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all right in today's lecture we're going to look at a sort of an extended example of designing a data type so we're going to build a fraction class and along the way we're going to look at a few things we haven't talked about we'll look at a few details about how constructors work in Java how methods can operate on objects of the same type and we'll look at a private helper method so normal instance methods of a class or declare public but you can also make methods that are declared private and all this will be surrounding the development of a fraction class and it's developed you know it represents fractions like 1/4 or 3/4 and things like that and towards the end you get a chance to implement some of the some of the methods I'm not going to show you in the lecture today just as a reminder where do we get our objects from in in Java well any kind of sort of non primitive object we've got to create we have to use that new operator and instantiate the object and how do objects like our little colored ball object here how do they come into being well the object store the constructor is what creates the object to begin with and here's an example constructor at the bottom we have a ball constructor it takes three parameters the XY and the radius and often what the code inside of a constructor will do is assign the instance variables of the ball class in this example to the parameters that were passed into the to the constructor method now if you do not you do not have to declare a constructor all right so a constructor method is a method that's in your in your class but it's named the same as the class and it has no return type that's what makes it a constructor you don't have to declare one and here's an example of a fraction class where we have not declared a constructor and the rule in Java is if you do not declare a constructor one will be assigned to you or one will automatically be created for you this is called the default no arg constructor there's no code associated with it and all it's going to end up doing is you're going to create an object and that object is going to have whatever the default value so in this case we've got the numerator and the denominator instance variables they're just going to either be assigned the value I give so I could say private int num equal five if I wanted to but if you don't specify something there then it's going to take the default default value for that datatype in the case of an integer that's going to be zero so first rule if you don't declare a constructor you'll get one automatically it just won't do anything if you've got a no arg constructor when you instantiate your object so in this example we're instantiate in a fraction called a equals new so the new operator is signifying we're calling the constructor here's the name I mean the name of the class and then you must give a parameter list even if you want to call that default in the large constructor Java requires you put the empty parentheses there and basically say I'm calling a constructor it's just the constructor doesn't take any parameters and in fact in the example I just showed you in the prior slide there is no actual method you could point at in the fraction class but nonetheless to create that fraction object with the Norg constructor you need to use those empty empty parentheses importantly remember whenever you create an object an array of reference types we've created here we're saying please Java I want to fracks variable and that data type is an array of type fraction and please make me two entries so I'm calling a new operator but here the difference is I'm using the square bracket so this is saying make me an array that has the potential to hold two fraction objects now it doesn't actually contain any fraction objects until I go through and instantiate an object into each element of that fracks array all right so typical style is you first create the memory for the array and then you go through either explicitly or in a in a for loop or something you go through all those you iterate through them and create the create the objects in each of the slots in the array normally you're going to you know in most classes except for really simple classes you're going to declare your own constructor all right but you need to understand once you do that Java will kind of pull the rug out and take away that default no our constructor all right as soon as I declare a constructor so it's a public method it has the same name as the class it has no return type all right in this case it takes the numerator and the denominator the fraction and then assigns the instance variables equal to it that's fine and now whenever I create a fraction object I must pass it to integers a numerator denominator that default no art constructor has essentially been then been destroyed or I don't know it's not there anymore okay and if you had code that depended on the the no art constructor you've now broken it and here's a client and it was calling um you know there's no art constructor up here as well as the nor constructor here in the array and none of this will compile anymore with the fraction class I just showed you because the only constructor form as soon as you declare one the only constructor you have are the ones you've explicitly declared in the class and you'll get an error like this constructor not found this does not match the only constructor all right it could find takes two parameters and into the net not too hard to fix if you really in fact want a default Norgan maybe you want to whenever you create a fraction if you don't specify you know you could have a fraction that just has a zero in the numerator and a zero in the denominator you have to actually not explicitly declare it and that's what I've done here I've declared a constructor that just just doesn't do anything and I've got it this is an overload situation I've got two methods with the same name just different parameter lists and that's got to be the case right they have to differ in the number or in