February 11, 1999CSPP503 Week 6 Bring on the Caffeine CSPP503 Week 6

February 11, 1999CSPP503 Week 6 Are You Java’s Type? Type: a set of values that are semantically similar Java is a strongly typed language – Every variable and every expression has a type that is known at compile time. – Strong typing helps detect errors at compile time.

February 11, 1999CSPP503 Week 6 Inside a Primitive Type Actual values for integral types: byte: -128 to 127 short: to int: to long: to char: from '\u0000' to '\uffff’ (from 0 to 65535) Why use int instead of long ?

February 11, 1999CSPP503 Week 6 Building a Boolean from a Number Can’t say (why not?): if (x) System.out.println(“Congratulations, it’s a Boole!”); Convert an integer x (following the C language convention that any nonzero value is true): if (x != 0) System.out.println(“Congratulations, it’s a Boole!”);

February 11, 1999CSPP503 Week 6 Building a Boolean from an Object Object reference obj can be converted (any reference other than null is true): obj! = null

February 11, 1999CSPP503 Week 6 The Other Kind of Type Reference types – Variables of reference types don’t hold values, but references to values – Classes, interfaces and arrays are all reference types

February 11, 1999CSPP503 Week 6 A Graphical View int counter Airport midway The data of the midway object

February 11, 1999CSPP503 Week 6 Classes, Interfaces and Arrays, oh my! Classes we’ve already seen Interfaces are programming contracts – An interface is a set of constants and methods – In Java, a class implements an interface

February 11, 1999CSPP503 Week 6 Implementing an Interface An example from the Comparison applet: public class Comparison extends Applet implements ActionListener Comparison promises to do everything an ActionListener does

February 11, 1999CSPP503 Week 6 Comparison Does What It Has To What does an ActionListener have to do? actionPerformed public abstract void actionPerformed(ActionEvent e) Provides an implementation for the interface: public void actionPerformed(ActionEvent e) { number1 = Integer.parseInt(input1.getText() ); number2 = Integer.parseInt(input2.getText() ); repaint(); }

February 11, 1999CSPP503 Week 6 Comparison Object has Two Types Comparison is a class that implements an interface: public class Comparison extends Applet implements ActionListener

February 11, 1999CSPP503 Week 6 Interface Type Example From the Comparison applet: input2 = new TextField( 10 ); input2.addActionListener( this ); add( input2 ); // put input2 on applet What is addActionListener()? public synchronized void addActionListener(ActionListener l)

February 11, 1999CSPP503 Week 6 Sideline Events Events are things that happen to programs (other than errors) // Process user's action on the input2 // text field public void actionPerformed(ActionEvent e) { number1 = Integer.parseInt( input1.getText() ); number2 = Integer.parseInt( input2.getText() ); repaint(); }

February 11, 1999CSPP503 Week 6 Class ? Type Variables have types Objects (and arrays) don’t have a type, but belong to a class Usually we’ll consider them the same

February 11, 1999CSPP503 Week 6 Casting Against Type A value could be two different types – Is 12 an int or a float? Compiler isn’t smart, so it’s conservative (signals an error) Override the compiler with a cast – Cast says: Treat this variable as the type I say – To cast in Java, write: (newType) variable

February 11, 1999CSPP503 Week 6 Examples of casting // Casting a float literal to a type int. // Without the cast operator, this would be a // compile-time error, because it’s a narrowing // conversion: int i = (int)12.5f; // From class average applet (Figure 2.9) if ( counter != 0 ) { average = (double) total / counter; System.out.println( "Class average is " + average ); } else...

February 11, 1999CSPP503 Week 6 Can’t Always Cast Can’t do this: if ((boolean) x) System.out.println(“Congratulations, it’s a Boole!”); Sometimes casts are automatic, and are called conversions

February 11, 1999CSPP503 Week 6 One of Two Ways Create an expression in a context where the type of the expression is not appropriate and either: – Error at compile time ( if statement has any type other than boolean ) – May be able to accept a type that is related to the type of the expression

February 11, 1999CSPP503 Week 6 Funky Conversions What does this print? class Test { public static void main(String[] args) { int big = ; float approx = big; System.out.println(big -(int)approx); }

February 11, 1999CSPP503 Week 6 What’s an Array An array is a group of values of the same type (usually stored in consecutive memory locations) Arrays are objects – Can be assigned to variables of type Object Dynamically created

