COMP 111. Introduction to Computer Science and Object-Oriented Programming. Week 10

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Emil Lee
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1 COMP 111 Introduction to Computer Science and Object-Oriented Programming

2 Tonight Wrapping, Boxing and a Whole Array of Goodies

3 A View of Variables int count; Height hgt1; int[] score; 0 count hgt1 score primitive value object reference array reference

4 A View of Variables count = 5; 5 count primitive value hgt1 = new Height(); 0 feet hgt1 inches 0 object reference int[] score = new int[3]; score [0] [1] [2] array reference

5 A View of Variables count = 9; 9 count primitive value hgt1.setfeet(5); 5 feet hgt1 inches 0 object reference score[1] = 10; score [0] [1] [2] array reference

6 A View of Variables count = count + 1; 10 count primitive value this.inches = 4; 5 feet hgt1 inches 0 object reference score[2] = score[1] - 1; score [0] [1] [2] array reference

7 So What Is An Array? An array has a name has a type has a length and is a reference to a collection of indexed values? variables? slots?

8 Variations classes can have instance fields that are arrays an instance of Gradebook J. Doe name [0] [1] [2] score

9 Variations arrays can have class types Height[] tall = new Height[3]; [0] [1] [2] 5 inches feet 10 tall[1] = new Height(5,10); tall[2] = new Height(6,4); 6 inches feet 4

10 Some Examples On BlueJ

11 Array Algorithms You can calculate with numbers You can manipulate Strings What can you do with arrays? count the elements count the elements matching a condition find element an element matching a condition calculate min, max, sum average, etc. and much more

12 Array Lists Arrays have some limitations While we don t need to fill them They have a maximum number of elements We have to program the operations (except for.????) Array lists Can grow and shrink as needed Provide useful methods

13 ArrayLists All element of same type Must be a class type (not primitive type) Starts out empty Has a size (starts out zero)

14 Examples Constructing ArrayList<Customer> store1 = new ArrayList<Customer> (); // newly created list, size is 0 Adding store1.add( new Customer( William Dodds, )); // size is 1 now store1.add( new Customer( Martha Shavers, )); // size is 2 now Accessing Customer cust = store1.get(1); // retrieves second object int store1size = store1.size(); // returns size (2) Customer newcust = new Customer( Lee Cash, ); store1.set(1, newcust); // overwrites Martha with Lee

15 Accessing Use get method Index starts at 0 BankAccount anaccount = accounts.get(2); gets the third element of the array list Bounds error if index is out of range Most common bounds error: int i = accounts.size(); anaccount = accounts.get(i); // Error

16 Adding Before add(i,a)

17 Removing remove removes an element at an index accounts.remove(i)

18 Wrappers primitive types are not objects so can t use directly in ArrayLIst so Java has wrapper classes

19 Wrapper Class ArrayList<Double> data = new ArrayList<Double>(); data.add(29.95); double x = data.get(0);

20 Wrapper Classes Are classes Start with cap Each contains a single value of corresponding type Use where objects needed

21 Auto-Boxing Automatic conversion between primitive types and the corresponding wrapper classes is automatic. Double d = 29.95; // auto-boxing; same as Double d = new Double(29.95); double x = d; // auto-unboxing; same as double x = d.doublevalue(); Auto-boxing even works inside arithmetic expressions Double e = d + 1; Means: auto-unbox d into a double add 1 auto-box the result into a new Double store a reference to the newly created wrapper object in e

22 Auto-Boxing A recent feature of Java Automatic conversion between primitive types corresponding wrapper class Double d = 29.95; Double d = new Double(29.95); double x = d; double x = d.doublevalue(); Double e = d + 1;

23 Enhanced For Loop Recent addition to Java Shortcut for a common loop for (typevariable : collection) { statements }

24 Examples double[] data = new double[100]; assume that array is then populated double sum = 0; for (double e : data) { sum = sum + e; }

25 Examples ArrayList<BankAccount> accounts = new ArrayList<BankAccount>(); assume that array list is then populated double sum = 0; for (BankAccount a : accounts) { sum = sum + a.getbalance(); }

26 Two-Dimensional Arrays int[][] table = new int[2][3]; The array identified by table will have 2 rows (the row is the FIRST subscript) 3 columns (the column is the SECOND subscript). [0][0] [1][0] [0][1] [1][1] [0][2] [1][2]

27 Examples table[0][0] = 22; table[0][1] = 1301;... table[1][2] = 43; for (int i = 0; i < 2; ++i) for (int j = 0; j < 3; ++j) System.out.println(table[i][j]);

Arrays and Array Lists

Arrays and Array Lists Advanced Programming ICOM 4015 Lecture 7 Reading: Java Concepts Chapter 8 Fall 2006 Slides adapted from Java Concepts companion slides 1 Lecture Goals To become familiar with using