Equals Method Implementation

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From the course by Duke University

Java Programming: Principles of Software Design

644 ratings

Duke University

Java Programming: Principles of Software Design

644 ratings

Course 4 of 5 in the Specialization Java Programming and Software Engineering Fundamentals

Solve real world problems with Java using multiple classes. Learn how to create programming solutions that scale using Java interfaces. Recognize that software engineering is more than writing code - it also involves logical thinking and design. By the end of this course you will have written a program that analyzes and sorts earthquake data, and developed a predictive text generator. After completing this course, you will be able to: 1. Use sorting appropriately in solving problems; 2. Develop classes that implement the Comparable interface; 3. Use timing data to analyze empirical performance; 4. Break problems into multiple classes, each with their own methods; 5. Determine if a class from the Java API can be used in solving a particular problem; 6. Implement programming solutions using multiple approaches and recognize tradeoffs; 7. Use object-oriented concepts including interfaces and abstract classes when developing programs; 8. Appropriately hide implementation decisions so they are not visible in public methods; and 9. Recognize the limitations of algorithms and Java programs in solving problems. 10. Recognize standard Java classes and idioms including exception-handling, static methods, java.net, and java.io packages.

From the lesson

N-Grams: Predictive Text

In this module, you will explore some of the underlying concepts of predictive text. The first lesson will introduce random character generation and then how to train the character selection based on an input text. The second lesson will extend this concept to complete words. By the end of this module, you will be able to: (1) base random text generation on the frequency of characters in a training text, (2) collect a set of characters that occur in a text after randomly chosen initial character(s) to create a semi-random text, (3) extend the predictive text generation to use whole words, and (4) implement your own .equals method to compare complex data types.

Introduction3:25

Order-One Concepts6:11

Order-One Helper Functions8:42

WordGram Class4:20

WordGram Class Implementation4:54

Equals and HashCode Methods5:09

Equals Method Implementation10:41

Meet the Instructors

Robert Duvall
Lecturer
Computer Science
Owen Astrachan
Professor of the Practice
Computer Science
Andrew D. Hilton
Associate Professor of the Practice
Electrical and Computer Engineering
Susan H. Rodger
Professor of the Practice
Computer Science

We're going to see where it might be useful to write your own equals operator.

So we're going to be at word gram tester which I have here, and

I have one method called test word gram and and

if you see here we're just going to use sort of a simple string.

We have this string here that just says, this is a test, this is a test,

this is a test of words.

So we've got some repetition in there.

And what we're going to do with that long string is we're going to create

some word grams of size four, and then we're just going to print them out.

So I'm going to just run this.

So I'll create my object and run test word gram and

you can see we've got several word grams.

They're all four letter words.

And you can also notice that if we want to index zero, the first one, this is a test,

is the same as the one at index four and also at index eight.

So, that's just printing out the word gram.

So, if we go back to the code You can see that, we go through and

we just print out the index position, the links to the word gram and the word gram.

How do we do that?

lets go look at the word gram class and you'll see in there it has a link method.

That's how it knows how to print out links and for all of those it was four,

because there's four words, and it also has a two string method,

so it knows how to print out each word gram, and

essentially what it's doing, is it's going through and just printing out

all the words in the word gram by building the string and printing out the string.

So now, I've also got, we want to focus on equals, so

I've also got this other method, here,

called test word gram equals, and let's see what that does.

Again, we're generating word grams from the same string that we had before.

And this time when we generate them, we're actually creating an array

list of type word gram and we're storing them in there instead of printing them, so

you can see that in the first for loop,

then what we're doing is we get the first word gram, and we're

going to compare it to all the other word grams to see if any of them are equal.

If you remember, the first one at index zero and the one at index four and

index eight were all the same.

This is a test.

So let's see what happens if we just run this.

So, I'm sorry, compiled,

So this time we're running test word gram equals.

Oh and you can see it only matched the first one.

And now and if you remember the very first one we're getting here in our code.

And then we're comparing it with the first one and then all the others.

So it only matches the ones that are exactly the same,

they're actually the same object.

But it's not matching the other ones that have the same words in them.

So let's see what we have here.

So we're going to try and fix this so here again, here's our code, you can see,

we're getting the first one, and then we're printing out checking, and

then we go through the loop, starting at zero, so

we are checking the first one with itself, and then

we compare first double equals with list.get(k), and

we only found one match even though we should have found three matches.

So what we're going to do, let's first try to change this to something else.

Instead let's try using the .equals operator.

So if we say first.equals.

Let's see if that works.

So instead of using the double equals, we're going to use the dot equals.

So we'll compile it, let's see if that works better.

We run it and we still only get one match, the first match,

because they're the same object.

So what we'd like to do is we'd like to write our own equals operator.

So we're going to go back to the word gram class over here.

And we're going to add a new method.

Equals operator is going to be a return type boolean.

We're going to have to pass in object o.

And we're going to create a word gram, calling it other.

And essentially what we're doing is we will convert the object that we pass in,

so that it lets it know it's a word gram.

So we will cast word gram oh,

now I'm just going to make sure this works, so

I'm just going to have it return true.

And we'll make sure this works before we finish writing the equals method.

Because all this should do, is it should always return true.

So let's see what happens.

We should get all the matches.

Yes okay so we compile it, and we come over here and.

So now if we run it, we get everything's a match because we just always return true.

So that's not quite what we wanted, but

at least we know that we are getting that equals operator.

Because before when we ran it without that equals operator in there,

we only got one match.

So we know the code we just put in there is forcing it to at least run

the equals method that we just wrote.

Alright, so now we need to develop it a little bit further.

Okay, so the first thing we should probably do with word grams is make sure

that if we're comparing two word grams they have the same number of

words in them.

So let's do that first.

So we'll check the length of the object,

this if that is not equal to

the other.length.

So again, we're passing in some word other.

And we want to compare it to the actual object that it's comparing to.

If the links are different, hey, we know right away they're not equal.

So at that point, we want to return false.

Now if they are the same, then what we need to do is we need to go through and

compare word by word to make sure the first words in each are equal,

the second words in each are equal, and so on.

So we need a loop for that.

So we're going to have to create a for loop.

And then my words dot length is the length of a n gram.

Okay, so we'll iterate through all the words in my words.

And we know we have an index position for each of those.

And then what we want to check is we want to check and see,

we compare the first two words, if they don't match,

we know right away the n grams, the word grams aren't equal.

So we'll say if it's not true,

that my words, k, in the k's position,

dot equals.

Here we're using the dot equals for two strings.

Then we want to know the corresponding word in other,

which is going to be at other.wordAt(k).

So if that is not true, that means that those two words do not match.

Then we know again that's false.

And we can just return right away, without checking in any of the other words.

So we'll just say return false again.

And if we manage to get through checking all the words, and

we don't find any of them false, that means they all match,

then we'll get to the bottom of our method, and return true.

So let's try this out.

Okay. So now, we got it compiling and

let's see what happens when we run it.

And it worked, because it found the three matches that

are identical at slot zero, four, and eight.

So here's a case where sometimes you need to write your own equal operator and

we wrote it, and we saw that it worked, and that's pretty exciting.

Happy programming!

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