Working with Text and Numbers in Java The Math & String Classes

The Math Class The Math class is a special class called a static class, there is no need to create a Math object, instead we ask the class directly Static class Math.PI//value of PI Non static class String name = new String(”Joe”); You can now use name in your code

Math class All of the buttons you find on your calculator are in the Math class sin, cos, tan sqrt, pow, etc. Of course you can find them all in the JavaDoc Official Oracle Documentation Another things you’ll need to use in the Math class is random numbers

Math Methods double Math.ceil( double value ) This method rounds up a floating point number It also returns a floating point so be sure to cast if you’re putting the result in an int Ex. double ceil = Math.ceil( 10.9 ); //returns 11.0 double ceil = Math.ceil( 3.1 ); //returns 4.0 double ceil = Math.ceil( -1.9 ); //returns -1.0 int ceil = (int)Math.ceil( -1.2 ); //returns -1.0

Math Methods double Math.floor( double value ) This method rounds down a floating point number It also returns a floating point so be sure to cast if you’re putting the result in an int Ex. double floor = Math.floor( 3.1 ); //returns 3.0 double floor = Math.floor( 10.9 ); //returns 10.0 double floor = Math.floor( -1.2 ); //returns -2.0 int floor = (int)Math.floor( -1.9 ); //returns -2.0

Math Methods double Math.pow( double base, double exponent ) This method returns the power of (base)exponent It also returns a floating point so be sure to cast if you’re putting the result in an int Ex. double power = Math.pow( 2, 3 ); //returns 8.0 double power = Math.pow( 3, 4 ); //returns 81.0 double power = Math.pow( 0.1, 5 ); //returns 0.00001 int power = (int)Math.pow( 1.5, 2 );//returns 2.25 (it is cast to 2)

Math Methods double Math.abs( double value ) This method returns the absolute value of value It also returns a floating point so be sure to cast if you’re putting the result in an int Ex. double absolute = Math.abs( 100 ); //returns 100.0 double absolute = Math.abs( -4.5); //returns 4.5 double absolute = Math.abs( 0.57 ); //returns 0.57 int absolute = (int)Math.abs( -1234 ); //returns 1234

Math Methods double Math.sqrt( double value ) This method returns the square root of value It also returns a floating point so be sure to cast if you’re putting the result in an int Ex. double root = Math.sqrt( 100 ); //returns 10.0 double root = Math.sqrt( 2 ); //returns 1.4142 double root = Math.sqrt( 0.25 ); //returns 0.5 int root = (int)Math.sqrt( 16.0 ); //returns 4

Making a Formula in Java The quadratic formula: Using the order of operations we get, x1 = ( –1*b + Math.sqrt( Math.pow( b, 2 ) – 4*a*c ) ) / (2*a) Can you rewrite the other solution? x2 = (–1*b – Math.sqrt( Math.pow( b, 2 ) – 4*a*c )) / (2*a)

Math.random() Random numbers can be generated from the random method of the Math Class static double random(); returns a random number between 0.0 (inclusive) and 1.0 (exclusive) Ex. double randomNumber = Math.random() * 100; This is a random number between 0.0 and 99.999999

Using Math.random Ex. double randomNumber = Math.random() * 35; Between 0.0 and 34.9999999999 double randomNumber = Math.random() * 50 + 10; Between 10.0 and 59.999999999 int randomNumber = (int)(Math.random() * 20) + 5; Between 5 and 24 don’t forget to cast with ints Notice the range is 5 to 24! Math.random() * 20 returns 0 to 19.999999999999… (int)(Math.random() * 20) chops off all decimals Returns 0 to 19

Be Careful When Casting! (int)Math.random() * 20 + 5; returns 5 every time your program is run: Generate a random value, say 0.774312 Cast it to an it Now it has the value 0 0 * 20 = 0 0 + 5 = 5

Using Math.random If you want to generate a number in the range [a,b)  not including b (as doubles) Math.random()*(b – a) + a If you want to generate a number in the range [a,b] (as ints) (int)(Math.random()*(b – a+1)) + a

Math Programming Exercises Create a program that asks the user to click the first time to indicate the center of a circle Tell the person to click again to indicate the a point on the radius of the circle The way this is done is you must calculate the distance between the two points and use that for the radius (along with the center they already clicked) when you create the circle.

Math Programming Exercise Have the user click on 3 points on the screen to draw a triangle. Display a message to the user that tells them the length of each side, its perimeter and its area. To calculate the length or each side you can use the distance formula on the previous slide To label the three sides you can use the midpoint of the two end points: To calculate the area you can use Heron’s Formula Label the area in the center of the triangle using its Centroid: Make sure to create methods for each of your tasks (not just voids)!

