COMP206-08S
General
Programming 2
Geoff Holmes and Bernhard
Pfahringer
Lectures
-
Today
Random Numbers
Timing
Assignment 1 Review
Department of Computer Science
2
Random Numbers




import java.util.Random;
Implements a random number generator that
appears to generate random numbers
Construct object of Random class
Use methods
nextInt(n) – random number 0 -> n-1
 nextDouble() – random number 0 -> 1 (exclusive)


Constructors use system clock – no params or you
can “seed” the generator
Department of Computer Science
3
Example


Random generator = new Random( );
int oneToTen = 1 + generator.nextInt(10);
Department of Computer Science
4
Useful utility
import java.util.Random;
public class ArrayUtil {
private static Random generator = new Random();
public static int[] randomIntArray(int length, int n) {
int[] a = new int[length];
for(int i=0; i<a.length; i++)
a[i] = generator.nextInt(n);
return a;
}
public static void print(int[] a) {
for (int x : a)
System.out.print(x + " ");
System.out.println();
}
}
Department of Computer Science
5
Test for Array Util
public class TestArrayUtil {
public static void main(String[] args) {
// 20 numbers in range 0 -> 99
int a[] = ArrayUtil.randomIntArray(20, 100);
ArrayUtil.print(a);
}
}
Department of Computer Science
6
Timing program runs



Want to be able to assess how long something
takes to run
Can be complicated due to threads
We will make use of a system call



System.currentTimeMillis()
This provides the current time in milliseconds
so we need to do some work to convert to the
elapsed time of the thing we are assessing.
Can get elapsed time while running
Department of Computer Science
7
Stop Watch Class
public class StopWatch {
private Boolean isRunning;
private long startTime, elapsedTime;
public StopWatch()
{
reset();
}
public void start()
{
if (isRunning) return;
isRunning = true;
startTime = System.currentTimeMillis();
}
Department of Computer Science
8
public void stop()
{
if (!isRunning) return;
isRunning = false;
long endTime = System.currentTimeMillis();
elapsedTime = elapsedTime + endTime - startTime;
}
public long getElapsedTime()
{
if (isRunning) {
long endTime = System.currentTimeMillis();
return elapsedTime + endTime - startTime;
}
else
return elapsedTime;
}
public void reset()
{
elapsedTime = 0;
isRunning = false;
}
}
Department of Computer Science
9
Test program for StopWatch
public class TestStopWatch {
public static void main(String[] args)
{
StopWatch timer = new StopWatch();
timer.start();
try {
Thread.sleep(6000);
}
catch (InterruptedException exception) { }
timer.stop();
System.out.println("Time to sleep ="+timer.getElapsedTime());
}
}
Department of Computer Science
10
Assignment 1 Review

Java naming conventions
All variables start with a lowercase letter and new
words start with an uppercase letter
 Eg int num, int numFound
 Class names uppercase letter first (then as above)
 Eg Circle, Student, BinarySearchTree
 Method names as per variables
 Eg processFile, main, findOutliers

Department of Computer Science
11
Question 1 – check chars
public class palindrome {
public static void main (String [] args) {
String s = args[0];
for (int i=0; i<s.length()/2; i++) {
if (!(s.charAt(i) == (s.charAt(s.length()-1-i)))) {
System.out.println(s+" is not a palindrome");
return;
}
}
System.out.println(s+" is a palindrome");
}
}
Department of Computer Science
12
Question 1 – use String methods
public static void main (String[ ] args) {
StringBuffer s = new StringBuffer (args[0]);
StringBuffer t = new StringBuffer (args[0]);
if (s.toString().equals(t.reverse().toString()))
System.out.println(s+" is a palindrome");
else
System.out.println(s+" is not a palindrome");
}
Department of Computer Science
13
Complex Numbers
public class Complex {
private float real;
private float imag;
public Complex(float r, float i)
{
real = r;
imag = i;
}
public Complex()
{
real = 0;
imag = 0;
}
Department of Computer Science
14
public Complex add(Complex a)
{
Complex result = new Complex();
result.real = a.real + this.real;
result.imag = a.imag + this.imag;
return result;
}
public String toString()
{
String result = new String();
result = "Real part = " + this.real;
result += " Imag part = " + this.imag;
return result;
}
public boolean equals (Complex y)
{
boolean result;
result = (this.real == y.real && this.imag == y.imag);
return result;
}
}
Department of Computer Science
15
equals


