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

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apcs ab java


Control Structures

October 17, 2005

if statements review
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))

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(“nonzero amount”);


if(amount == 0)

amount = 500;

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


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
  • 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
while loop
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 >>



boolean keepLooping = true;

while (keepLooping){


int choice = getUserInput();

if(choice == 0){ // 0 is the “exit” choice

keepLooping = false;



System.out.println(“Good choice”);

// do other stuff



System.out.println(“Thanks for playing”);

for loops
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
class exercise
Class Exercise
  • How would we write a method that would print the numbers between 1 and 100, but only in increments of 10?
apcs ab java1


Control Structures

October 20, 2005

switch statement
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
  • 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)
apcs ab java2


Java API & Strings

October 21, 2005

  • 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
java api
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/
why this is cool
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
strings are immutable
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 Dow”

name = name + “ Dow”;

other string methods java api
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’;

java packages
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
using packages
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.*;

string project schedule
String Project/Schedule
  • Codebreaker due Thursday