COMP 14 Introduction to Programming Mr. Joshua Stough February 28, 2005 Monday/Wednesday 11:00-12:15 Peabody Hall 218

Announcements Previous year’s test, or practice problems, forthcoming\ –Review ppt’s.

Mid-Term Wrap-Up Common Mistakes You're not in math class –  instead of <= –2(i+5) instead of 2 * (i+5) –num 2 instead of num*num –num^2 instead of num*num Changing loop control variable for computation inside loop Not testing your code

Mid-Term Wrap-Up for (int i = 10; i<=50; i+=10) { System.out.print (i + "\t"); i = i + 5; System.out.print (i + "\t"); i = i * 2; System.out.print (i + "\t"); i = i * i; System.out.println (i); } int temp; temp = i + 5; temp temp = temp * 2; temp temp = temp * temp; temp

COMP 14 So Far... Problem Solving Mathematical Calculations Output User Input File I/O Selection (if, if-else, switch) Loops (while, do...while, for)

COMP 14 Next... Object-Oriented Design Writing Methods –pieces of code that we give a name Writing Classes –organize related pieces of information Arrays –access multiple pieces of information with a single identifier Goal: Not sure yet. –Javascript GUI Blackjack game, personal projects

Object-Oriented Design Problem Statement Write a program to input the length and width of a rectangle and calculate and print the perimeter and area of the rectangle nouns - objects, data members verbs - actions, operations (methods)

class Rectangle Data Members and Operations Last Step: design and implement an algorithm for each operation class name data members operations (methods)

Classes and Objects Rectangle A class (the concept) length = 15, width = 3 An object (the realization) length = 20, width = 6 length = 15, width = 15 Multiple objects from the same class

Anatomy of a Class A class contains data declarations and method declarations int width; int length; Data declarations Method declarations (operations)

Rectangle.java So Far public class Rectangle { // data members private int width; private int length; // methods // setLength // setWidth // getLength //... // computeArea // print }

What is a Method? Group of programming statements that we give a name Statements usually work together to perform a specific task/operation Every method belongs to a class

Why Use Methods? To divide complex programs into manageable pieces Abstraction –provide an operation to be performed on an object (ex: computeArea) Code Re-use –write a small piece of code that can be used (called) multiple times (saves typing)

Methods Pre-defined methods –provided by Java standard library –we've used these before –Math class (Math.pow, Math.sqrt,...) –Integer class (Integer.parseInt,...) User-defined methods –you write these

String Methods Method Description (pgs ) boolean equals(String str) int length() String toLowerCase() char charAt(int index) the result can be stored in a boolean variable or used in a boolean expression (return type) the method needs to be given another String (parameter) method name method specification, not syntax of use

Using String Methods list of String methods: Chapter 3 (pgs ) and Appendix E (pgs ) Steps: –create a String object –put data into the object –use the dot operator to "call" the method String line; line = keyboard.readLine(); String greeting = "Hello"; int len = line.length(); greeting = greeting.toUpperCase(); char ch = greeting.charAt(3);

Two Types of Methods Value-returning methods –"return", "evaluate to", "result in" some value –like mathematical functions –only returns a single value Void methods –performs operations but returns no value

Value-Returning Methods Think mathematical function f(x) = 2x + 5f(x, y) = 2x + y f(1) = 7f(1, 5) = 7 f(2) = 9f(2, 3) = 7 f(3) = 11f(3, 1) = 7 Can have multiple arguments (parameters) Only one result of the function

Using Methods What You Need To Know 1.Name of the method 2.Number of parameters 3.Data type of each parameter 4.Data type of value computed (returned) by the method 5.The code required to accomplish the task * Only need to know 1-4 for predefined methods

Value-Returning Methods Uses Save the value for future calculation Use the value in a calculation Print the value So, often used in expressions: x = Math.pow (y, 2); z = a + Math.pow (y, 2) + x; System.out.println (Math.pow (y, 2));

Control Flow Program control flow –execution always begins with the first statement in the method main –other methods execute only when called Method control flow –when a method is invoked, the flow of control jumps to the method and the computer executes its code –when complete, the flow of control returns to the place where the method was called and the computer continues executing code

Method Declaration Specifies the code that will be executed when the method is invoked (or called) Located inside a class definition Contains –method header –method body

Method Header public static int countCharInWord (char ch, String word) method name return type formal parameter list The parameter list specifies the type and name of each parameter The name of a parameter in the method declaration is called a formal argument A method declaration begins with a method header visibility modifiers

int number = countCharInWord ('e', "Heels"); 2 Return Type Indicates the type of value that the method evaluates to (or, sends back to the calling location) –primitive data type –class name –void reserved word indicating that nothing is returned When a value is returned, the method call is replaced with the returned value

Method Body The method header is followed by the method body { int count = 0; for (int i = 0; i<word.length(); i++) { if (word.charAt(i) == ch) { count++; } return count; } The return expression must be consistent with the return type ch and word are local data They are created each time the method is called, and are destroyed when it finishes executing public static int countCharInWord (char ch, String word)

Parameters Each time a method is called, the actual parameters in the call are copied into the formal parameters int num = countCharInWord ('e', "Heels"); { int count = 0; for (int i = 0; i<word.length(); i++) { if (word.charAt(i) == ch) { count++; } return count; } public static int countCharInWord (char ch, String word)

The return Statement Tells the computer to "return" back to where the call was originally made. Specifies the value that will be returned return expression; The data type of expression must match the method's return type Methods with return types of void usually don't (but can) have a return statement Value-returning methods must have at least one return statement

Using return public static double larger (double x, double y) { double max; if(x >= y) max = x; else max = y; return max; } public static double larger (double x, double y) { if(x >= y) return x; else return y; } These are equivalent methods.

Void Methods Does not return a value Has a return type of void Similar in structure to value-returning methods Call to method is always a stand-alone statement Can use return statement to exit method early

The main Method The main method looks just like all other methods public static void main (String[] args) modifiersreturn type method name parameter list

Code Re-Use Methods can also be used to save typing If you have a section of code that does one task and it needs to be done multiple times, you can write it as a method Example: you want to print the following heading to start your program *********************************** ************ Blackjack ************ ***********************************

public class MyProgram { public static void main (String[] args) { printStars(); System.out.print ("************"); System.out.print (" Blackjack "); System.out.println ("************"); printStars(); } public static void printStars() { System.out.print ("***************"); System.out.println ("*****"); }

printStars What if we wanted to change the number of stars printed on the line? give the number of stars as a parameter

public class MyProgram { public static void main (String[] args) { printStars(35); System.out.print ("************"); System.out.print (" Blackjack "); System.out.println ("************"); printStars(35); } public static void printStars(int numStars) { for (int i = 0; i<numStars; i++) { System.out.print ("*"); } System.out.println(); }

printStars What if we wanted to change the character printed (something besides stars?) change the name of the method (because it may not print stars) --> printChars give the character as a parameter

public class MyProgram { public static void main (String[] args) { printChars(35, '#'); System.out.print ("###############"); System.out.print (" Blackjack "); System.out.println ("###############"); printChars(35, '#'); } public static void printChars(int num, char ch) { for (int i = 0; i<num; i++) { System.out.print (ch); } System.out.println(); }

printStars (35); printStars (30 + 5); int num = 35; printStars (num); Parameters Formal parameters –variable declarations in the method header –automatic local variables for the method Actual parameters –actual values that are passed to the method –can be variables, literals, or expressions

Next Time in COMP 14 More methods –more parameter passing –method overloading –practice writing methods Reading: Ch 7 (pgs , ),