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COMP 110 Classes Luv Kohli October 1, 2008 MWF 2-2:50 pm Sitterson 014
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Announcements Lab 4 due 2
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Questions? 3
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Today in COMP 110 Briefly go over Program 2 Classes 4
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Program 2 5 12.7+3.3 0123456789 Operand 1Operand 2Operator
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Program 2 6 12.7+3.3 0123456789 Assume the String above is in the variable calculation. int firstSpace = calculation.indexOf(‘ ’); int lastSpace = calculation.lastIndexOf(‘ ’); double operand1 = Double.parseDouble(calculation.substring(0, firstSpace)); double operand2 = Double.parseDouble(calculation.substring(lastSpace + 1)); char operator = calculation.charAt(firstSpace + 1);
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Program 2 Now you can determine which calculation to perform using a switch or if/else double result = 0.0; switch (operator) { case ‘+’: result = operand1 + operand2; break; case ‘-’: result = operand1 – operand2; break; // and so on } 7
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Exponent ^ double result = 0.0; switch (operator) { case ‘^’: result = 1.0; for (int count = 0; count < (int) operand2; count++) result *= operand1; break; case ‘-’: result = operand1 – operand2; break; // and so on } 8
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== generally bad for floating-point Floating-point numbers are imprecise ◦ After doing many computations, value may not be exactly the same ◦ 5.0 * 3.0 might be different from 1.0 + 2.0 + 3.0 + 4.0 + 5.0 Okay for this assignment for the divide by 0 extra credit 9
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num++ vs. ++num num++ is NOT num + 1 num++ does num = num + 1 So does ++num, BUT, there is a difference int num1 = 5; System.out.println(num1++); // outputs num1 (5), then increments num1 int num2 = 5; System.out.println(++num2); // increments num2, then outputs num2 (6) 10
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Class A class is the definition of a kind of object ◦ A blueprint for constructing specific objects 11 Class Name: Automobile Data: amount of fuel speed license plate Methods (actions): accelerate: How: Press on gas pedal. decelerate: How: Press on brake pedal.
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Objects, Instantiation 12 Object Name: patsCar amount of fuel: 10 gallons speed: 55 miles per hour license plate: “135 XJK” Object Name: suesCar amount of fuel: 14 gallons speed: 0 miles per hour license plate: “SUES CAR” Object Name: ronsCar amount of fuel: 2 gallons speed: 75 miles per hour license plate: “351 WLF” Instantiations, or instances, of the class Automobile
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UML (Universal Modeling Language) 13 Automobile - fuel: double - speed: double - license: String + accelerate(double pedalPressure): void + decelerate(double pedalPressure): void Class name Data Methods (actions)
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Objects Important: classes do not have data; individual objects have data Classes specify what kind of data objects have 14
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Class files and separate compilation Each Java class definition goes in its own, SEPARATE.java file ClassName save the file as ClassName.java Student.java includes the class Student 15
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Class files and separate compilation What happens when you compile a.java file? ◦.java file gets compiled into a.class file Contains Java bytecode Same filename except for.class instead of.java (note to self: show this) You can compile a Java class before you have a program that uses it 16
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class Student 17 Class Name: Student - Name - Year - GPA - Major - Credits - GPA sum + getName + getMajor + printData + increaseYear How: increase year by 1 + calcGpa How: average grades
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class Student 18 Class Name: Student - name: String - year: int - gpa: double - major: String - credits: int - gpaSum: double + getName(): String + getMajor(): String + printData(): void + increaseYear(): void + calcGpa(double grade): void
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Defining a class public class Student { public String name; public int classYear; public double GPA; public String major; //... public String getMajor() { return major; } public void increaseYear() { classYear++; } 19 Class name Data (instance variables) Methods
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Creating an object Create an object jack of class Student Student jack = new Student(); Scanner keyboard = new Scanner(System.in); Create an object keyboard of class Scanner 20 Create an object Return memory address of object Assign memory address of object to variable
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Instance variables Data defined in the class are called instance variables public String name; public int classYear; public double GPA; public String major; 21 public: no restrictions on how these instance variables are used (more details later – public is actually a bad idea here) type: int, double, String… variables
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Using instance variables: inside the class definition 22 public class Student { public String name; public int classYear; public double GPA; public String major; //... public String getMajor() { return major; } public void increaseYear() { classYear++; }
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Using public instance variables outside a class public static void main(String[] args) { Student jack = new Student(); jack.name = “Jack Smith”; jack.major = “Computer Science”; System.out.println(jack.name + “ is majoring in ” + jack.major); Student apu = new Student(); apu.name = “Apu Nahasapeemapetilon”; apu.major = “Biology”; System.out.println(apu.name + “ is majoring in ” + apu.major); } jack.name and apu.name are two different instance variables because they belong to different objects 23
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Methods Two kinds of methods ◦ Methods that return a value Examples: String’s.substring() method, String’s.indexOf() method, etc. ◦ Methods that return nothing Example: System.out.println() 24
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Methods public String getMajor() { return major; } public void increaseYear() { classYear++; } 25 returns a String returns nothing return type
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Defining methods that return nothing Method heading: keywords ◦ public: no restriction on how to use the method (more details later) ◦ void: the method returns nothing Method body: statements executed when the method is called (invoked) ◦ Must be inside a pair of braces public void increaseYear() { classYear++; } 26
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Method printData As usual, inside a block (defined by braces), you can have multiple statements public void printData() { System.out.println(“Name: ” + name); System.out.println(“Major: ” + major); System.out.println(“GPA: ” + gpa); } 27
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Calling methods that return nothing object, followed by dot, then method name, then () Use them as Java statements Student jack = new Student(); jack.classYear = 1; jack.increaseYear(); System.out.println(“Jack’s class year is ” + jack.classYear); 28
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Defining methods that return a value Method heading: keywords ◦ public: no restriction on how to use the method (more details later) ◦ Type: the type of value the method returns Method body: statements executed ◦ Must be inside a pair of braces ◦ Must have a return statement public String getMajor() { return major; } 29
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return statement A method that returns a value must have at least one return statement Terminates the method, and returns a value to the caller Syntax: ◦ return Expression; Expression can be any expression that produces a value of type specified by the return type in the method heading 30
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Methods that return a value public String getClassYear() { if (classYear == 1) return “Freshman”; else if (classYear == 2) return “Sophomore”; else if... } 31
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Calling methods that return a value object, followed by dot, then method name, then () (same as before) Use them as a value of the type specified by the method’s return type Student jack = new Student(); jack.major = “Computer Science”; String m = jack.getMajor(); System.out.println(“Jack’s full name is ” + jack.getName()); System.out.println(“Jack’s major is ” + m); 32
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return statement Can also be used in methods that return nothing Terminates the method Syntax: ◦ return; public void increaseYear() { if (classYear >= 4) return; classYear++; } 33
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Friday More about classes 34
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