CS 1302 Chapter 9 (Review) Object & Classes

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1 CS 1302 Chapter 9 (Review) Object & Classes Reference Sections , Defining Classes for Objects 1. A class is a blueprint (or template) for creating objects. A class defines the state (the current conditions) and behavior (the things it can do) that objects have. State is implemented with instance variables (also called data field, field, property, attribute) Behavior (also called services, responsibilities) is implemented with methods. 2. The Unified Modelling Language (UML) is a standardized graphical language for modelling object-oriented software. UML is composed of a number of different diagrams, however we will only consider class, object, and package diagrams. 3. A UML class diagram is used to model the classes in a system. As shown above on the right, a class diagram has three compartments: the top box shows the name of the class, the middle box shows the state and the bottom box shows the behaviors. On the right, we show the complete notation used for a class diagram. Notice that the notation is a little bit different than Java because UML is for any object-oriented language. For example: Methods are shown as: method(paramname : datatype) : returntype Instance variables has this format varname : datatype We will discuss the dependency arrow later. 1

2 4. We can show a class in various degrees of detail as shown below, depending on the stage of modeling. When you first start to analyze a problem, you may only be concerned with documenting the classes. Later, you may add state and behavior. Finally, you may assign visibility, parameters, datatypes, and return types. 5. A UML object diagram shows an example of objects that might exist in a system when it is running. For example, the object diagram on the right shows that we have a dog reference variable of type Dog and that this dog has a name of Spot. The full notation is shown below Example 1. A class is need to represent a box. A box has a length, width, and height which are supplied when the box is first created. The length, width, and height cannot be directly changed except through setters (mutators). They can also be retrieved. A box has a method that can calculate the volume. It also has a method that accepts a length, width, and height of a product that it might contain. The method should return true if the product will fit in the box and false otherwise. a. Draw a class diagram to represent the box. b. Draw an object diagram that shows a box. c. Write the box class. d. Write a main to test the class. 2

3 Homework: 1. A class is needed to represent a savings account. Savings accounts have an owner and a balance which are initialized when the account is first created. The owner can never be changed, however the owner can be retrieved. The balance can be retrieved also. The balance cannot be changed directly except through deposit and withdrawal actions which accept an amount of money, which must be positive. A deposit increases the balance by the amount of money that is passed in and a withdrawal decreases the balance by the amount of money that is passed in. A withdrawal should not be executed if it would decrease the balance below zero. In addition this class has a method that accepts an interest rate in decimal form, which must be positive and increases the balance by this rate. a. Draw a class diagram for this class. b. Draw an object diagram that shows a savings account. c. Draw an object diagram that shows a savings account with a name and balance. d. Write this class. e. Write a main to test this class. 2. The default value for data fields: reference type is null, numeric is 0, and boolean is false. There is no default value for local variables. 3. When we create an object we usually assign it to a variable. This type of variable is called a reference type and the variable is called a reference type variable. For example: 4. A reference type variable is not the actual object (although we frequently refer to it as the object). It really is a pointer to a location in memory where the actual object resides. This is shown in the table below on the left. first row of the table on the left, below. As shown in the second row, when assign a new reference variable (b2) to an existing one (b1), we create another reference to the same box. Thus, if we use that new reference (b2) to change the box, the original reference (b1) reflects this change (because they both refer to the same box) as shown in the third row of of the table. Reference Types Primitive Types Code Memory Code Memory Box b1 = new Box(4,6,2); int x = 3; Box b2 = b1 int y = x; b2.length = 6; y = 5; 3

4 The primitive data types in Java are: byte, short, int, long, float, double, boolean, char. All other types are reference types. Use of the assignment operator with primitives works differently as shown in the figure on the right, above. Homework: 2. Consider the three lines of code below. Draw an object diagram showing what is in memory after each line is executed. Thus, there will be three diagrams. Box b1 = new Box(2.0,3.0,4.0); Box b2 = new Box(1.0,5.0,3.0); b1=b2; 9.7 Static Variables & Methods 1. A static variable is shared by all instances of a class. We use the class name to reference a static variable. For example: Math.E or Math.PI 2. A static method is shared by all instances of a class, thus, it cannot depend on on the values of instance variables for a particular object. For example, the String class defines methods like substring and indexof which operate on a particular instance of a string. The String class also defines the static method, valueof that turns an integer (or double, Boolean, etc) into a string. We use the class name to reference a static method. For example: 3. We don t use static variables and methods very often in domain classes. For instance, the Box and SavingsAccount classes we have defined earlier would not need either. As far as this class goes you will never need to define your own static variables or methods. The one exception is when we write driver classes to test our code we frequently use static methods. For example: public class BoxDriver { public static void main(string[] args) { boxtest1(); public static void boxtest1() { Box b = new Box(2,3,4); System.out.println(b.volume()); b.setlength(10.5); System.out.println(b.volume()); 4

When we call a static method from the class where it is defined we do not have to precede the call with the class name as shown above. 9.8 Visibility 1. A package is a way to group related classes.

