Module 4: Characters, Strings, and Mathematical Functions

Transcription

1 Module 4: Characters, Strings, and Mathematical Functions

2 Objectives To solve mathematics problems by using the methods in the Math class ( 4.2). To represent characters using the char type ( 4.3). To encode characters using ASCII and Unicode ( 4.3.1). To represent special characters using the escape sequences ( 4.4.2). To cast a numeric value to a character and cast a character to an integer ( 4.3.3). To compare and test characters using the static methods in the Character class ( 4.3.4). To introduce objects and instance methods ( 4.4). To represent strings using the String objects ( 4.4). To return the string length using the length() method ( 4.4.1). To return a character in the string using the charat(i) method ( 4.4.2). To use the + operator to concatenate strings ( 4.4.3). To read strings from the console ( 4.4.4). To read a character from the console ( 4.4.5). To compare strings using the equals method and the compareto methods ( 4.4.6). To obtain substrings ( 4.4.7). To find a character or a substring in a string using the indexof method ( 4.4.8). To program using characters and strings (GuessBirthday) ( 4.5.1). To convert a hexadecimal character to a decimal value (HexDigit2Dec) ( 4.5.2). To revise the lottery program using strings (LotteryUsingStrings) ( 4.5.3). To format output using the System.out.printf method ( 4.6). Module 4: Characters, Strings, and Mathematical Functions page 2

3 Motivations Suppose you need to estimate the area enclosed by four cities, given the GPS locations (latitude and longitude) of these cities, as shown in the following diagram. How would you write a program to solve this problem? You will be able to write such a program after completing this module. Module 4: Characters, Strings, and Mathematical Functions page 3

4 Summary: Motivations This module introduced additional basic features of Java These features include Strings, Characters, and additional Mathematical Functions Mathematical Functions Java provides many useful methods in the Math class for performing common mathematical functions. Module 4: Characters, Strings, and Mathematical Functions page 4

5 Common Mathematical Functions Method: A group of statements that performs a specific task You have already seen several methods: pow(a, b): to compute a b random(): generate a random number Both of these methods are inside the Math class There are many other beneficial methods inside the Math class as well Module 4: Characters, Strings, and Mathematical Functions page 5

6 Class constants: PI E The Math Class Class methods can be categorized as: Trigonometric Methods Exponent Methods Service methods You can use these constants as Math.PI and Math.E in any program. rounding, min, max, abs, and random Methods Module 4: Characters, Strings, and Mathematical Functions page 6

7 Trigonometric Methods The parameter of sin, cos, and tan is an angle in radians The return value of asin, acos, and atan is a degree in radians Module 4: Characters, Strings, and Mathematical Functions page 7

8 Trigonometric Methods Examples: Math.toDegrees(Math.PI / 2); //returns 90.0 Math.toRadians(30); Math.sin(0); //returns 0.0 //returns (same as π/6) Math.sin(Math.toRadians(270)); // returns -1.0 Math.sin(Math.PI / 6); // returns 0.5 Math.sin(Math.PI / 2); // returns 1.0 Math.cos(0); // returns 1.0 Math.cos(Math.PI / 6); // returns Math.cos(Math.PI / 2); // returns 0 Math.asin(0.5); Math.acos(0.5); Math.atan(1.0); // returns (same as π/6) //returns (same as π/3) // returns (same as π/4) Module 4: Characters, Strings, and Mathematical Functions page 8

9 Exponent Methods Module 4: Characters, Strings, and Mathematical Functions page 9

10 Exponent Methods Examples: Math.exp(1); // returns Math.log(Math.E); // returns 1.0 Math.log10(10); // returns 1.0 Math.pow(2, 3); // returns 8.0 Math.pow(3, 2); // returns 9.0 Math.pow(4.5, 2.5); // returns Math.sqrt(4); // returns 2.0 Math.sqrt(10.5); // returns 4.24 Module 4: Characters, Strings, and Mathematical Functions page 10

