Ifi7107 lesson4

Lesson 4

2

OutlineFormatting Output

Boolean Expressions

The if Statement

Comparing Data

The switch Statement

Anatomy of a Method

Anatomy of a Class

Encapsulation

3

The switch Statement• The switch statement provides another way to

decide which statement to execute next

• The switch statement evaluates an expression, then attempts to match the result to one of several possible cases

• Each case contains a value and a list of statements

• The flow of control transfers to statement associated with the first case value that matches

4

The switch Statement• The general syntax of a switch statement is:

switch ( expression ){ case value1 : statement-list1 case value2 : statement-list2 case value3 : statement-list3 case ...

}

switchandcaseare

reservedwords

If expressionmatches value2,control jumpsto here

5

The switch Statement• Often a break statement is used as the last

statement in each case's statement list

• A break statement causes control to transfer to the end of the switch statement

• If a break statement is not used, the flow of control will continue into the next case

• Sometimes this may be appropriate, but often we want to execute only the statements associated with one case

6


switch (option){ case 'A': aCount++; break; case 'B': bCount++; break; case 'C': cCount++; break;}

• An example of a switch statement:

7

The switch Statement• A switch statement can have an optional

default case

• The default case has no associated value and simply uses the reserved word default

• If the default case is present, control will transfer to it if no other case value matches

• If there is no default case, and no other value matches, control falls through to the statement after the switch

8

The switch Statement• The type of a switch expression must be integers,

characters, or enumerated types

• As of Java 7, a switch can also be used with strings

• You cannot use a switch with floating point values

• The implicit boolean condition in a switch statement is equality

• You cannot perform relational checks with a switch statement

• See GradeReport.java

9

//********************************************************************// GradeReport.java Author: Lewis/Loftus//// Demonstrates the use of a switch statement.//********************************************************************

import java.util.Scanner;

public class GradeReport{ //----------------------------------------------------------------- // Reads a grade from the user and prints comments accordingly. //----------------------------------------------------------------- public static void main (String[] args) { int grade, category;

Scanner scan = new Scanner (System.in);

System.out.print ("Enter a numeric grade (0 to 100): "); grade = scan.nextInt();

category = grade / 10;

System.out.print ("That grade is ");

continue

10

continue

switch (category) { case 10: System.out.println ("a perfect score. Well done."); break; case 9: System.out.println ("well above average. Excellent."); break; case 8: System.out.println ("above average. Nice job."); break; case 7: System.out.println ("average."); break; case 6: System.out.println ("below average. You should see the"); System.out.println ("instructor to clarify the material " + "presented in class."); break; default: System.out.println ("not passing."); } }}

11

continue

switch (category) { case 10: System.out.println ("a perfect score. Well done."); break; case 9: System.out.println ("well above average. Excellent."); break; case 8: System.out.println ("above average. Nice job."); break; case 7: System.out.println ("average."); break; case 6: System.out.println ("below average. You should see the"); System.out.println ("instructor to clarify the material " + "presented in class."); break; default: System.out.println ("not passing."); } }}

Sample RunEnter a numeric grade (0 to 100): 91That grade is well above average. Excellent.

12

Outline

Formatting Output

Boolean Expressions

The if Statement

Comparing Data


Anatomy of a Method

Anatomy of a Class

Encapsulation

13

Method Declarations

• Let’s now examine methods in more detail

• A method declaration specifies the code that will be executed when the method is invoked (called)

• When a method is invoked, the flow of control jumps to the method and executes its code

• When complete, the flow returns to the place where the method was called and continues

• The invocation may or may not return a value, depending on how the method is defined

14

//********************************************************************// Guessing2.java Author: Adapted from Lewis/Loftus//// Demonstrates the use of a block statement in an if-else.//********************************************************************

import java.util.*;

public class Guessing2{ static void checkGuess(int, answer, int, guess) { if (guess == answer) System.out.println ("You got it! Good guessing!"); else { System.out.println ("That is not correct, sorry."); System.out.println ("The number was " + answer); } }

//----------------------------------------------------------------- // Plays a simple guessing game with the user. //----------------------------------------------------------------- public static void main (String[] args) { final int MAX = 10; int answer, guess;

continue

15

Continue

Scanner scan = new Scanner (System.in); Random generator = new Random();

answer = generator.nextInt(MAX) + 1;