February 11, 1999CSPP503 Week 6 An Array of Examples From Figure 5.3 int n[]; // declare an array of integers // initialize instance variables public void init() { n = new int[ 10 ]; //dynamic allocation }

February 11, 1999CSPP503 Week 6 The Declaration of Array Array declarations int[] ai; // array of int short[][] as;// array of array of short Object[] ao, // array of Object otherAo; // array of Object

February 11, 1999CSPP503 Week 6 Declaring Arrays Can’t specify the length in a declaration int test [] = new int [100]; // Okay int test2 [100]; // No good int test3 [100]=new int [100]; // No good

February 11, 1999CSPP503 Week 6 Some Array Definitions These create array objects: Exception ae[] = new Exception[3]; Object aao[][] = new Exception[2][3]; int[] factorial = { 1, 1, 2, 6, 24 }; String[] aas = {"array", "of", "String"};

February 11, 1999CSPP503 Week 6 An Array Example int face; int frequency[]; // initialize instance variables public void init() { frequency = new int[ 7 ]; for (int roll=1; roll <= 6000; roll++ ) { face = 1 + (int) (Math.random() * 6 ); ++frequency[ face ]; }

February 11, 1999CSPP503 Week 6 An Array of Observations Arrays know their own length test.length == 100 – What kind of loops do we usually use with arrays? Arrays start at index 0 – Last index == ?? Arrays must be indexed by int values ( short, byte, or char are okay, but long is no good)

February 11, 1999CSPP503 Week 6 Operating on an Array Square brackets are an operator Brackets have very high precedence – Same as ()’s Example -17*studentGrades[12] * 2

February 11, 1999CSPP503 Week 6 "Can he do that?" What’s wrong with the following code? int scores[] = {98, 76, 84, 97, 101, 78}; int counter=0; for (; counter <= scores.length; counter++) { System.out.println("Test #" + counter + ": " + scores[counter]); }

February 11, 1999CSPP503 Week 6 Array Initializers Use {}’s to set initial values – int grades[] = {87, 92, 76, 94}

February 11, 1999CSPP503 Week 6 Searching & Sorting Data The Second Law of Thermodynamics Sorting restores order to the world Sorting in everyday life – Bank checks by account Searching is easier when sorted – Reverse directories of phone numbers

February 11, 1999CSPP503 Week 6 Sorting in Computers Usually start with an array of items to be sorted – Integers – Social Security numbers – Last names and first names Lots of data – All the SSN’s in the U.S. – All of GM’s employees

February 11, 1999CSPP503 Week 6 Different Ways to Sort Bubble sort – Compare two neighbors, swap if out of order

February 11, 1999CSPP503 Week 6 The Code for Bubble Sort public void sort() { int hold; // temporary holding area for swap // passes for ( int pass = 1; pass < a.length; pass++ ) // one pass for ( int i = 0; i < a.length - 1; i++ ) // one comparison if ( a[ i ] > a[ i + 1 ] ) { // one swap hold = a[i]; a[ i ] = a[ i + 1 ]; a[ i + 1 ] = hold; } Nested for loops

February 11, 1999CSPP503 Week 6 There’s More Than One Way to Sort a Cat Other sorts – Quicksort – Heapsort – Bucket sort Different sorts have different characteristics – Memory usage – Speed

February 11, 1999CSPP503 Week 6 Searching Here, There & Everywhere Need to find a value? Search for it! Linear search – Is it the next one? No, then look again Yes, stick a fork in me, ‘cuz I’m done – Works on unsorted items Binary search – Cut in half every time

February 11, 1999CSPP503 Week 6 Arrays of Arrays Arrays in multiple dimensions – Not just a list – A table (an array of arrays) – A cube (an array of arrays of arrays)

February 11, 1999CSPP503 Week 6 An Example An array of arrays int b[][] = { {1,2}, {3, 4, 5}} Row 0:12 Row 1:345 A 3x3 array int b[][]; b = new int[3][3];

February 11, 1999CSPP503 Week 6 Where Do You Want To Go Today? Building new objects Accessors this and That

February 11, 1999CSPP503 Week 6 Caution, Objects Under Construction! Objects have structure and structure has to be built new means create a new instance of an object How are objects actually built? – Objects know how to do things, including how to build themselves