11 Rounding Methods Math class contains five rounding methods: double ceil(double x) x rounded up to its nearest integer. This integer is returned as a double value. double floor(double x) x is rounded down to its nearest integer. This integer is returned as a double value. double rint(double x) x is rounded to its nearest integer. If x is equally close to two integers, the even one is returned as a double. int round(float x) Return (int)math.floor(x+0.5). long round(double x) Return (long)math.floor(x+0.5). Module 4: Characters, Strings, and Mathematical Functions page 11

12 Rounding Methods Examples: Math.ceil(2.1); //returns 3.0 Math.ceil(2.0); //returns 2.0 Math.ceil(-2.0); //returns 2.0 Math.ceil(-2.1); //returns -2.0 Math.floor(2.1); //returns 2.0 Math.floor(2.0); //returns 2.0 Math.floor(-2.0); //returns 2.0 Math.floor(-2.1); //returns -3.0 Math.rint(2.1); //returns 2.0 Math.rint(2.0); //returns 2.0 Math.rint(-2.0); //returns 2.0 Math.rint(-2.1); //returns -2.0 Math.rint(2.5); //returns 2.0 Math.rint(-2.5); //returns -2.0 Math.round(2.6f); //returns 3 Math.round(2.0); //returns 2 Math.round(-2.0f); //returns -2 Math.round(-2.6); //returns -3 Module 4: Characters, Strings, and Mathematical Functions page 12

13 min and max methods min, max, and abs min(a, b): returns minimum of a and b max(a, b): returns the maximum of a and b abs method: abs(a): returns the absolute value of a Examples: Math.max(2, 3); //returns 3 Math.max(2.5, 3); //returns 3 Math.min(2.5, 4.6); //returns 2.5 Math.abs(-2); //returns 2 Math.abs(-2.1); //returns 2.1 Module 4: Characters, Strings, and Mathematical Functions page 13

14 The random Method Generates a random double value This value is greater than or equal to 0.0 and less than <= Math.random() < 1.0 Examples: (int)(math.random() * 10); returns a random integer between 0 and (int)(math.random() * 50); returns a random integer between 50 and 99 a + Math.random() * b; returns a random number between a and a+b, excluding a+b Module 4: Characters, Strings, and Mathematical Functions page 14

15 Evaluate the following method calls: a. Math.sqrt(4) b. Math.pow(2,3) c. Math.rint(-2.5) d. Math.ceil(7.1) e. Math.ceil(-2.8) f. Math.floor(-2.2) g. Math.floor(9.9) h. Math.round(2.5) i. Math.round(Math.abs(-3.5)) Module 4: Characters, Strings, and Mathematical Functions page 15

16 Character Data Type Java allows you to process characters using the character data type, char char represents a single character a character literal is enclosed in single quotation marks Examples: char letter = A ; Assigns the character A to the char variable letter char numchar = 4 ; Assigns the digit character 4 to the char variable numchar Module 4: Characters, Strings, and Mathematical Functions page 16

17 Unicode and ASCII code Computers use binary numbers internally a character is stored in a computer as a sequence of 0s and 1s Mapping a character to its binary representation is called encoding There are different ways to encode a character Java uses Unicode Unicode is an encoding scheme Originally designed for 16-bit character encoding This allowed for 2 16 = 65,536 characters Module 4: Characters, Strings, and Mathematical Functions page 17

18 Unicode Unicode and ASCII code So Unicode originally allowed only 65,536 characters Is that enough characters to represent all the characters in the world? The answer is no. So Unicode has been extended to allow 1,112,064 different types of characters These are called supplementary characters We will not discuss those characters in CPCS-202 For simplicity, we consider only the original 16-bit Unicode characters (stored in a char variable) Module 4: Characters, Strings, and Mathematical Functions page 18