System.out.print ("I'm thinking of a number between 1 and " + MAX + ". Guess what it is: ");

guess = scan.nextInt(); checkGuess(answer,guess);

}}

16

Continue

Scanner scan = new Scanner (System.in); Random generator = new Random();

answer = generator.nextInt(MAX) + 1;

System.out.print ("I'm thinking of a number between 1 and " + MAX + ". Guess what it is: ");

guess = scan.nextInt(); checkGuess(answer,guess);

}}

Sample RunI'm thinking of a number between 1 and 10. Guess what it is: 6That is not correct, sorry.The number was 9

17

myMethod();

myMethodcompute

Method Control Flow• If the called method is in the same class, only

the method name is needed

18

doIt

helpMe

helpMe();

obj.doIt();

main

Method Control Flow• The called method is often part of another

class or object

19

Method Header• A method declaration begins with a method header

char calc (int num1, int num2, String message)

methodname

returntype

parameter list

The parameter list specifies the typeand name of each parameter

The name of a parameter in the methoddeclaration is called a formal parameter

20

Method Body• The method header is followed by the method body


{ int sum = num1 + num2; char result = message.charAt (sum);

return result;}

The return expressionmust be consistent withthe return type

sum and resultare local data

They are created each time the method is called, and are destroyed when it finishes executing

21

The return Statement• The return type of a method indicates the

type of value that the method sends back to the calling location

• A method that does not return a value has a void return type

• A return statement specifies the value that will be returned

return expression;

• Its expression must conform to the return type

22

Parameters• When a method is called, the actual

parameters in the invocation are copied into the formal parameters in the method header


{ int sum = num1 + num2; char result = message.charAt (sum);

return result;}

ch = obj.calc (25, count, "Hello");

23

Local Data• As we’ve seen, local variables can be declared

inside a method

• The formal parameters of a method create automatic local variables when the method is invoked

• When the method finishes, all local variables are destroyed (including the formal parameters)

• Keep in mind that instance variables, declared at the class level, exists as long as the object exists

24

//********************************************************************// testMethod.java Author: Isaias Barreto da Rosa//// Demonstrates the use of methods.//********************************************************************

package testmethod;import java.util.Scanner;public class TestMethod { //----------------------------------------------------------------- // Receives one integer as parameter and multiply it by 1000 // if it is positive and by 500 if it is negative //----------------------------------------------------------------- static int executeFormula(int x) { int result; if (x>0) result = x*1000; else result = x*500; return result; }

continue

25

//----------------------------------------------------------------- // Reads one integers from the user and multiply it by 1000 // if it is positive and by 500 if it is negative (by calling // executeFormula) //-----------------------------------------------------------------

public static void main(String[] args) { int value; int r; Scanner scan = new Scanner(System.in); System.out.println("write a number"); value = scan.nextInt(); r = executeFormula(value); System.out.println("The result is: "+r); }}

26

//----------------------------------------------------------------- // Reads one integers from the user and multiply it by 1000 // if it is positive and by 500 if it is negative (by callin TestMethod) //-----------------------------------------------------------------

public static void main(String[] args) { int value; int r; Scanner scan = new Scanner(System.in); System.out.println("write a number"); value = scan.nextInt(); r = executeFormula(value); System.out.println("The result is: "+r); }}

Sample RunEnter a number : 2The result is .2000Enter a number: -2The result is : -1000

27

Lexicographic Ordering• Lexicographic ordering is not strictly alphabetical

when uppercase and lowercase characters are mixed

• For example, the string "Great" comes before the string "fantastic" because all of the uppercase letters come before all of the lowercase letters in Unicode

• Also, short strings come before longer strings with the same prefix (lexicographically)

• Therefore "book" comes before "bookcase"

28

Outline

Formatting Output

Boolean Expressions

The if Statement

Comparing Data


Anatomy of a Method

Anatomy of a Class

Encapsulation

29

Writing Classes• The programs we’ve written in previous

examples have used classes defined in the Java standard class library

• Now we will begin to design programs that rely on classes that we write ourselves