A package not only provides a way to organize classes, it also provides protection as we will see below. 2. We declare a package with a package statement as the first line in a file.

5 When we call a static method from the class where it is defined we do not have to precede the call with the class name as shown above. 9.8 Visibility 1. A package is a way to group related classes. The UML package diagram shown on the right shows a package named warehouse_stuff which contains three classes: Box, PackingTape, and PackingPeanuts. A package not only provides a way to organize classes, it also provides protection as we will see below. 2. We declare a package with a package statement as the first line in a file. By convention, package names are all lower-case with an underscore separating words. For example: Box.java PackingTape.java PackingPeanuts.java package warehouse_stuff; package warehouse_stuff; package warehouse_stuff; public class Box {... public class PackingTape {... public class PackingPeanuts {... All classes in a package must be inside a folder with the same name as that of the package. 3. Example Suppose we have a class A in package pack1 and a class B in package pack2 as shown in the figure on the right. If class A needs to use class B, then it must import it as shown below. A.java package pack1; import pack2.b; B.java package pack2; public class B { public class A { public static void main(... ) { B b = new B(); You can import all classes in a package with, for example: import pack.*; however, by convention, it is recommended that you only import the specific class(es) you need. 4. There are 2 visibility (accessibility) modifiers for classes: UML Java Example Meaning + public public class Box {... The class is available in any package [blank] [blank] class Box {... The class is available only in containing package 5

6 5. There are 4 visibility modifiers for members (methods and instance variables) in a class. In order of most to least visible, they are: UML Java Meaning + public Available in any class # protected [We learn about this in Ch 11] [blank] [blank] Available in any class in the same package - private Available only in the class (and not in subclasses). 6. Classes and methods can access classes and methods in other packages if they are imported and if the accessibility allows it. For example, consider the packages and classes shown in the UML diagram on the right and note the following: Class A can access class C because C is public. It can also access class B because it is in the same package. Class B can access classes A and C. Class C can access class A, but not B because class B has package-level accessibility. Methods in C can access the x method in A, but not they y method because it is private, and not z because it has package-level accessibility. 7. We can also define subpackages. For example: package pack1.subpack; public class A {... In this case, class A would be defined in a subpack folder and the subpack folder would be inside the pack1 folder. For a class in another package to access class A, you need to import the package and subpackage. For example: package another_package; import pack1.subpack.a;... 6

7 9.9 Data Encapsulation 1. In general we want to encapsulate the state of a class. What this means is that we do not want to allow a client (another class using your class) to directly change the values of instance variables. To make the data in a class encapsulated, we make all instance variables private and provide getters and setters as shown in the figure below. Not Encapsulated public class Box { public double length;... Box b = new Box(2,4,6); b.length = 10; Encapsulated public class Box { private double length; public void setlength(double length) { if( length > 0 ) this.length = length; public double getlength() { return length;... Box b = new Box(2,4,6); b.setlength(10); 2. You may say, what s the big deal? Why do I have to do all this extra work? Real systems are composed of many classes written by different teams and developers. Your classes may be used by other developers and vice versa. If your Box class is not encapsulated, you may know not to assign a negative value to the length, but others using your class may not know that. It is better to practice defensive programming, to take steps to make sure others use your classes correctly. One way to do this is to protect (encapsulate) the state of your classes. In addition to inadvertent misuse of a class, encapsulation also protects against deliberate misuse of a class. For example, you wouldn t define a BankAccount class with a public balance method because then clients could change the balance to any value they wanted. Instead, you would define a deposit method which logs the amount of the deposit in a database and updates the private balance. As an analogy, when you go to the store to purchase something and the cashier says, $10 please, you could just hand your wallet to the cashier in which case your wallet would be public: public class You { public double wallet; But of course, your wallet is private! public class You { private double wallet; public void paycashier(double amount) { // take money out of wallet and hand to cashier 7

3. When we want a property to be read-only (i.e. we allow a client to retrieve the value of a property, but not change it) we simply omit the setter. Homework: 3.

