
    3. Creating a for loop
      3m 39s
  8. 19m 28s
    1. Cleaning up with string concatenation
      4m 30s
    2. Finding patterns in strings
      8m 3s
    3. Introduction to regular expressions
      6m 55s
  9. 19m 58s
    1. Working with arrays
      5m 46s
    2. Array behavior
      5m 29s
    3. Iterating through collections
      5m 18s
    4. Collections in other languages
      3m 25s
  10. 10m 50s
    1. Programming style
      5m 55s
    2. Writing pseudocode
      4m 55s
  11. 25m 55s
    1. Input/output and persistence
      3m 6s
    2. Reading and writing from the DOM
      8m 11s
    3. Event driven programming
      7m 47s
    4. Introduction to file I/O
      6m 51s
  12. 24m 25s
    1. Introduction to debugging
      5m 57s
    2. Tracing through a section of code
      7m 5s
    3. Understanding error messages
      3m 21s
    4. Using debuggers
      8m 2s
  13. 14m 16s
    1. Introduction to object-oriented languages
      5m 18s
    2. Using classes and objects
      6m 28s
    3. Reviewing object-oriented languages
      2m 30s
  14. 11m 14s
    1. Memory management across languages
      5m 11s
    2. Introduction to algorithms
      4m 2s
    3. Introduction to multithreading
      2m 1s
  15. 29m 20s
    1. Introduction to languages
      1m 42s
    2. C-based languages
      4m 40s
    3. The Java world
      3m 13s
    4. .NET languages: C# and Visual Basic .NET
      6m 17s
    5. Ruby
      3m 4s
    6. Python
      2m 56s
    7. Objective-C
      4m 3s
    8. Libraries and frameworks
      3m 25s
  16. 1m 2s
    1. Where to go from here
      1m 2s

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