19 Unicode Unicode and ASCII code Again, we only consider 16-bit Unicode characters Remember: one byte is eight bits So a 16-bit Unicode requires two bytes, preceded by \u Also, because reading 16 bits is cumbersome and difficult, Unicode is usually expressed in hexadecimal Remember: Decimal number system: 10 digits from 0 to 9 Hexadecimal number system: 16 digits from 0 to F 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, A, B, C, D, E, F We need 4 bits to represent a Hex digit Example: 15 in decimal is 1111 in binary and F in Hex Module 4: Characters, Strings, and Mathematical Functions page 19

20 Unicode Unicode and ASCII code Hex system: Module 4: Characters, Strings, and Mathematical Functions page 20

21 Unicode Unicode and ASCII code Unicode is usually expressed in Hexadecimal 16 bits in Unicode Each hexadecimal requires 4 bits So that means each Unicode is expressed in 4 Hex digits! From \u0000 to \uffff Example: Module 4: Characters, Strings, and Mathematical Functions page 21

22 Unicode and ASCII code ASCII Most computers use ASCII American Standard Code for International Exchange 8-bit encoding scheme Used to represent all uppercase and lowercase letters, all digits, all punctuation marks, and control characters 128 characters total Unicode includes ASCII code \u0000 to \u007f representing the 128 ASCII characters Module 4: Characters, Strings, and Mathematical Functions page 22

23 Unicode and ASCII code ASCII You can use both ASCII and Unicode in your program The following statements are equivalent: char letter = A ; char letter = \u0041 ; // A s Unicode value is 0041 Both statements assign character A to the char variable letter Module 4: Characters, Strings, and Mathematical Functions page 23

24 Escape Sequences for Special Characters What if you want to print a message with quotation marks printed in the output. Can you do this: System.out.println("He said "Java is fun""); Answer: No! This statement will give a compile error. The compiler will see the quotation before the word Java. And the compiler will assume this is the END of the string! So it gets confused when it sees more characters after How do we overcome this problem? Escape Sequences Module 4: Characters, Strings, and Mathematical Functions page 24

25 Escape Sequences for Special Characters Escape Sequences: Java uses a special notation to represent special characters This notation is called an escape sequence Example escape sequences are shown in the table below: Module 4: Characters, Strings, and Mathematical Functions page 25

26 Escape Sequences for Special Characters Escape Sequences: So we use a backslash (\) followed by a character or a combination of digits Example: System.out.println("He said \" Java is fun\""); The output is: He said "Java is fun" Note that the symbols \ and " together represent one character Module 4: Characters, Strings, and Mathematical Functions page 26

27 Escape Sequences for Special Characters Escape Sequences: So what if you wanted to actually print a backslash Example: what if you wanted to print the following: \t is a tab character So we actually want to print the backslash to the screen How can you do this? Backslash is an escape character Using a backslash signals to the compiler that a special character should be printed So how do we actually print the backslash: we escape it! System.out.println("\\t is a tab character"); Module 4: Characters, Strings, and Mathematical Functions page 27

28 Casting between char and Numeric Types Implicit casting: Implicit casting can be used if the result of casting fits into the target variable. Otherwise, explicit casting is required Example: Unicode of 'a' is 97 This is within the range of a byte (and of course an int) So the following are okay: byte b = 'a'; // same as byte b = (byte)'a'; int i = 'a'; char c = 97; // same as int i = (int)'a'; // same as char c = (char)97; Module 4: Characters, Strings, and Mathematical Functions page 28

29 Casting between char and Numeric Types Explicit casting: The following would be incorrect: byte b = '\ufff4'; Why? Because the right side is 16 bits. A byte is 8 bits Clearly, 16 bits does not fit into 8 bits To force the assignment, use explicit casting: byte b = (byte)'\ufff4'; Remember: a char uses two bytes So any positive integer between 0000 and FFFF in hexadecimal can be cast into a char implicitly. Module 4: Characters, Strings, and Mathematical Functions page 29

30 Casting between char and Numeric Types Numeric operators All numeric operators can be applied to char operands A char operand is automatically cast into a number if the other operand is a number or a character But if the other operand is a string, the character is concatenated with the string Module 4: Characters, Strings, and Mathematical Functions page 30