The equals method can be either overloaded or
overridden
Here it is overloaded – so there are two equals
methods
boolean equals(Complex other) – defined in
Complex
 boolean equals(Object other) – defined in Object


boolean equals(Complex x, Complex y) is a
different method entirely
Department of Computer Science
16
Equals – formal defn
public boolean equals(Object otherObject) {
if (otherObject == null) return false;
if (getClass() != otherObject.getClass())
return false;
Complex other = (Complex) otherObject;
return (this.real == other.real && this.imag ==
other.imag);
}
Department of Computer Science
17
Question 3 - Scanner
import java.util.Scanner;
public class TestComplex {
public static void main (String [] args)
{
Scanner in = new Scanner(System.in);
int xr = in.nextInt();
int xi = in.nextInt();
Complex x = new Complex(xr,xi);
String op = in.next();
int yr = in.nextInt();
int yi = in.nextInt();
Complex y = new Complex(yr, yi);
Complex result;
if (op.equals("+")) {
result = x.add(y);
System.out.println("result is "+result.toString());
}
else System.out.println("Something went wrong");
}
}
Department of Computer Science
18
Scanner methods






Scanner(InputStream in)
Scanner(Reader in)
boolean hasNext()
String next()
int nextInt()
String nextLine()
Department of Computer Science
19
One line input
import java.util.Scanner;
import java.util.StringTokenizer;
public class TestComplex {
public static void main (String [] args)
{
Scanner in = new Scanner(System.in);
String s = in.nextLine();
StringTokenizer tokens = new StringTokenizer(s);
String openBracket = tokens.nextToken();
int xr = Integer.parseInt(tokens.nextToken());
int xi = Integer.parseInt(tokens.nextToken());
Complex x = new Complex(xr, xi);
String op = tokens.nextToken();
int yr = Integer.parseInt(tokens.nextToken());
int yi = Integer.parseInt(tokens.nextToken());
Complex y = new Complex(yr, yi);
String closeBracket = tokens.nextToken();
Complex result;
if (op.equals("+")) {
result = x.add(y);
System.out.println("result is "+result.toString());
}
else System.out.println("Something went wrong");
}
}
Department of Computer Science
20
Question 4
import java.util.ArrayList;
public class TestShapes {
public static void main(String[] args) {
ArrayList<Shape> shapes = new ArrayList<Shape>();
shapes.add(new Circle(4));
shapes.add(new Triangle(5,10));
shapes.add(new Rectangle(4,5));
double totalArea = 0;
for (Shape x : shapes) {
totalArea += x.area();
}
System.out.println("Total area is "+totalArea);
}
}
Department of Computer Science
21
Implement Shape as an Interface
public interface Shape {
double area();
}
Department of Computer Science
22
And each shape implements area
public class Circle implements Shape {
private double radius;
public Circle() {
radius = 0;
}
public Circle(double r) {
radius = r;
}
public double area() {
return Math.PI*radius*radius;
}
}
Department of Computer Science
23
Similarly for others
public class Rectangle implements Shape {
private double width, length;
public Rectangle() {
width = 0;
length = 0;
}
public Rectangle(double w, double l) {
width = w;
length = l;
}
public double area() {
return width*length;
}
}
Department of Computer Science
24
And Triangle
public class Triangle implements Shape {
private double base, height;
public Triangle() {
base = 0;
height = 0;
}
public Triangle(double b, double h) {
base = b;
height = h;
}
public double area() {
return 0.5*base*height;
}
}
Department of Computer Science
25