Math Programming Exercises Create a program that will draw a random polygon every time it is run. Use a minimum of 5 points. Your points must be drawn on the screen! Do not restrict the values to being integers (the graphics window can draw floating point coordinates) You can do this by randomly generating the coordinates of each point you put in the JPolygon. Make the color of the JPolygon random also Do this by generating three random numbers (red, green and blue) for the fill and border color. The numbers should be from 0 to 255

The String Class Like all standard java classes, there is a javadoc for the String class that can be helpful to you. The methods our class is responsible for are: length() indexOf( String ) indexOf( String, int ) substring( int, int ) substring( int )

How Java Deals With Text Strings in Java are stored in memory as a consecutive sequence of characters Each character has an index to refer to its location The first index in every String is zero Ex. String phrase = “JAVA IS COOL”; Character J A V I S C O L Index 1 2 3 4 5 6 7 8 9 10 11

The length method Strings can tell you how long they are by using the length method: Scanner in = new Scanner( System.in ); System.out.println( “Please Enter Your Name: “ ); String name = in.nextLine(); int nameLength = name.length();//count the characters in name System.out.println( “There are “ + nameLength + ” characters “ + “in your name.” );

The indexOf method There are two indexOf methods, the first is a method where the string will find the first occurrence of an expression within the word. Ex. String example = “TESTING”; int result = example.indexOf( “S” );// returns 2 int result = example.indexOf( “TIN” );// returns 3 int result = example.indexOf( “T” );// returns 0, first T found int result = example.indexOf( “Q” );// returns -1 if not found Character T E S I N G Index 1 2 3 4 5 6

The indexOf Method The other indexOf method takes an extra parameter that tells it where to start looking Ex. String example = “TESTING”; Character T E S T I N G Index 0 1 2 3 4 5 6 int result = example.indexOf( “T”, 1 );// returns 3, first T after index 1 int result = example.indexOf( “E”, 2 );// returns -1, no E after index 2 int result = example.indexOf( “N”, 3 );// returns 5, first N after index 3

indexOf Practice Let’s try a few indexOf examples together I’ll put the examples on the whiteboard and your job is to tell me what value is returned Remember, strings are 0-based if the expression is not found -1 is returned The first location is returned unless you tell the method where to start

The substring Method The substring method allows you to extract pieces of a string The first version we’ll look at takes one parameter, where to start the substring Ex. String example = “substring method”; System.out.println( example.substring( 10 ) );//prints “method” System.out.println( example.substring( 3 ) );//prints “string method” System.out.println( example.substring( 13 ) );//prints “hod”

The substring Method The second version of substring takes two parameters, pay CAREFUL attention! The place to start (inclusive) One space past the end (exclusive) String example = “substring method”; System.out.println( example.substring( 0,3 ) );//prints “sub” System.out.println( example.substring( 3,9 ) );//prints “string” System.out.println( example.substring( 10,16 ) );//prints “method”

substring Practice The substring method can be tricky Especially when there are two parameters The first one is the starting index The second one is ONE PAST the index you want to end at If you only specify the starting position, then substring will go to the end of the string Let’s try some together

Other String Methods I encourage you to refer to the JavaDoc online for Strings, there are many methods that you may find helpful as we create new projects. Later on I will review with you the methods: equals compareTo Some suggestions that can be helpful: replace trim valueOf toUpperCase toLowerCase matches You’ll also need to learn about a very useful programming concept called regular expressions in order to use this method

String Exercises Write a program that separates your full name (first, middle and last) using the substring method Ex. String name = “John Jacob Smith”; System.out.println… Output will be: John Jacob Smith

String Exercises Create a program that reads a line of text from the user and tells them the first and last characters which were entered. Ex. Enter your text: Hello World! First Character: H Last Character: !

String Exercises Create a program that reads in a word from a user and separates it into two halves then puts it back together in the wrong order: Ex. Please enter your text: NetBeans First Half: NetB Second Half: eans Scrambled: eansNetB

String Exercises Write a program to retrieve a specific character for a user. Ex. Enter your text: This is a test Which character do you want me to find: 4 The character is: “s” *Remember, Strings are zero based but the user is giving you the position as though it started at 1

String exercises Repeat the first substring program but this time ask the user for their full name and split their name apart using the substring and indexOf methods (hint, the “ “ marks the start/end of each word)