31 Casting between char and Numeric Types Numeric operators Examples: int i = '2' + '3'; // (int)'2' is 50 and (int)'3' is 51 System.out.println("i is " + i); // i is 101 int j = 2 + 'a'; // (int)'a' is 97 System.out.println("j is " + j); // j is 99 System.out.println("Module " + '2'); System.out.println(j + " is the Unicode for character " + (char)j); Output: i is 101 j is 99 Module 2 99 is the Unicode for character c Module 4: Characters, Strings, and Mathematical Functions page 31

32 Comparing and Testing Characters Two characters can be compared using relational operators just like comparing two numbers This is done by comparing the Unicode values Examples: 'a' < 'b' is true because the Unicode for 'a' (97) is less than the Unicode for 'b' (98). 'a' < 'A' is false because the Unicode for 'a' (97) is greater than the Unicode for 'A' (65). '1' < '8' is true because the Unicode for '1' (49) is less than the Unicode for '8' (56). Module 4: Characters, Strings, and Mathematical Functions page 32

33 Comparing and Testing Characters Often you must test whether a character is a number, a letter, or even if it is uppercase or lowercase. The following code tests whether a character ch is an uppercase letter, a lowercase letter, or a digit: if (ch >= 'A' && ch <= 'Z') System.out.println(ch + " is an uppercase letter"); else if (ch >= 'a' && ch <= 'z') System.out.println(ch + " is a lowercase letter"); else if (ch >= '0' && ch <= '9') System.out.println(ch + " is a numeric character"); Module 4: Characters, Strings, and Mathematical Functions page 33

34 Methods in the Character Class Testing characters is common Therefore, Java gives the following methods inside the Character class Module 4: Characters, Strings, and Mathematical Functions page 34

35 Methods in the Character Class Using the Character class: Examples: System.out.println("isDigit('a') is " + Character.isDigit('a')); System.out.println("isLetter('a') is " + Character.isLetter('a')); System.out.println("isLowerCase('a') is " + Character.isLowerCase('a')); System.out.println("isUpperCase('a') is " + Character.isUpperCase('a')); System.out.println("toLowerCase('T') is " + Character.toLowerCase('T')); System.out.println("toUpperCase('q') is " + Character.toUpperCase('q')); Output: isdigit('a') is false isletter('a') is true islowercase('a') is true isuppercase('a') is false tolowercase('t') is t touppercase('q') is Q Module 4: Characters, Strings, and Mathematical Functions page 35

36 Evaluate the following : System.out.println('a' < 'b'); System.out.println('a' <= 'A'); System.out.println('a' > 'b'); System.out.println('a' >= 'A'); System.out.println('a' == 'a'); System.out.println('a'!= 'b'); true false false true true true Module 4: Characters, Strings, and Mathematical Functions page 36

37 What is the output of the following program: public class Test { public static void main(string[] args) { char x = 'a'; char y = 'c'; System.out.println(++x); System.out.println(y++); System.out.println(x - y); } } Output: b c -2 Module 4: Characters, Strings, and Mathematical Functions page 37

38 Write code that generates a random lowercase letter. Remember: lowercase letters are 97 to 122 in decimal So 26 possible values (letters) public class RandomLowercase{ public static void main(string[] args) { int x = 97 + (int)(math.random() * 26); char randomchar = x; System.out.println("Random char: " + randomchar); } } Module 4: Characters, Strings, and Mathematical Functions page 38

39 The String Type A char represents one character So how do we represent a sequence (a string) of characters? Use the data type called String Example: The following code declares variable message to be a string with the value "Welcome to Java" String message = "Welcome to Java"; Module 4: Characters, Strings, and Mathematical Functions page 39

40 String details: The String Type String is a predefined class in the Java library Just like the classes System and Scanner The String type is not a primitive type It is known as a reference type And the variable declared by a reference type is known as a reference variable that references an object String message = "Welcome to Java"; Here, message is a reference variable that references a string object with the contents "Welcome to Java" Module 4: Characters, Strings, and Mathematical Functions page 40

