Primitive Types, Strings, and Console I/O Variables and Expressions The Class String Keyboard and Screen I/O Documentation and Style Reading => Section 1.2 1 ©Silberschatz, Korth and Sudarshan Variables and Values Variables are memory locations that store data such as numbers and letters. The data stored by a variable is called its value. The value is stored in the memory location. A variables value can be changed. 2 ©Silberschatz, Korth and Sudarshan Variables and Values public class EggBasket Output: { If you have public static void main (String[] args) 6 eggs per basket and { 10 baskets, then int baskets, eggsPer, totalEggs; the total number of eggs is 60 baskets = 10; eggsPer = 6; totalEggs = baskets * eggsPer; System.out.println ("If you have"); System.out.print(eggsPer ); System.out.println(" eggs per basket and"); System.out.print(baskets); System.out.println(" baskets, then"); System.out.print("the total number of eggs is “); System.out.println(totalEggs); } } 3 ©Silberschatz, Korth and Sudarshan Variables and Values public class EggBasket Output: { If you have public static void main (String[] args) 6 eggs per basket and { 10 baskets, then int baskets, eggsPer, totalEggs; the total number of eggs is 60 baskets = 10; eggsPer = 6; totalEggs = baskets * eggsPer; System.out.println ("If you have"); System.out.println (eggsPer + " eggs per basket and"); System.out.println (baskets + " baskets, then"); System.out.println ("the total number of eggs is " + totalEggs); } } 4 ©Silberschatz, Korth and Sudarshan Variables and Values Variables: baskets eggsPer totalEggs Assigning values: baskets = 10; eggsPer = 6; totalEggs = baskets * eggsPer; A variable must be declared before it is used. When you declare a variable, you provide its name and type. int baskets, eggsPer, totalEggs; A variable’s type determines what kinds of values it can hold, e.g., integers, real numbers, characters. 5 ©Silberschatz, Korth and Sudarshan Syntax and Examples Variable declaration syntax: type variable_1, variable_2, …; Examples: int styleChoice, numberOfChecks; double balance, interestRate; char jointOrIndividual; A variable is declared just before it is used or at the beginning of a “block” enclosed in { }: public static void main(String[] args) { /* declare variables here */ : } 6 ©Silberschatz, Korth and Sudarshan Types in Java In most programming languages, a type implies several things: A set of values A (hardware) representation for those values A set of operations on those values Example - type int: Values – integers in the range -2147483648 to 2147483647 Representation – 4 bytes, binary, “two’s complement” Operations – addition (+), subtraction (-), multiplication (*), division (/), etc. Two kinds of types: Primitive types Class types 7 ©Silberschatz, Korth and Sudarshan Types in Java A primitive type: values are “simple,” non-decomposable values such as an individual number or individual character int, double, char A class type: values are “complex” objects a class of objects has both data and methods “BIG bad John.” is a value of class type String ‘November 10, 1989’ is a value of class type date (non-Java) 8 ©Silberschatz, Korth and Sudarshan Java Identifiers An identifier is a name given to something in a program: a variable, method, class, etc. created by the programmer, generally Identifiers may contain only: letters digits (0 through 9) the underscore character (_) and the dollar sign symbol ($) which has a special meaning the first character cannot be a digit. 9 ©Silberschatz, Korth and Sudarshan Java Identifiers Legal Examples: count name address count1 count2 count3 x y z i j k myFavoriteHobby My_Favorite_Hobby Illegal Examples: 1count _name 7-11 netscape.com util.* 10 ©Silberschatz, Korth and Sudarshan Java Identifiers, cont. Java is case sensitive i.e., stuff, Stuff, and STUFF are different identifiers. Keywords or reserved words have special, predefined meanings: abstract assert boolean break byte case catch char class const continue default do double else enum extends false final finally float for goto if implements import instanceof int interface long native new null package private protected public return short static strictfp super switch synchronized this throw throws transient true try void volatile while Keywords cannot be used as identifiers Identifiers can be arbitrarily long. 11 ©Silberschatz, Korth and Sudarshan Naming Conventions A common recommendation is to choose names that are helpful, or rather readable, such as count or speed, but not c or s. Sometimes short names are appropriate. Class types: begin with an uppercase letter (e.g. String). Primitive types: begin with a lowercase letter (e.g. int). Variables of both class and primitive types: begin with a lowercase letters (e.g. myName, myBalance). multiword names are “punctuated” using uppercase letters. 