APCS-AB: Java

1. APCS-AB: Java Control Structures October 17, 2005

2. If Statements (Review) if ( << conditional >> ) { << body >> } else if ( << conditional >> ) { << body >> } else { << body >> } • The << conditional >> can be any true or false conditional • A simple boolean like (true) • A check for equality like (x == 5) • A greater than or equal to like (x >= 1) • A combination of the above with &&(and) , ||(or), or another conditional (( x==5 && y == 2) || (z > 42))

3. If/Else • Remember, the brackets are technically optional • BUT only if you want to execute ONE statement after the if or else statement if (amount == 0) System.out.println(“okay”); else System.out.println(“nonzero amount”); ____________________________________________ if(amount == 0) amount = 500; System.out.println(“amount equal to 0”); else amount = 200; System.out.println(“amount was not equal to 0”); • In this bottom example, both print statements will print, regardless of the value of amount

4. Looping • The if/else code structure lets us change the flow of the program, depending on certain conditions • Looping always us to easily repeat an action, until a condition has been met • What situations can you imagine in which this would be really helpful? • There are two kinds of loops in Java • While they are technically interchangeable, each is syntactically geared to a specific kind of situation

5. While loop • While loops logically follow the pattern of “while something is true then perform the following set of actions” • This is useful in a situation in which you don’t know how many times you need to do something, but you know what the end result needs to be • The syntax is simple: while ( << conditional >> ) { << body >> }

6. Example boolean keepLooping = true; while (keepLooping){ printMenu(); int choice = getUserInput(); if(choice == 0){ // 0 is the “exit” choice keepLooping = false; } else{ System.out.println(“Good choice”); // do other stuff } } System.out.println(“Thanks for playing”);

7. For Loops • We use for loops when we want to do a set of statements a predetermined number of times • The syntax for a for loop is: for ( <starting value>; <conditional>; <update statement>) { << body >> } for (int x = 0; x < 10; x++) { System.out.println(“x is: “ + x); } • The conditional is the same as it is in a while loop • The update statement is optional, but usually is used to increment or change the looping variable

8. Class Exercise • How would we write a method that would print the numbers between 1 and 100, but only in increments of 10?

9. APCS-AB: Java Control Structures October 20, 2005

10. Switch Statement • The someValue needs to be an int or a char • If no case value is matched, then the optional default case is executed -- but it’s a good idea to always have the default case even if you don’t expect to use it

11. Schedule • Today: Work on finishing loop lab in class • Homework: Mini-project first, loop lab if there is time • Friday - quiz postponed; String Manipulation lecture • Monday: Work Day • Tuesday: Programming Quiz (One problem to solve, replaces Friday Quiz) • Wednesday: Work Day/ Review • Thursday: Cumulative Java Quiz (Written)

12. APCS-AB: Java Java API & Strings October 21, 2005

13. Checkpoint • Loop Lab • How many of the tasks have you completed? • Graphics Mini-Project • Due today, extensions (one free late, or 10% each day late) count weekend days, so get it to me over the weekend if you can

14. Java API • API = application programming interface • In Java, it is the list of all the classes available, with details about the constructors, methods, and usually a description of how to use the class • I had you download the full API to your computers at home, there is also a scaled down version that only has the methods and classes that are used for the APCS test • That is available online at: http://www.cs.duke.edu/csed/ap/subset/doc/

15. Why this is Cool • There is so much code in Java that is already written for you - you just have to • Know that it is out there • Figure out how to use it • The API gives a standard way to look at classes and methods so that any Java programmer can understand how to use a class without having to see the code

16. String Class (APCS subset)

17. Strings are immutable • Once a string is created, it cannot change • So string methods always return new strings -- that way you can just change the pointer String name = “Jane”; X “Jane” String name “Jane Dow” name = name + “ Dow”;

18. Other String Methods (Java API) • In addition to what the AP people think you need to know, there are some other cool String methods • boolean equalsIgnoreCase(String str) • String replace (char oldChar, char newChar) • boolean endsWith (String suffix) • boolean startsWith (String prefix) • String toUpperCase() • String toLowerCase() • String concat(String str) • String trim() //takes off white space from front & back • Note: to make a char: char ch = ‘A’;

19. Java Packages • All Java classes are grouped into libraries (or packages) • String is part of the java.lang package, which is pre-loaded when you are programming in Java • We’ve already seen one other library, the java.util library, where Scanner is • Some of the other standard Java Libraries: • java.applet java.util • java.awt java.math • java.io java.net • java.lang javax.swing

20. Using Packages • Everything in java.lang is available for use • So it’s as if somebody already did: import java.lang.*; • To use other packages, we need to import either the specific class or the entire package (just like we did for Scanner class) • To import a class we use the whole package name: import java.util.Scanner; import java.io.File; • To import an entire library we use the asterisk: import java.util.*; import java.io.*;

21. String Project/Schedule • Codebreaker due Thursday