41 String details: The String Type Declaring a String variable: String firstname; Assign a value to the String variable: firstname = "Muhammad Alzahrani"; Most important: How to use the methods in the String class The following page shows some of the common methods that can be used to operate on Strings Module 4: Characters, Strings, and Mathematical Functions page 41

42 Simple Methods for String Objects String details: Strings are objects in Java For this reason, these methods are called "instance methods" A non-instance method is called a static method All methods in the Math class are static methods They are not tied to a specific object instance Module 4: Characters, Strings, and Mathematical Functions page 42

43 Simple Methods for String Objects String details: Again, string methods are instance methods This means that they are tied to a specific object/string Therefore, you must invoke them from a specific object/string Syntax: referencevariable.methodname(arguments) Recall: syntax to invoke a static method: ClassName.methodName(arguments) Example: Math.pow(2, 3); // result is 8 So instead of mentioning the Class of the method We mention the specific variable Module 4: Characters, Strings, and Mathematical Functions page 43

44 Simple Methods for String Objects Getting String Length Use the length() method to return the number of characters in a string Example: String message = "Welcome to Java"; System.out.println("The length of " + message + " is " + message.length()); Output: The length of Welcome to Java is 15 Module 4: Characters, Strings, and Mathematical Functions page 44

45 Simple Methods for String Objects Getting Characters from a String The s.charat(index) method can be used to retrieve a specific character in a string s The index is between 0 and s.length()-1 Example: message.charat(0); // Returns the character W Module 4: Characters, Strings, and Mathematical Functions page 45

46 Simple Methods for String Objects Concatenating Strings You can use the concat method to concatenate two strings Example: Strings s3 = s1.concat(s2); concatenates s1 and s2 into s3 String concatenation is very common in Java Therefore, Java gives is the plus (+) operator for this Example: String s3 = s1 + s2; Module 4: Characters, Strings, and Mathematical Functions page 46

47 Simple Methods for String Objects Concatenating Strings Remember: you can concatenate a number with a string At least one of the operands must be a string Examples: String message = "Welcome " + "to " + "Java"; String s = "Module " + 2; s becomes "Module 2" String s1 = "Supplement " + 'B'; s1 becomes "Supplement B" Module 4: Characters, Strings, and Mathematical Functions page 47

48 Simple Methods for String Objects Concatenating Strings The augmented += operator can also be used for concatenation with strings Example: String message = "Welcome to Java"; message += ", and Java is fun."; System.out.println(message); Output: "Welcome to Java, and Java is fun." Module 4: Characters, Strings, and Mathematical Functions page 48

49 Simple Methods for String Objects Concatenating Strings Final example: If i=1 and j=2, what is the output of the following: System.out.println("i + j is " + i + j); Output: "i + j is 12" Why? In Java, we read from left to right So we have the String "i + j is " concatenated with the int i The result: a new String ("i + j is 1") This new String is the concatenated with the int j You can force addition by enclosing the i + j in parentheses Module 4: Characters, Strings, and Mathematical Functions page 49

50 Simple Methods for String Objects Converting Strings Methods: tolowercase() method returns a new string with all lowercase letters touppercase() method returns a new string with all uppercase letters trim() method returns a new string after removing all whitespace characters from both ends of the string Final example: "Welcome".toLowerCase(); // returns a new string welcome "Welcome".toUpperCase(); // returns a new string WELCOME "\t a b c ".trim(); // returns a new string a b c Module 4: Characters, Strings, and Mathematical Functions page 50

51 Reading a String from the Console How to read a string from the console Use the next() method on a Scanner object System.out.print("Enter three words separated by spaces: "); String s1 = input.next(); // assume we made Scanner object String s2 = input.next(); String s3 = input.next(); System.out.println("s1 is " + s1); System.out.println("s2 is " + s2); System.out.println("s3 is " + s3); Module 4: Characters, Strings, and Mathematical Functions page 51