12 ©Silberschatz, Korth and Sudarshan Primitive Types Four integer types: byte short int (most common) long Two floating-point types: float double (most common) One character type: char One boolean type: boolean 13 ©Silberschatz, Korth and Sudarshan Primitive Types, cont. 14 ©Silberschatz, Korth and Sudarshan Examples of Primitive Values Integer values: 0 -1 365 12000 Floating-point values: 0.99 -22.8 3.14159 5.0 Character values: `a` `A` `#` ` ` Boolean values: true false 15 ©Silberschatz, Korth and Sudarshan Motivation for Primitive Types Why are there several different integer types? storage space operator efficiency More generally, why are there different types at all? reflects how people understand different kinds of data, e.g., letter vs. numeric grades. helps prevent programmer errors. 16 ©Silberschatz, Korth and Sudarshan Assignment Statements An assignment statement is used to assign a value to a variable: int answer; answer = 42; The “equal sign” is called the assignment operator. In the above example, the variable named answer is assigned a value of 42, or more simply, answer is assigned 42. 17 ©Silberschatz, Korth and Sudarshan Assignment Statements, cont. Assignment syntax: variable = expression ; where expression can be a literal or constant (such as a number), another variable, or an expression which combines variables and literals using operators 18 ©Silberschatz, Korth and Sudarshan Assignment Examples Examples: int amount; int score, numberOfCards, handicap; int eggsPerBasket; char firstInitial; : score = 3; firstInitial = ‘W’; amount = score; score = numberOfCards + handicap; eggsPerBasket = eggsPerBasket - 2; => Some note that the last line looks weird in mathematics. Why? 19 ©Silberschatz, Korth and Sudarshan Assignment Evaluation The expression on the right-hand side of the assignment operator (=) is evaluated first. The result is used to set the value of the variable on the left-hand side of the assignment operator. score = numberOfCards + handicap; eggsPerBasket = eggsPerBasket - 2; 20 ©Silberschatz, Korth and Sudarshan Simple Input Sometimes data is needed and obtained from the user at run time. Simple keyboard input requires: import java.util.*; or import java.util.Scanner; at the beginning of the file. 21 ©Silberschatz, Korth and Sudarshan Simple Input, cont. A “Scanner” object must be initialized before inputting data: Scanner keyboard = new Scanner(System.in); To input data: eggsPerBasket = keyboard.nextInt(); which reads one int value from the keyboard and assigns it to the variable eggsPerBasket. 22 ©Silberschatz, Korth and Sudarshan Simple Input, cont. class EggBasket2 23 ©Silberschatz, Korth and Sudarshan Command-Line Arguments Frequently input is provided to a program at the command-line. public class UseArgument { public static void main(String[] args) { System.out.print(“Hi, ”); System.out.print(args[0]); System.out.println(“. How are you?”); } } Sample interaction: % javac UseArgument.java % java UseArgument Alice Hi, Alice. How are you? % java UseArgument Bob Hi, Bob. How are you? 24 ©Silberschatz, Korth and Sudarshan Command-Line Arguments Frequently multiple values are provided at the command-line. public class Use3Arguments { public static void main(String[] args) { System.out.print(“The first word is ”); System.out.print(args[0]); System.out.print(“, the second is ”); System.out.print(args[1]); System.out.print(“, and the third is ”); System.out.println(args[2]); } } Sample interaction: % javac Use3Arguments.java % java Use3Arguments dog cat cow The first word is dog, the second is cat, and the third is cow 25 ©Silberschatz, Korth and Sudarshan Command-Line Arguments Command-line arguments can be numeric. public class IntOps { public static void main(String[] args) { int a = Integer.parseInt(args[0]); // Notice the variable declaration int b = Integer.parseInt(args[1]); // Notice the