week7 1

APCS-AB: Java

Control Structures

October 17, 2005

week7 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))

week7 3

If/Else

• Remember, the brackets are technically optional BUT only if you want to execute ONE statement after the if or else

statementif (amount == 0)

System.out.println(“okay”);else

System.out.println(“nonzero amount”);____________________________________________

if(amount == 0)amount = 500;System.out.println(“amount equal to 0”);

elseamount = 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

week7 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

week7 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 >>

}

week7 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”);

week7 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

week7 8

Class Exercise

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

week7 9

APCS-AB: Java

Control Structures

October 20, 2005

week7 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

int someValue;

// someValue gets a value

switch ( someValue ) {

case 0:

//do something

break;

case 1:

//do somethingbreak;

default:

//do something

}

char someValue;

// someValue gets a character

switch ( someValue ) {

case ‘A’:

//do something

break;

case ‘B’:

//do somethingbreak;

default:

//do something

}

week7 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)

week7 12

APCS-AB: Java

Java API & Strings

October 21, 2005

week7 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

week7 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/

week7 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

week7 16

String Class (APCS subset)

week7 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”;

String name

“Jane”

“Jane Dow”

X

name = name + “ Dow”;

week7 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’;

week7 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

week7 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.*;

week7 21

String Project/Schedule

• Codebreaker due Thursday