52 Reading a String from the Console How to read a complete line from the console Use the nextline() method on a Scanner object Scanner input = new Scanner(System.in); System.out.println("Enter a line: "); String s = input.nextline(); System.out.println("The line entered is " + s); Module 4: Characters, Strings, and Mathematical Functions page 52

53 Reading a Character from the Console How to read a single character from the console Use the nextline() method to read a string and then invoke the charat(0) method on the string Scanner input = new Scanner(System.in); System.out.print("Enter a character: "); String s = input.nextline(); char ch = s.charat(0); System.out.println("The character entered is " + ch); Module 4: Characters, Strings, and Mathematical Functions page 53

54 Comparing Strings The String class has many methods you can use to compare two strings. Module 4: Characters, Strings, and Mathematical Functions page 54

55 String equality Comparing Strings How can you check if two strings are equal? Note: you cannot use the == operator Example: if (s1 == s2) else System.out.println("s1 and s2 are the same object"); System.out.println("s1 and s2 are different object"); This will only tell us if two string reference variables point to (refer to) the same object Module 4: Characters, Strings, and Mathematical Functions page 55

56 String equality Comparing Strings So how can you check if two strings are equal? Meaning, how to check if they have same contents? Use the equals() method Example: if (s1.equals(s2)) else System.out.println("s1 and s2 have same contents"); System.out.println("s1 and s2 are not equal"); Module 4: Characters, Strings, and Mathematical Functions page 56

57 String equality Comparing Strings Example: String s1 = "Welcome to Java"; String s2 = "Welcome to Java"; String s3 = "Welcome to C++"; System.out.println(s1.equals(s2)); // true System.out.println(s1.equals(s3)); // false Module 4: Characters, Strings, and Mathematical Functions page 57

58 Comparing Strings compareto() method We can use the compareto() method to compare two strings This allows us to alphabetically order the strings Syntax: s1.compareto(s2); Returns the value 0 if s1 is equal to s2 Returns a value less than 0 if s1 is "less than" s2 Returns a value greater than 0 if s1 is "greater than" s2 Example: "abc".compareto("abg"); // returns -4 Module 4: Characters, Strings, and Mathematical Functions page 58

59 Program 1: OrderTwoCities Write a program that prompts the user to enter two cities and then displays them in alphabetical order. Remember: Step 1: Problem-solving Phase Step 2: Implementation Phase Module 4: Characters, Strings, and Mathematical Functions page 59

60 Program 1: OrderTwoCities Step 1: Problem-solving Phase Algorithm: 1. Prompt the user to enter an two Strings 2. Compare the two strings using compareto() method 3. Print the cities in correct alphabetical order Module 4: Characters, Strings, and Mathematical Functions page 60

61 Program 1: OrderTwoCities Step 2: Implementation Module 4: Characters, Strings, and Mathematical Functions page 61

62 Program 1: OrderTwoCities Run the Program: Note: Some city names have multiple words Such as New York Therefore, we used nextline() to scan the city name Instead of next() Click here to view and trace code Module 4: Characters, Strings, and Mathematical Functions page 62

63 Obtaining Substrings You can get a substring, from a string, by using the substring method in the String class Example: String message = "Welcome to Java"; String message = message.substring(0, 11) + "HTML"; System.out.println(message); Output: Welcome to HTML Module 4: Characters, Strings, and Mathematical Functions page 63

64 Obtaining Substrings Java gives you two substring methods: substring(beginindex) Returns this string's substring that begins with the character at the specified beginindex and extends to the end of the string substring(beginindex, endindex) Returns this string's substring that begins with the character at the specified beginindex and extends to the character at index endindex 1 NOTE: the character at endindex is NOT part of the substring Module 4: Characters, Strings, and Mathematical Functions page 64

