Object Oriented Programming

Object Oriented Programming Lecture 5: Arrays and Strings Mustafa Emre İlal emreilal@iyte.edu.tr

Recap • Assignment 4 • Flow Control • Conditions / Decisions • Loops

Today • Assignment 04 • Arrays – Strings • Thinking in Java – Chapter 4,5

Arrays • Need for an Array [group] • To group elements of a particular type • To apply a procedure to individual elements within a larger group • Example: assume we have 50 Circles • We do not want to create 50 different variables. • What would the code to find the sum of their areas look like? • Why 50? Why any pre-determined number? Why not let the user determine the number?

Java Arrays • The Array is an ordered group data elements of a single data type (primitive or reference). • Java arrays are objects (reference type) • Arrays can be declared and initialized seperately • Declaration is by specifying the data type • Initialization requires the number of elements to be stored. • int[] intArray; //square brackets declare the array • intArray = new int[10]; // memory for 10 int’s are allocated and intArray points to the array note the “new” statement • int intArray[] = new int[10] // alternative – same result

Index • The value within the square brackets define the order of the element of interest within the array. (Its index) • Java arrays start counting the index at 0. (not 1) • intArray[2] = 7; // the third element in the array is 7

.length • Java arrays are objects. • Slightly slower. • Lots of convenience • Each array has a variable “.length“. This variable remembers the storage capacity of the array. • Very convenient in setting up loops int index = 0; while ( index < intArray.length ) { System.out.println (intArray[index++]); }

Initialization of Elements • Nothing to be done for primitives • Object arrays are reference types • A new object needs to be initialized at the reference point. Circle[] circleArray = new Circle[10]; for (int j=0; j<10; j++) { circleArray[j] = new Circle(); }

Multi Dimensional Arrays • Arrays do not have to represent a one dimensional relationship (queues) between elements. • Multi-dimensional structures (tables, time-series) can also be abstracted • Note that humans have difficulty in imagining beyond three dimensions Cell[][] spreadsheet = new Cell[25][50]; spreadsheet[row][column]; classroom[][][] university= new classroom[10][8][4]; university[building][floor][room]

Pre-known Data • Alternative way to initialize an array involves providing a listing of all the data to be stored. int[] fibonacci = {0,1,1,2,3,5,8,13}; • For multi-dimensional arrays: int[][] matrix= { {0,1,2},{1,0,1},{34,23,6} };

java.lang.Vector • Array size is fixed once the array is initialized. • When more data needs to be added than the size allows, a new array of a larger size needs to be initialized and the elements of the old array copied into the new one. • Vector Class is a data structure that has no size limitation. Vectors are utilized through the methods the class provides. Some methods: public Vector(int initialSize); public Vector(); // create empty vector public void addElement(Object obj); public Object elementAt(int index); public void insertElementAt(int index); public void removeElementAt(int index); public int indexOf(Object obj);

Character Arrays (String) • Text is an array of characters (string) • Java provides the String class for convenient manipulation of these character arrays. • Using the String class is almost as easy as using primitive data types.

String • Points to watch out for • The content cannot be subjected to comparison by the “==“ operator. the .equals() method needs to be used.firstString.equals(String secondString) • String object does not allow altering the content directly. Useful to use helper classes

StringBuffer • String class is not designed to modify the character contents (value) but to manage the container • StringBuffer class should be used to add to or strip characters from the array.

StringBuffer methods • Some examples: • public int charAt(int index) • public StringBuffer append(“anytype” t) • public StringBuffer insert(int offset, “anytype” t) • public deleteCharAt(int index) • public delete(int start, int end) • public StringBuffer subString(int start, int end) • public String reverse() • …

preparation - chapter 4,5 • Chapter 4 • Constructor • Overloading • this() for calling one constructor from others • static • finalize() the method that is called before the object is deleted • Chapter 5 • package * • public – private – protected – "friendly"

Assignment 05 • Add the following to the Circle class • Variable for name • Variable for color • Substantial changes in CircleApplication • Create 100 circles • Each should have a name in the form of “Circle-XXX" according to its order of creation. • Use Math.random() to assign the following properties • Color: Either Red , Green, Blue, Yellow, Puple, Cyan, Orange, Brown, Black, or White (use an array for color list) • Coordinates for the center and the radius

Assignment 05 • Reports to be displayed: • List the circles in alphabetical order, displaying the properties of each. • Sum of all areas and the average of all areas • The number of circles for each color • List of red circles sorted in order of size. • The farthest and closest circles (in terms of their centers) to a randomly determined point.

Next week • Preparation: Thinking in Java Chapter 6

Object Oriented Programming