8 3. When we want a property to be read-only (i.e. we allow a client to retrieve the value of a property, but not change it) we simply omit the setter. Homework: 3. Draw a class diagram for an encapsulated employee class that has a name and hourly pay rate and has a method that accepts the number of hours worked and returns the pay. The name is read-only. Also, write the code for this class Passing Objects to Methods 1. All variables in Java are passed by value. This means that when you pass an argument to a method, the method receives a copy of the argument. In the case of a reference variable, a copy of the pointer to the object is passed. 2. Consider the method on the right and what happens in memory when the code is executed as shown below. The calling method defines a box reference and when it is passed to the method, a local copy is made, b for use public static void shrinkbox( Box b ) { b.setlength( b.getlength()-1 ); inside the method. We use the local reference to manipulate the box. When the method finishes and control returns to the calling program, the reference b is no longer available (it is garbage collected), and we continue to use the box reference. 8

9 3. As another example is shown on the right. This method creates a new box, but as we see (below), it is short lived, when we return to the calling program, the original reference, box public static void changebox( Box b ) { b = new Box(9,9,9); Homework: 4. Modify the SavingsAccount class from homework problem 1 so that it has a method, mergeaccount that accepts another SavingsAccount object. This method should add the balance of this instance to the balance of the calling object if the two accountowners are the same. Write a main to test the class. 9

10 9.11 Array of Objects 1. Frequently, we say things like, we need an array of Boxes. Technically, the array of Boxes is really an array of references to Box objects. Below, boxes is a reference to an array that resides in memory. The array itself contains references to Box objects that reside elsewhere in memory. Box[] boxes = new Box[3]; boxes[0] = new Box(4,4,4); boxes[1] = new Box(9,9,9); boxes[2] = new Box(6,6,6); 2. Next we consider the case where we have two references to the same array and we use one reference to change an element in the array. As we saw earlier (in a different context), both references reflect the change as (of course) they are both pointing to the same array. Example (continued from above): 10

11 Homework: 5. Write a class, SavingsAccountTester with the following static methods: a. totalbalance accepts an array of SavingsAccount objects and returns the sum of the balance in each object. b. applyinterest accepts an array of SavingsAccount objects and an interest rate and applies this interest rate to all the objects. c. accountsreport accepts an array of SavingsAccount objects and returns a formatted string showing the name and balance of each account, one per line. The format should be like this: Gibson, $12, Watson, $6, Write a main to test the class Immutability 1. Sometimes it is useful to define a Class that doesn t allow the state to be changed. In other words, the state is set when the object is created, but can never be changed after that. Such a class (object) is called immutable. 2. Example This box class is immutable: public class ImmutableBox { private double length, width, height; public double getlength() { return length; public double getwidth() { return width; public double getheight() { return height; public ImmutableBox( double l, double w, double h ) { length = l; width = w; height = h; public double volume() { return public String tostring() {... 11

12 3. Example This student class is not immutable. The getscores method returns the array of scores, which is a reference variable. Thus, scores can be changed. public class Student { private String name; private int[] scores; private double average=0.0; public static void main(string[] args) { public Student(String name, int[] scores) { this.name = name; this.scores = scores; for(int score : scores) { average += score; average /= scores.length; public String getname() { return name; public int[] getscores() { return scores; public double getaverage() { return average; 4. Thus, the requirements for a class to be immutable are: All instance variables must be private. There are no setters (or other methods) that change the values of instance variables. No methods can return a reference to an instance variable that is a mutable reference type. 4. The String class is immutable. This means that a String can never be changed. When you change a string a new String is created and assigned to the reference as shown in the figure on the right. Code String s = "dog"; s += " and cat"; Memory 5. This means that we need to be aware that a String (unlike most objects) cannot be changed in a method. Consider the method shown on the right. Then, consider what happens in memory as the code is executed: public static void doesntchangestring(string str){ str += " and cat"; 12

Code Memory String s = "dog"; doesntchangestring(s); // Inside doesntchangestring() str += " and cat"; // Back in main If we did want to change a string, we could have the method return the changed