the types of the parameters those overloaded constructors take all right another thing we haven't maybe seen or maybe you haven't seen is passing in parameters of the same type as yourself and here's a special circumstance one called a copy constructor often sometimes what you want to do is you've got an object of a certain type and you want to create a new object that's an identical copy and a copy constructor does just that in this constructor I'm passing in a reference to another fraction object and what do I do well I copy the data I say my numerator is equal to that guy's numerator my denominator is equal to that guy's denominator and now I'm a copy I'm the same fraction but I'm a different object and so somebody could come along and do something change me make me me the reciprocal or do something to my set you know the one fraction object without impacting the other one right that's one thing I wanted to show you on the slide is the idea of a copy constructor so it's a common common thing you'll see in a lot of a lot of software the other thing I wanted to show you is how it's getting there um it turns out you can get to private instance variables of parameters that are the same type because I'm a fraction I'm an instance method and I'm a constructor in this case but I could be you know any other instance method in that fraction class because I'm inside the fraction 5 well excuse me because I'm inside that fraction class any instance method of the fraction class if you pass it another fraction object it's allowed to access the private instance and variables so I can just use the dot and then the name of the instance variable and get to that instance variable and you might think you can't do that because of data encapsulation but you kind of trust I guess the idea is you at least in Java you trust objects of your own type so if you're an instance method of type fraction you're allowed to look at sort of the private you know the private data of other fraction objects often by convention I will call the parameter like this other and there's nothing special about the name other it's a variable name like any I could I could you know change this parameter here and I could call it you know Oh or I could call it you know foo or whatever it's just the parameter name into the method but often it helps if you call it other than you kind of think of you know I'm my object and the other guy is the other object so it helps you keep it a little straight all right let's do some fractional some grade-school fraction computations and the first I'm going to give how I'm going to do the easy ones and you're going to get through the hard ones multiplying fractions given to fraction objects return a new fraction that is the multiplication of the two alright we need to fraction objects so do we need a method with the tick's of the parameter two fractions not if we're going to use instance methods all right we're creating a new fraction by taking a and multiplying it by B so there are two fractions involved there's the fraction that's running the instance variable or the instance method multiplied and then there's the fraction parameter B that's getting passed into that method and then they're going to return a new fraction object and I'll show you the code and here's the goal right we want to have one half time two thirds so half of two thirds is 1/3 that's our goal let's take our first stab at implementing this multiply and if the multiply of method is returning a fraction object you're going to need to new a fraction object inside of that method and that's what's going on here we're creating a new local variable called result it's of type fraction I'm in stanching it instantiating it by calling a new operator and what shall I create the new fraction to be well what's the multiplication of two fractions it's my numerator times the other guy's numerator over the other guy's denominator times the sorry my denominator times the other guy's denominator all right this is what this fraction C equals x is numerator would be the numerator of a denominator would be the denominator of a other num would be bees numerator and other dot denominator would be bees numerator that would get put into the result object and then that's what's actually you know the variable C is going to refer to as that that result object does it work mmm not so much here's the code I'm doing the multiply and what am I getting well whenever you see something like this all right what we're doing is we're printing an object of type fraction then we're printing out you know double quote star double quote and then the fraction B then equal sign and the fraction C if we don't do anything these are of reference type and Java will go ahead and print but it's default format is just to print the class name at and then memory address all right not terribly useful at least you can tell it's kind of working but it's kind of not what we hope for as far as output from from our fractions how do we fix that well in Java there's a two string method and what you can do is the fraction class is allowed to over override that fraction class or that two string method so if you declare our two string method in your fraction class whenever you call system's out I'll print 'ln with a fraction datatype it's going to call to string and to string for this to work it needs to be public it needs to return a string and it not needs to be named to string exactly like this and take no parameters this will be very common thing we will be doing in a lot of the classes we'll develop because often we want to print them out or in order to debug them it's handy to be able to just send the send the object into a print statement and get some sensible representation in this case okay I'm I'm putting the numerator and then putting a little flash in the denominator and then I return this string and that becomes the output from system dot out dot print line so system dot out dot print line automatically will call this so you don't have to call dot to string you just get that automatically these lines here