65 Finding a Character or a Substring in a String The String class provides several versions of indexof and lastindexof methods to find a character or a substring in a string Module 4: Characters, Strings, and Mathematical Functions page 65

66 Finding a Character or a Substring in a String The String class provides several versions of indexof and lastindexof methods to find a character or a substring in a string Examples: "Welcome to Java".indexOf('W') returns 0. "Welcome to Java".indexOf('o') returns 4. "Welcome to Java".indexOf('o', 5) returns 9. "Welcome to Java".indexOf("come") returns 3. "Welcome to Java".indexOf("Java", 5) returns 11. "Welcome to Java".indexOf("java", 5) returns -1. Module 4: Characters, Strings, and Mathematical Functions page 66

67 Finding a Character or a Substring in a String The String class provides several versions of indexof and lastindexof methods to find a character or a substring in a string Examples: "Welcome to Java".lastIndexOf('W') returns 0. "Welcome to Java".lastIndexOf('o') returns 9. "Welcome to Java".lastIndexOf('o', 5) returns 4. "Welcome to Java".lastIndexOf("come") returns 3. "Welcome to Java".lastIndexOf("Java", 5) returns -1. "Welcome to Java".lastIndexOf("Java") returns 11. Module 4: Characters, Strings, and Mathematical Functions page 67

68 Finding a Character or a Substring in a String Practical Example Suppose a string s contains the first name and last name of a student, separated by a space How can you extract the first name and last name? Can we use a method to find the space? YES! We can use the indexof(' ') This will give us the index of the first space Then, because we know this index, we can use the substring() method to find the first name and the last name Module 4: Characters, Strings, and Mathematical Functions page 68

69 Finding a Character or a Substring in a String Practical Example Solution: String s = "Kim Jones"; int k = s.indexof(' '); String firstname = s.substring(0, k); String lastname = s.substring(k + 1); Module 4: Characters, Strings, and Mathematical Functions page 69

70 Conversion between Strings and Numbers You can convert a numeric string into an int How? Use the Integer.parseInt() method Example: int value = Integer.parseInt("152"); You can also convert a string to a double Use the Double.parseDouble() method Example: double value = Double.parseDouble("827.55"); Module 4: Characters, Strings, and Mathematical Functions page 70

71 Conversion between Strings and Numbers Can you convert a number to a String? Yes. You can do it a complicated way We won't bother showing you! OR, you do it the EASY way: int number = 7; String s = number + ""; kinda like a "hack" But it works great! We concatenate a number with the empty string Result: we get a string representation of the number! Module 4: Characters, Strings, and Mathematical Functions page 71

72 Suppose that s1, s2, and s3 are three strings: String s1 = "Welcome to Java"; String s2 = "Programming is fun"; String s3 = "Welcome to Java"; What are the results of the following expressions? a. s1 == s2 b. s1 == s3 c. s1.equals(s2) d. s1.equals(s3) e. s1.compareto(s2) f. s2.compareto(s3) g. s2.compareto(s2) h. s1.charat(0) false false false true Greater than 0 Less than 0 0 (cause contents are equal) W Module 4: Characters, Strings, and Mathematical Functions page 72

73 Suppose that s1, s2, and s3 are three strings: String s1 = "Welcome to Java"; String s2 = "Programming is fun"; String s3 = "Welcome to Java"; What are the results of the following expressions? a. s1.indexof('j') b. s1.indexof('j') c. s1.indexof("to") d. s1.lastindexof('a') e. s1.length() f. s1.substring(5) g. s1.substring(5,12) h. s1.endswith("java") 11-1 (meaning, not found) 8 (the starting index of "to") me to Java me to J true Module 4: Characters, Strings, and Mathematical Functions page 73

74 Show the output of the following expressions: a) System.out.println("1" + 1); 11 b) System.out.println('1' + 1); 50 c) System.out.println("1" ); 111 d) System.out.println("1" + (1 + 1)); 12 e) System.out.println('1' ); 51 Module 4: Characters, Strings, and Mathematical Functions page 74