comments…lol int sum = a + b; int prod = a * b; int quot = a / b; int rem = a % b; System.out.println(a + " + " + b + " = " + sum); System.out.println(a + " * " + b + " = " + prod); System.out.println(a + " / " + b + " = " + quot); System.out.println(a + " % " + b + " = " + rem); } } Sample interaction: % javac IntOps.java % java IntOps 1234 99 1234 + 99 = 1333 1234 * 99 = 122166 1234 / 99 = 12 1234 % 99 = 46 26 ©Silberschatz, Korth and Sudarshan Literals Values such as 2, 3.7, or ’y’ are called constants or literals. Integer literals can be preceded by a + or - sign, but cannot contain commas. There are two distinct properties that every integer literal has: format – either decimal, hexadecimal or octal, and type – either long or int Both the format and the type of an integer literal can be determined by…looking at it! 27 ©Silberschatz, Korth and Sudarshan Integer Literal Formats Decimal: 0, 10, 37, 643, -47, 829, etc. first digit must NOT be zero, for any non-zero integer Hexadecimal: consists of the leading characters 0x or 0X followed by one or more hexadecimal digits, i.e., 0 through F (lower and upper case equivalent) each letter may be either upper case or lower case 0xA (decimal 10), 0x0 (decimal 0), 0xFF (decimal 255), 0xf0 (decimal 240), 0x34A2, 0Xff decimal (255), etc. Octal: Consists of the leading digit 0 followed by one or more octal digits, i.e., 0 through 7 010 (decimal 8), 023 (decimal 19), 037 (decimal 31) 28 ©Silberschatz, Korth and Sudarshan Integer Literal Formats What is the format? 0 2 0372 0xDadaCafe 1996 0x00FF00FF 0A3C 0945 450xA 0x5FGC3 29 ©Silberschatz, Korth and Sudarshan Integer Literal Formats The format used to specify an integer literal has no impact on its corresponding internal value. All of the following println statements output the same thing: int x; x = 16; System.out.println("Value:" + x); x = 020; System.out.println("Value:" + x); x = 0x10; System.out.println("Value:" + x); 30 ©Silberschatz, Korth and Sudarshan Integer Literal Type Now lets talk about the type of an integer literal… An integer literal is of type long if it is suffixed with an letter L or l; otherwise it is of type int. note that capital L is preferred Integer literals of type long: 2L 0777L 0372L 0x100000000l 0xDadaCafeL 0xC0B0L 1996L 2147483648L 0x00FF00FFL 0l 31 ©Silberschatz, Korth and Sudarshan Integer Literal Type See the program: http://www.cs.fit.edu/~pbernhar/teaching/cse1001/literals 32 ©Silberschatz, Korth and Sudarshan Floating Point Literals Just like with integer literals… Floating point literals can also be preceded by a + or - sign, but cannot contain commas. There are two distinct properties that every floating point literal has: format – either fully expanded notation or scientific notation type – either float or double Both the format and the type of a floating point literal can be determined by…looking at it! 33 ©Silberschatz, Korth and Sudarshan Floating Point Literal Formats Floating-point constants can be written: with digits after a decimal point, as in 3.5 or using scientific notation Examples: 865000000.0 can be written as 8.65e8 0.000483 can be written as 4.83e-4 The number in front of the “e” does not need to contain a decimal point, e.g. 4e-4 Floating point numbers can also be specified in hexidecimal, but we won’t go there… 34 ©Silberschatz, Korth and Sudarshan Floating Point Literal Type Now lets talk about the type of a floating point literal… An floating point literal is of type float if it is suffixed with an letter F or f; otherwise it is of type double. Floating point literals of type float: 2.5F 0.0f 8.65e8f 4e-4F 3f +35.4f -16F 35 ©Silberschatz, Korth and Sudarshan Assignment Compatibilities Java is said to be strongly typed, which means that there are limitations on mixing variables and values in expressions and assignments. int x = 0; long y = 0; float z = 0.0f; x x y z y = = = = = y; z; z; 3.6; 25; // // // // // illegal illegal illegal illegal (3.6 is of type double) legal, but…why? 36 ©Silberschatz, Korth and Sudarshan Assignment Compatibilities Sometimes automatic conversions between types do take place: short s; int x; s = 83; x = s; double doubleVariable; int intVariable; intVariable = 7; doubleVariable = intVariable; 37 ©Silberschatz, Korth and Sudarshan Assignment Compatibilities, cont. In general, a value (or expression) of one numeric type can be assigned to a variable of any type further to the right, as follows: byte --> short --> int --> long --> float --> double but not to a variable of any type further to the left. Makes sense intuitively because, for example, any legal byte value is a legal short value. On the other hand, many legal short values are not legal byte values. 