APCS-A: Java

1. APCS-A: Java November 8, 2005

2. 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

3. 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.*;

4. Math Class • Defined in the java.lang package • All methods in the Math class are static methods (aka class methods) • This means that you do not need an object of the class to call the methods - you invoke them directly through the name of the class • Some useful methods: • static double sqrt (double power) • static double pow (double num, double power) • static double random () • static double cos (double num) • static double sin (double num) • static double tan (double num)

5. static • Static Methods - invoked by class name, no object needed • Can be used when no object state is needed to do an action • Static Variables (also called Class Variables) are shared among all instances of a class • There is one copy of the variable for all objects in the class • This is different from an instance variable in which each instance (object) has its own version of the variable

6. enum • Enumerated type, lists all the possible values of a variable of the type • The values given are identifiers • Examples: enum Season {winter, spring, summer, fall} enum Grade {A, B, C, D, F} enum STA-grade {Aplus, A, Bplus, B, Cplus, C, Dplus, D, F} • Using an enum type: STA-grade myGrade = STA-grade.Aplus; Season time = Season.fall;

7. Song length : int artist : Artist album : Album play() : void toString(): String More about Classes & Design • Need to be consistent with our models and design of classes. One way to do this is to use UML (Unified Modeling Language) diagrams to visualize programs • Use box to represent class • Top box - Class name • Middle box - instance variables • Bottom box - methods

8. Encapsulation • The instance data of an object should only be modified by that object • Keep the data private • Make other objects use getter (accessor) and setter (mutator) methods to access and change data • This guarding of data is called encapsulation public private Violate encapsulation Enforce encapsulation variables Provide services to clients Support other methods in class methods Figure 4.5

9. Wednesday • Finishing up lecture from yesterday…

10. Class Relationships • Dependency (one class “uses” another) • But how does a class gain access to another object? • One class can instantiate (create) the other object • One class can gain access to another by getting that object as a parameter to a method • The more classes depend on one another, the more changes in one class can impact another (which can be troublesome) • Classes can depend upon objects of the same class (ie one object of a class interacts with another object of the same class)

11. Class Relationships • Composition (aggregation) - one object is made up of other objects • Can be described as a “has-a” relationship • A special type of dependency

12. Homework • Write a class that will represent a student at STA or NCS. This is a very simplistic model, but this student only knows what form he or she is in, and can only do math • The student should be assigned to a form when constructed • Yesterday we talked about enums. They can be useful in many situations, but here we will use it to define the range of legal values for form assignment • The student class should have a method called “doMath” that will do the type of math appropriate for their grade level (defined below) • The function should use input and output as we have previously done (input using Scanner) • The method should make appropriate calls to static methods of the Math class

13. More Homework • The following is the mapping of forms to the math they can do • C Form - parrot back the number you are given • B Form - give the IEEE remainder (std 754) of 2 nums • A Form - rounding a number • I Form - providing the absolute value of a number • II Form - exponentiation • III Form - Square roots • IV Form - Sin functions • V Form - provide the larger of two numbers • VI Form - Quadratic Formula

14. APCS-A: Java Arrays (related information in Chapter 7 of Lewis & Loftus) November 11, 2005

15. Grouping objects • Fixed-sized collections • When you know in advance how many items will be stored in a collection (and that stays fixed for the life of the collection) • A fixed-sized collection is an array • It’s kind of like a tray of cups – each cup can hold an object or a primitive data type • Note: the array itself is an object • Flexible-sized collections • When you don’t know in advance how many items you will need to store • Will go into more details about these in a few weeks

16. int array called c -423 c[0] 3 c[1] c[2] 34 c[3] 54 32 c[4] 2 c[5] 9 c[6] 4 c[7] 3453 c[8] Arrays • An array is a group of variables (called elements or components) containing values that all have the same data type • To refer to a particular element in an array, use the array’s name and the position number of the element in the array

17. int array called c -423 c[0] 3 c[1] c[2] 34 c[3] 54 32 c[4] 2 c[5] 9 c[6] 4 c[7] 3453 c[8] Anatomy of an array • Array names are the same as any other variable name • An array with 9 elements (or variables) • First element has index zero (the zeroth element) • ith element of array c is c[i -1] • Index must be a positive integer (or an integer expression that can be promoted to an int)

18. Advantages to using arrays • It is easy to access individual items in an array (and it’s efficient) • Arrays are able to store objects or primitive type values (int, double, float, etc) • (We’ll see later that the flexible-sized collections can only store objects)

19. Declaring and creating arrays • Array objects occupy space in memory. All objects in Java must be created with a keyword new • When you declare an array you must tell Java the type of the array elements and the number of the elements that will appear in the array • Example declaration for an integer array called hourArray: int hourArray[] = new int[24]; // declares and creates an array to hold 24 int elements

20. An array hourArray :int[ ] 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23

21. How do we fill an array? • All values are initially set to zero/null when we initialize an array • One way to fill the array is to use an array initializer, a comma-separated list of expression enclosed in braces: when you declare it int n[] = {10, 23, 34, 235, 234}; • The array length is automatically determined by the number of elements in the initializer list

22. Class Exercise • Write code to declare and initialize an array to hold the months of a year • Write code to declare and initialize an array to hold the number of days in each month of the year

23. How else do we fill an array? • Any ideas about how else we could fill an array? • A large number of the things we want to do with arrays involve manipulating the entire array. Unfortunately, there are very few built-in operations that are defined to work on the contents of an entire array. Instead, we must define those processes in terms of what they require us to do to the individual elements of the array. This task is then repeated once for each element of the array. Since the size of the array is fixed once the array is allocated, it is a natural application of a for-loop.

24. How else do we fill an array? • Arrays and for-loops go together like peanut butter and jelly! • For filling, for printing values, for checking values, etc • We could use a for loop and access each array element to set a value to it: int myArray = new int[10]; for(int i =0; i < myArray.length; i++){ myArray[i] = 5 * i; } • Note: unlike the String class when we access the length of the array we are not calling a method, rather we are accessing one of the array’s instance variables

25. Exercise • Print out the names and lengths of each of the months

26. Mistakes • It is not uncommon to make a programming mistake that causes the bounds of a loop to be mis-defined and the loop index variable to stray beyond the range of the array. • For example, what if in your for loop you wrote: for(int i = 0; i <= array.length; i++){ System.out.println(array[i]); }

27. Lab/Homework • Lab: LightsOut game - we will work on this in class today and Monday • Homework: Chess Design Project - Due next Wednesday • Note: Quizzes the next two Tuesdays.