February 11, 1999CSPP503 Week 6 Constructor Methods Constructors are special methods that build new objects – Declaration looks just like a method declaration that has no result type public class Circle { double x_center, y_center, radius; public Circle(double x, double y, double r) { x_center = x; y_center = y; radius = r; }

February 11, 1999CSPP503 Week 6 A Few Notes on Constructors I Don't have to declare a constructor – Java creates a default constructor public class Circle { public double x_center, y_center, radius; // Uses the default constructor public double Area() { return d * radius * radius; }

February 11, 1999CSPP503 Week 6 A Few Notes on Constructors II Constructors have the same name as the class No return type (why not?) Constructor declarations are not members – Never inherited, so no hiding or overriding (More on this later)

February 11, 1999CSPP503 Week 6 Multiple Constructors public Circle(double x, double y, double r) { x_center = x; y_center = y; radius = r; } public Circle() { x_center = 0; y_center = 0; radius = 1; } public Circle(double x, double y) { x_center = x; y_center = y; radius = 1; }

February 11, 1999CSPP503 Week 6 Handling Multiple Constructors How does Java tell the constructors apart? – Method signatures – Signatures include: Name of the method Number formal parameters Types of formal parameters – Doesn't include the name of the parameter

February 11, 1999CSPP503 Week 6 Method Signatures For this method: public Circle(double x, double y, double r) Signature is: Circle(double, double, double) Different methods, same signature public Circle(double x, double y) public Circle(double x, double r)

February 11, 1999CSPP503 Week 6 Back to Multiple Constructors Multiple constructors are okay – Each constructor has to have a unique signature (All of a class's methods have to have unique signatures, not just constructors) Method overloading: multiple versions of the same method (but with different signatures)

February 11, 1999CSPP503 Week 6 The Linda Tripp Problem How do we keep private data private? Avoid having object1 mess with object2 's data Example: Student objects know student's grades, but one student shouldn't modify another student's grades. Storing invalid data – Stuffing 12-hr time into a 24-hr time object

February 11, 1999CSPP503 Week 6 If It's Public... Start with: public class Circle { public double x_center, y_center; public double radius, area; public Circle(double x, double y) { x_center = x; y_center = y; radius = 1; area = }

February 11, 1999CSPP503 Week 6 …other objects can mess with it Create a circle Circle duPont = new Circle(100d, 100d); // Change the area, but leave r the same duPont.Area = 11; // Now a circle with radius 1 has area 11

February 11, 1999CSPP503 Week 6 Using private Declare variables to be private public class Circle { private double x_center, y_center; private double radius, area; : } Now only Circle can modify the variables (x_center, y_center,...)

February 11, 1999CSPP503 Week 6 Granting Access Other classes might need to read or write the values stored in private members Create accessor functions – Set (or settor) – Get (or gettor) Control access through get and set

February 11, 1999CSPP503 Week 6 Setting and Getting What's the difference between using get and set methods, and just making everything public ? Two benefits – Protect instance variables from outside meddling – Insulates class users from changes in variables

February 11, 1999CSPP503 Week 6 Example from Fig 6.5 // blah, blah, blah. Set invalid values to zero. public void setTime( int h, int m, int s ) { setHour( h ); // set the hour setMinute( m ); // set the minute setSecond( s ); // set the second } // set the hour public void setHour( int h ) { hour = ( ( h >= 0 && h < 24 ) ? h : 0 ); } // set the minute public void setMinute( int m ) { minute = ( ( m >= 0 && m < 60 ) ? m : 0 ); } // set the second public void setSecond( int s ) { second = ( ( s >= 0 && s < 60 ) ? s : 0 ); }

February 11, 1999CSPP503 Week 6 Me, a Name I Call Myself How can objects refer to themselves? – Every object has a built-in reference to itself, called this this can be used only in: – Body of a method or constructor – Initializer of an instance variable – Anywhere else is a compile-time error

February 11, 1999CSPP503 Week 6 This is Bad, public class Circle { public double x, y; public double r, area; public Circle(double x, double y) { x = x;// This doesn't do much, because we're y = y; // just assigning parameters to themselves r = 1; area = }

February 11, 1999CSPP503 Week 6 But this is okay public class Circle { public double x, y; public double r, area; public Circle(double x, double y) { this.x = x;// Now we're assigning the instance this.y = y; // variables to the parameters r = 1; area = } Better to avoid this mess altogether by using different names for data members and method parameters