38 ©Silberschatz, Korth and Sudarshan Assignment Compatibilities, cont. Example – all of the following are legal, and will compile: byte b; short s; int i; long l; float f; double d; s = b; i = b; l = i; f = l; // This one is interesting, why? d = f; b = 10; 39 ©Silberschatz, Korth and Sudarshan Assignment Compatibilities, cont. Example – NONE (except the first) of the following will compile: byte b; short s; int i; long l; float f; double d; d = 1.0; f = d; l = f; i = l; s = i; b = s; 40 ©Silberschatz, Korth and Sudarshan Type Casting A type cast creates a value in a new type from an original type. A type cast can be used to force an assignment when otherwise it would be illegal (thereby over-riding the compiler, in a sense). Example: double distance; distance = 9.0; int points; points = distance; // illegal points = (int)distance; // legal 41 ©Silberschatz, Korth and Sudarshan Type Casting, cont. The value of (int)distance is 9, but the value of distance, both before and after the cast, is 9.0. The type of distance does NOT change and remains double. What happens if distance contains 9.7? Any value right of the decimal point is truncated (as oppossed to rounded). Remember to “cast with care,” because the results can be unpredictable. int x; long z = ?; x = (int)z; 42 ©Silberschatz, Korth and Sudarshan Characters as Integers Like everything else, each character is represented by a binary sequence. The binary sequence corresponding to a character is a positive integer. Which integer corresponds to each character is dictated by a standardized character encoding. Each character is assigned a unique integer code The codes are different for upper and lower case letters, e.g., 97 may be the integer value for ‘a’ and 65 for ‘A’ Some characters are printable, others are not Why should different computers and languages use the same code? ASCII and Unicode are the most common character codes. 43 ©Silberschatz, Korth and Sudarshan Unicode Character Set Most programming languages use the ASCII character encoding. American Standard Code for Information Interchange (ASCII) (only) encodes characters from the north American keyboard uses one byte of storage Java uses the Unicode character encoding. The Unicode character set: uses two bytes of storage includes many international character sets (in contrast to ASCII) codes characters from the North American keyboard the same way that ASCII does Hey, lets google ASCII!!!! 44 ©Silberschatz, Korth and Sudarshan Assigning a char to an int A value of type char can be assigned to a variable of type int to obtain its Unicode value. Example: char answer = ’y’; System.out.println(answer); System.out.println((int)answer); >y >121 See the program at: http://www.cs.fit.edu/~pbernhar/teaching/cse1001/charTest 45 ©Silberschatz, Korth and Sudarshan Initializing Variables A variable that has been declared, but not yet given a value is said to be uninitialized. int x, y, z; x = y; x = z + 1; Some languages automatically initialize a variable when it’s declared, other languages don’t. Others (Java) initialize in some circumstances, but not in others. Variables declared to be of a primitive type are NOT automatically initialized. In other cases Java will initialize variables; this will be discussed later. 46 ©Silberschatz, Korth and Sudarshan Initializing Variables Some languages report an error when a variable is used prior to initialization by the program. Sometimes at compile time, other times at run-time. Some languages won’t report an error, and will even let you use an uninitialized variable. In such cases the initial value of the variable is arbitrary! The program might appear to run correctly sometimes, but give errors on others. Java: The compiler will (try) to catch uninitialized variables. => Always make sure your variables are initialized prior to use! 47 ©Silberschatz, Korth and Sudarshan Initializing Variables, cont. In Java, a variable can be assigned an initial value in it’s declaration. Examples: int count = 0; char grade = ’A’; // default is an A Syntax: type variable1 = expression1, variable2 = expression2, …; 48 ©Silberschatz, Korth and Sudarshan Arithmetic Operations Arithmetic expressions: Formed using the +, -, *, / and % operators Operators have operands, which are literals, variables or sub-expressions. Expressions with two or more operators can be viewed as a series of steps, each involving only two operands. The result of one step produces an operand which is used in the next step. Java is left-associative. Most of the basic rules of precedence apply. Example: int x = 0, y = 50, z = 20; double balance = 50.25, rate = 0.05; x = x + y + z; balance = balance + (balance * rate) 49 ©Silberschatz, Korth and Sudarshan Expression Type An arithmetic expression can have operands of different numeric types. x + (y * z) / w Note that this does not contradict our rules for assignment. Every arithmetic expression has a (resulting) type. k = x + (y * z) / w; // Does this compile? Given an arithmetic expression: If any operand in the expression is of type double, then the expression has type double. Otherwise, if any operand in the expression is of type float, then the expression has type float. Otherwise, if any operand in the expression is of type long, then the expression has type long. Otherwise the expression has type int. 50 ©Silberschatz, Korth and Sudarshan Expression Type, cont. Example: int hoursWorked = 40; double payRate = 8.25; double totalPay; Then the expression in the assignment: totalPay = hoursWorked * payRate is a double with a value of 500.0. 51 ©Silberschatz, Korth and Sudarshan Operators with integer and floating point numbers See the program: http://www.cs.fit.edu/~pbernhar/teaching/cse1001/expressions http://www.cs.fit.edu/~pbernhar/teaching/cse1001/integralConversion 52 ©Silberschatz, Korth and Sudarshan The Division Operator The division operator (/) behaves as expected. If one of the operands is a floating-point type then the result is of the same floating point type. 9.0 / 2 = 4.5 9 / 2.0 = 4.5 If both operands are integer types then the result is truncated, not rounded. 9/2 = 4 99 / 100 = 0 Note that the typing rules previously presented (slide 50) still apply! 53 ©Silberschatz, Korth and Sudarshan The mod Operator The mod (%) operator is used with operands of integer type to obtain the remainder after integer division. 14 divided by 4 is 3 with a remainder of 2. Hence, 14 % 4 is equal to 2. The mod operator has many uses, including determining: If an integer is odd or even (x % 2 = 0) If one integer is evenly divisible by another integer (a % b = 0) Frequently we want to map (a large number of) m “items,” number 1 through m, into n>=2 buckets. 54 ©Silberschatz, Korth and Sudarshan Example: Vending Machine Program Requirements: The user enters an amount between 1 cent and 99 cents. The program determines a combination of coins equal to that amount. For example, 55 cents can be two quarters and one nickel. => What are “requirements” anyway? Sample dialog: Enter a whole number from 1 to 99. The machine will determine a combination of coins. 87 87 cents in coins: 3 quarters 1 dime 0 nickels 2 pennies 55 ©Silberschatz, Korth and Sudarshan Example, cont. Algorithm (version #1, in pseudo-code): 1. Read the amount. 2. Find the maximum number of quarters in the amount. 3. Subtract the value of the quarters from the amount. 4. Repeat the last two steps for dimes, nickels, and pennies. 5. Print the amount and the quantities of each coin. Program Variables Needed: int amount, quarters, dimes, nickels, pennies; 56 ©Silberschatz, Korth and Sudarshan Example, cont. Interpreted literally, the algorithm leaves out one detail - the original amount is changed (and hence lost) by the intermediate steps. So we add an additional variable: int amount, originalAmount, quarters, dimes, nickles, pennies; and update the algorithm. 57 ©Silberschatz, Korth and Sudarshan Example, cont. Algorithm (version #2): 1. Read the amount. 2. Make a copy of the amount. 3. Find the maximum number of quarters in the amount. 4. Subtract the value of the quarters from the amount. 5. Repeat the last two steps for dimes, nickels, and pennies. 6. Print the original amount and the quantities of each coin. Typically pseudo-code is iteratively embellished or enhanced. 