13 Code Memory String s = "dog"; doesntchangestring(s); // Inside doesntchangestring() str += " and cat"; // Back in main If we did want to change a string, we could have the method return the changed string as shown on the right and the explanation of what is happening in memory as the code executes shown below: Code public static String changestring(string str){ str += " and cat"; return str; Memory String s = "dog"; s = changestring(s); // Inside changestring() str += " and cat"; return str; // Just after return 9.13 Scope of Variables 1. Read and understand 13

14 9.14 this 1. The this keyword is use to reference an item inside the object itself. It is sort of like using the pronoun my (my arm, my wallet, etc). 2. There are two common ways to use this, as the example below shows: We can use this to call another constructor in the same class. It must be the first line of the constructor. We can use this to differentiate between an instance variable and a parameter with the same name. Homework: 6. Modify the SavingsAccount class so that it has the following constructors: a. SavingsAccount() initializes the object to have a balance of 0 and an account owner, unknown. b. SavingsAccount(double initialbalance) initializes the object to have a balance of initialbalance and an account owner, unknown. c. SavingsAccount(String owner) initializes the object to have a balance of 0 and an account owner, accountowner. The constructor below already exists. The three above should call this constructor. SavingsAccount(String owner, double initialbalance) initializes the object to have a balance of initialbalance and an account owner, accountowner. 14

15 Miscellaneous Review Material 1. Java defines the % binary operator which provides the remainder when the two operands are divided. Some examples are shown on the right. Notice that x%y yields a value in the range: {0,1,2,, y 1. This can be useful as we show shortly. The % operator is also called the remainder operator and the modulus operator. Consider x%y, we typically speak this by saying, x mod y. 2. A problem that occurs regularly is determining if a number is odd or even. A useful trick is to take the number and mod it by 2. If the result is 0 then the number is even, otherwise it is odd. For example: int val = (int)(math.random()*1000); if(val%2==0) { // val is even else if(val%2==1) { // val is odd 0 % 5 = 0 1 % 5 = 1 2 % 5 = 2 3 % 5 = 3 4 % 5 = 4 5 % 5 = 0 6 % 5 = 1 7 % 5 = 2 8 % 5 = 3 9 % 5 = 4 10 % 5 = 0 0 % 8 = 0 1 % 8 = 1 2 % 8 = 2 3 % 8 = 3 4 % 8 = 4 5 % 8 = 5 6 % 8 = 6 7 % 8 = 7 8 % 8 = 0 9 % 8 = 1 10 % 8 = 2 3. Mod and integer division are useful together sometimes. For example, how do you translate seconds into minutes and seconds? int time = 143; int minutes = time/60; int seconds = time%60; 4. Example Suppose you have an array of integers, vals. Write a snippet of code to print the values 4 per line. Solution: Suppose that there are 14 values in the array. Then, The number of complete lines (rows) is: 14/4 = 3 The number of values in the last (incomplete) line (row) is: 14%4 = 2 15

16 Example 1. A dog has a name (which can t be changed) and a toughness (integer) that are initialized when the dog is first created. However, a dog can be created with just a name, in which case the toughness is set to 1. A dog can walk. When it walks it returns a message like this: dogname is walking. A dog can also bark by returning a message like: dogname is barking. A dog can also bark at another dog by returning a message like: dogname is barking at otherdogname. a. Model this situation with a class diagram. b. Write the code for this class. c. Write some code to test the class. d. Write a static method that accepts an array of dogs and has them all walk printing the results. e. Write a static method that accepts an array of dogs and has the first dog bark at all the other dogs printing the results. f. Write a static method that accepts an array of dogs and returns the dog that is the toughest (highest toughness). g. Write a static method that accepts an array of dogs and returns a new array that holds every other dog. For example, if the input is 5 dogs, then an array will be returned with the first, third, and fifth dogs. Or, if the input is 4 dogs, then it will return an array with the first and third dogs. h. Suppose you use the array returned in g to change the toughness of the first dog. Then, you print the toughness of the first dog in the array that was the input to g. What will this toughness be? i. Modify the classes to handle this situation: A dog can fight another dog. The one with the highest toughness wins. Dogs need to keep track of how many fights they have been in and how many they have won. 16

Chapter 7 Classes & Objects, Part B

Chapter 7 Classes & Objects, Part B These note present Dog simulation example that shows how we go about OO modeling. A number of new things are introduced. They also present the Person > BirthDate example.