75 Program 2: HexDigit2Dec Write a program that prompts the user to enter one hex digit and then displays this as a decimal value. Remember: Step 1: Problem-solving Phase Step 2: Implementation Phase Module 4: Characters, Strings, and Mathematical Functions page 75

76 Program 2: HexDigit2Dec Step 1: Problem-solving Phase Algorithm: 1. Prompt the user to enter a hex digit 2. Check to see if the input is exactly one digit 3. If so, confirm the input is between 0-9 or A-F Then print the decimal equivalent 4. Otherwise, print invalid input For step 3 above, we can use methods we've learned in this Module Module 4: Characters, Strings, and Mathematical Functions page 76

77 Program 2: HexDigit2Dec Step 2: Implementation Module 4: Characters, Strings, and Mathematical Functions page 77

78 Program 2: HexDigit2Dec Step 2: Implementation is A-F? is 0-9? Module 4: Characters, Strings, and Mathematical Functions page 78

79 Program 2: HexDigit2Dec Run the Program: Click here to view and trace code Module 4: Characters, Strings, and Mathematical Functions page 79

80 Program 3: Lottery Re-do the Lottery program from Module 3. This time, use strings for both the randomly generated number and the user input. The logic is easier than dealing with mod! Remember: Step 1: Problem-solving Phase Step 2: Implementation Phase Module 4: Characters, Strings, and Mathematical Functions page 80

81 Program 3: Lottery Step 1: Problem-solving Phase Notes: The user will win money according to the following rules: 1. If the user lottery number matches the winning lottery number in the exact order, the award is $10, If all digits in the user lottery number match all digits in the winning lottery number (but not in the exact order), the award is $3, If only one digit in the user lottery number matches the winning lottery number, the award is $1,000 User MUST enter two digits Module 4: Characters, Strings, and Mathematical Functions page 81

82 Program 3: Lottery Step 1: Problem-solving Phase Algorithm: 1. Randomly generate a winning lottery number Between 0 and 99. This is easy. int lottery = (int)(math.random() * 100); 2. Save that number as a String! 3. Prompt user for input 4. Scan the lottery number from user as a String! 5. Compare user number with winning number, and determine winning amount (if any) Here we must compare the actual digits!!! 6. Display results to user Module 4: Characters, Strings, and Mathematical Functions page 82

83 Program 3: Lottery Step 1: Problem-solving Phase So how do we compare digits? We are saving these two-digit numbers as strings Therefore, we can extract the individual digits using the String method charat() This is MUCH easier than mod! Example: String number = "49"; char c1 = number.charat(0); char c2 = number.charat(1); Module 4: Characters, Strings, and Mathematical Functions page 83

84 Program 3: Lottery Step 2: Implementation Module 4: Characters, Strings, and Mathematical Functions page 84

85 Program 3: Lottery Step 2: Implementation Module 4: Characters, Strings, and Mathematical Functions page 85

86 Run the Program: Program 3: Lottery Click here to view and trace code Module 4: Characters, Strings, and Mathematical Functions page 86

87 Formatting Output You can use the System.out.printf method to display formatted output When printing double values, often we do not need or want all the decimals. In fact, often we want only two (for money)! In Module 2, we learned that you can get two digits after the decimal as follows: double x = ; x = (int)(x * 100) / 100.0; However, if we print x, we will get 16.4 The final zero will not print! So how do we print the zero? Module 4: Characters, Strings, and Mathematical Functions page 87

88 Formatting Output You can use the System.out.printf method to display formatted output How to print with only two decimals? Easy! double x = ; System.out.printf("x is %4.2f", x); Output: x is Module 4: Characters, Strings, and Mathematical Functions page 88

89 Formatting Output You can use the System.out.printf method to display formatted output Another example: Module 4: Characters, Strings, and Mathematical Functions page 89

90 Formatting Output You can use the System.out.printf method to display formatted output Common format specifiers: Module 4: Characters, Strings, and Mathematical Functions page 90