would now call it's kind of like doing a dot to string and you could do that but there's no no particular reason to do that it will automatically happen for you now I forgot to mention so what's the problem well let's say we want to actually display this in lowest terms one half times two thirds is to six which is true but normally we'd reduce to six to the lowest terms and have it output one-third here is some code that does that and um first of all I've got the beginning of my multiply do the same thing as I did before I take the numerator and of the two and multiply the denominator is a two and I get my new fraction object but what I'm going to do is try and find the biggest number that divides evenly into both the numerator and the denominator and if that has any chance of succeeding right there's no sense of starting at you know the biggest number we start at whichever the numerator after doing this multiplication you know like in the last slide right I do that multiplication the numerator would be two and denominator be six and what I'll do is start my I at the minimum of those two and and if you know that is zero so this would happen if both the numerator and denominator zero I'll just give up alright because I don't want my program to crash alright it's usually bad in most programming languages if you try to divide by zero or use something like the percent the remainder operator if you've got a zero in there it's going to crash so a zero I just don't do anything leave it at Nino zero zero at time if it's not then what I do is I start at that I which is the minimum you know the the minimum of the two numerator denominator I see well this first is shorthand for does that number I evenly divided in the numerator okay or does not sorry we want to keep going as long as what we're looking for is the number I two evenly divide into both the numerator and the denominator and then we stop so as long as it doesn't evenly divide into the numerator or it doesn't evenly divide into the denominator then keep reducing it so we're going to start at something like two and say well this to divide into two yeah it does this two divided the six yeah it does then we're going to exit out and not reduce I anymore and divide two by two and six by two getting one-third all right that's a simple example so in a more complicated example I might start at a higher number and then work its way down until it finds that biggest common denominator and then it's going to divide both the numerator and denominator and this is okay so normally we'd be kind of worried here right we've got the slash and the right sides and integer and the left sides and integer and so that's integer division but it's okay in this circumstance because we know it's evenly divisible so energy division and normal floating point division would be the same thing and actually we want this to type to hit edge when we're done great that's the code for multiply and and we're going to reduce it to lowest terms and I should say like when I do this fraction class there are a lot of sort of design choices I've made and these are just some of the ways you could imagine building a fraction class to work it wouldn't necessarily be the case that I would always want to reduce you know multiplication to lowest terms I'm just showing you some of this for sake of an example and you know it's up to you when you design your own data types exactly what things do let's do division okay and to do division all right we're going to take the numerator the other guy and times it so I'm a fraction right I have a numerator and denominator and essentially what i'm doing here is multiplying by the reciprocal of the other guy so i flipped his denominator and his numerator and it's the same as multiply the except for that and i have the same issue with divide is i need to reduce it to lowest terms and so if you look at these two slides all right all i've done is done that swap in the top okay and this code to reduce it to lowest terms is identical alright and if you remember our saying repeated code is evil we really don't want to have this you know somewhat complicated algorithm to reduce the lowest terms in two places in our code and so i'm going to show you how to do that because you're going to have a similar issue and add and subtract and a lot of other methods are going to need to reduce a fraction to lowest terms and so rather than repeat that code what we're going to do is put it into a helper method here's the helper method it's I'm calling a helper method and oftens you know say that one it's a private method alright squared private which means no one outside of the fraction class can ever call reduce and that's again a design choice maybe you know you could make other choices when you build your fraction class but I decided I will take care of reducing the lowest terms inside of the other methods like multiply and what I've done is just move that code into here and I'm a fraction and all I'm going to do is look at my numerator and my denominator and I'm going to work my way backwards and find in number that evenly divides in both ok and I think I was almost thinking that yeah yep so that's what I've done and now I can change my multiply called the dot reduce on the result and then return the result so this will always return a multiplication that's in lowest terms and the advantage obviously is now I didn't show you the divide method but the divide method can do the identical thing so I've showed you the main things I wanted to show you today about no no our default constructors and other overloaded constructors and we talked about private help helper methods and now what I think you should do is complete the missing code from the web page I'll put a link to a fraction class it's it's similar to this it has more comments and I think I've put in some temporary code and all these methods just so it compiles and you can work on and it has big tests main that you can test out once you've implemented the equals method and the reciprocal method the add method and the subtract method and