[MUSIC] So now we're gonna dive in to working with strings in Java. This video is gonna introduce the basics, and this may be familiar to many of you, and if it is just go ahead and watch this really fast or even just skip it. But if not we wanna make sure we've got the basics down for everybody before we move forward. So by the end of this video you'll be able to describe how strings are represented in Java, and then perform some basic operations with strings. So, going back to our task which is to estimate the Flesch readability score for a piece of text. Again, our motivation here is that we wanna be able to count the number of words, syllables and sentences in a string and the way we are gonna represent in a document that is. And the way we're gonna represent a document is just as one big long string in Java. So let's take a look in more detail about how strings are represented. Here are some basics about strings. Strings in Java are objects, so they're not primitive types they're object types. And this means that they're represented in your computer's heap, in the memory, and the variables that refer to them have references. So I'm gonna draw you a memory model diagram, and if you took our first course in this specialization, this will look familiar to you. And if you didn't, this our typical way that we draw memory models of data in our programs. So we have a variable declaration. It gets a box. We have a new object, that's going to get created on the heap. And remember that little symbol next to that object, that's just our way of representing an actual object of a particular class. Then the assignment operator connects the variable to the object with this reference, which we represent as an arrow. So here we have one string variable and refers to a string object in memory, and that string object stores some text. How does that string object store in the text? It actually stores it as an array of characters. So we have one slot in the array for each character in the string, including that space between those two words. All right, we can add to this code. We have a second line of code here, which declares another variable and gives it the value of our first variable. So just like any other object, this results in two references to the exact same object in memory. Now you may get concerned at this point and say wait a minute, we have two objects, sorry two references to the exact same object. What if one of those references changes our string? Turns out, that's not possible. In Java, strings are immutable, which means that the objects that are in memory, that are the strings, can never be changed. So there's no way to change a string object once it's created. Now, this doesn't mean that you can't build strings. Obviously, you've probably done this already. For example, you can append strings together using the built in append method inside the string class. So here we have our first string, which says Hello World! And we append to that string, It's a great day. But it doesn't actually change that first string object. What it does is it creates a whole new string object, takes the text out of the first string object, puts it in the second one, and appends that text that we want to append onto the end. So one more object, no change to that first object. You can also use the + operator to do string appends in Java. I'm sure you've done this before, and this does the exact same thing as that appends method that I just showed you. Again, no change to that first object there. Now this could get messy with all these different string objects being created in memory. Strings could end up taking a lot of your heap space and Java is aware of this fact so they provide an optimization, which is something called an interned string. And I'm gonna tell you a little bit about how that works. This is a detail. You don't have to worry too much about it when you're programming, but it's good to keep in mind. So, we have on top the way that we created the string before, with that new operator, so we said new String ("Hello World!"). That gave us the first string object that you see over there in memory. The second and third lines also create or refer to string objects with the same text, Hello World!. However, this string here without the new string before it, creates what's called an interned string object. Which is a single string object the Java's going to use whenever we don't explicitly create a new string object with the keyword new and a call to the constructor. So that first line there explicitly creates a new string object, but the second and third lines just refer to the same interned hello world string, that Java will create for us if it doesn't exist. So there's that intern string, both of those variables refer to that. So, let's talk about ways of comparing strings. There are two ways we can consider comparing string objects in Java, .equals, which is a method on the string class, or equals-equals, which is just that operator we've seen before to compare the values of two variables. And these two ways behave quite differently when we consider the underlying representation of the string. So in this example, we have two strings that we've created by calling new and the constructor of the string class twice, but they store the exact same piece of text, Hello World! If we call .equals on one of the strings, so text.equals (text2), this will return true, because what .equals does is it goes into the string objects and compares them character by character, returning true if they represent the same piece of text. However, equals-equals behaves exactly the same way it's always behaved. It just compares the values of the variables themselves. And if we look in the box, we can see that the variables themselves store references to locations in memory. So these two variables store different references. Therefore equals-equals is going to return false when we compare them. The lesson here is that we almost always wanna use .equals to compare strings and not equals-equals. So that's a basic introduction to strings. They're really powerful objects in Java and they can do a lot of things. We're gonna look at just some of the things they can do in the next video