• The class that contains the main method is just the starting point of a program

• True object-oriented programming is based on defining classes that represent objects with well-defined characteristics and functionality

30

Examples of Classes

31

Classes• A class can contain data declarations and

method declarations

int size, weight;char category;

Data declarations

Method declarations

32

Classes• The values of the data define the state of an

object created from the class

• The functionalities of the methods define the behaviors of the object

33

Classes• We’ll want to design the Student class so

that it is a versatile and reusable resource

• Any given program will probably not use all operations of a given class

• See StudentManager.java

34

//********************************************************************// StudentManager.java Author: Isaias Barreto da Rosa//// Demonstrates the creation and use of a user-defined class.//********************************************************************

package studentmanager;import java.util.Scanner;public class StudentManager {

//----------------------------------------------------------------- // Compares the grades of two students and tells who is the best //----------------------------------------------------------------- static void compareStudents(Student st1, Student st2) { if (st1.getGrade() > st2.getGrade()) { System.out.print(st1.getName +" is a better student");} else { if (st2.getGrade() > st1.geGrade()) {System.out.print(st2.getName() +" is a better student");} else {System.out.println(st1.getNname() + " and " + st2.getName()

+ "are on the same level");} } }continue

35

Continue

public static void main(String[] args) { Student st1, st2, st3; String name; double grade; Scanner scan = new Scanner(System.in); System.out.println("Insert the first name for Student 1"); name = scan.nextLine(); System.out.println("Insert the grade for Student 1"); grade = scan.nextDouble(); st1 = new Student(name,grade); System.out.println("Insert the first name for Student 2"); name = scan.nextLine(); System.out.println("Insert the grade for Student 2"); grade = scan.nextDouble(); st2 = new Student(name,grade);

compareStudents(st1,st2); }}

36

continue

class Student{ private String name; private double grade; public final double MAXGRADE=20; //----------------------------------------------------------------- // Constructor: Sets the student’s name and initial grade. //----------------------------------------------------------------- public Student (String name1, double grade1) { name = name1; grade = grade1; } //----------------------------------------------------------------- // Returns the student's name //----------------------------------------------------------------- String getName() { return name; } //----------------------------------------------------------------- // Returns the student's grade //----------------------------------------------------------------- double getGrade() { return grade; } //----------------------------------------------------------------- // Increase the studens's grade //----------------------------------------------------------------- double increaseGrade() { if (grade < MAXGRADE);

{grade++;} return grade; } }

37

The Student Class• The Student class contains two data

values

– a String name that represents the student's name

– an double grade that represents the student’s grade

38

Constructors• As mentioned previously, a constructor is

used to set up an object when it is initially created

• A constructor has the same name as the class

• The Student constructor is used to set the name and the initial grade

39

Data Scope• The scope of data is the area in a program in

which that data can be referenced (used)

• Data declared at the class level can be referenced by all methods in that class

• Data declared within a method can be used only in that method

• Data declared within a method is called local data

• In the compareStudents class, the variable scan is declared inside the main method -- it is local to that method and cannot be referenced anywhere else

40

Instance Data• A variable declared at the class level (such as name)

is called class attribute or instance variable

• Each instance (object) has its own instance variable

• A class declares the type of the data, but it does not reserve memory space for it

• Each time a Student object is created, a new name variable is created as well

• The objects of a class share the method definitions, but each object has its own data space

• That's the only way two objects can have different states

41

Instance Data• We can depict the two Student objects from

the StudentManager program as follows:

st1 Johnname

st2 Maryname

Each object maintains its own name variable, and thus its own state

42

Quick CheckWhat is the relationship between a class and an object?

43

Quick CheckWhat is the relationship between a class and an object?

A class is the definition/pattern/blueprint of an object. It defines the data that will be managed by an object but doesn't reserve memory space for it. Multiple objects can be created from a class, and each object has its own copy of the instance data.

44

Quick CheckWhere is instance data declared?

What is the scope of instance data?

What is local data?

45

Quick CheckWhere is instance data declared?

What is the scope of instance data?

What is local data?

At the class level.

It can be referenced in any method of the class.

Local data is declared within a method, and is only accessible in that method.

Software

Ifi7107 lesson4