58 ©Silberschatz, Korth and Sudarshan Example, cont. How do we determine the number of quarters in an amount? There are 2 quarters in 55 cents, but there are also 2 quarters in 65 cents. That’s because 55 / 25 = 2 and 65 / 25 = 2. How do we determine the remaining amount? Using the mod operator: 55 % 25 = 5 and 65 % 25 = 15 Similarly for dimes, nickels and pennies Pennies are simply amount % 5. 59 ©Silberschatz, Korth and Sudarshan Example, cont. 60 ©Silberschatz, Korth and Sudarshan Testing the Program The program should be tested with several different amounts. Test with values that give zero values for each possible coin denomination. Test with amounts close to: “Extreme” or “boundary condition” values such as 0, 1, 98 and 99. Coin denominations such as 24, 25, and 26. 61 ©Silberschatz, Korth and Sudarshan Increment (and Decrement) Operators Used to increase (or decrease) the value of a variable by 1. count = count +1 count = count -1 The increment and decrement operations are easy to use, and important to recognize. The increment operator: count++ ++count The decrement operator: count- --count 62 ©Silberschatz, Korth and Sudarshan Increment (and Decrement) Operators “Mostly” equivalent operations: count++; ++count; count = count + 1; count--; --count; count = count - 1; Unlike the assignment versions, however, the increment and decrement operators are operators and have a resulting value…huh? 63 ©Silberschatz, Korth and Sudarshan Increment (and Decrement) Operators in Expressions After executing: int m = 4; int result = 3 * (++m); result has a value of 15 and m has a value of 5 After executing: int m = 4; int result = 3 * (m++); result has a value of 12 and m has a value of 5 64 ©Silberschatz, Korth and Sudarshan Increment and Decrement Operator, Cont. Common code: int n = 3; int m = 4; int result; What will be the value of m and result after each of these executes? (a) result = n * ++m; (b) result = n * m++; (c) result = n * --m; (d) result = n * m--; 65 ©Silberschatz, Korth and Sudarshan Precedence Rules—Binary Operators An expression (without parenthesis) is evaluated according to the (basic) rules of precedence: Highest precedence – unary operators +, -, ++, -- and ! Intermediate precedence – binary arithmetic operators *, / and % Lowest precedence – binary arithmetic operators + and Binary operators in Java are “left-associative:” When binary operators have equal precedence the operator on the left has higher precedence than the operator(s) on the right. Unary operators in Java are “right-associative:” When unary operators have equal precedence the operator on the right has higher precedence if x contains 10 -++x is -11 and x is 11 afterwards; same as –(++x) • -x++ is -10 and x is 11 afterwards; same as –(x++) • 66 ©Silberschatz, Korth and Sudarshan Use Parentheses Parentheses can be used to over-ride the rules of precedence: (cost + tax) * discount Parenthesis can also be used to clarify code, even when precedence is not over-ridden: balance + (interestRate * balance) Spaces also clarify code: balance + interestRate*balance but spaces do not dictate precedence. 67 ©Silberschatz, Korth and Sudarshan Sample Expressions 68 ©Silberschatz, Korth and Sudarshan The Class String As we already have seen, a String is a sequence of characters. We’ve used constants, or rather, literals of type String: “Enter a whole number from 1 to 99.” “Number of quarters:” “I will output a combination of coins” 69 ©Silberschatz, Korth and Sudarshan Declaring and Printing Strings Variables of type String can be declared and Initialized: String greeting; greeting = “Hello!”; Equivalent to the above: String greeting = “Hello!”; String greeting = new String(“Hello!”); Printing: System.out.println(greeting); 70 ©Silberschatz, Korth and Sudarshan Inputting Strings Variables of type String can be input: String Word; System.out.print(“Enter a word:”); Word = keyboard.next(); System.out.print(“The word entered was: “ + Word); 71 ©Silberschatz, Korth and Sudarshan Concatenation of Strings Two strings can be concatenated using the + operator: String greeting = “Hello”; String name = “Smith”; String sentence; sentence = greeting + “ officer ”; sentence = sentence + name; System.out.println(sentence); Any number of strings can be concatenated using the + operator. 72 ©Silberschatz, Korth and Sudarshan Concatenating Strings and Integers Strings can be concatenated with other types: String solution; solution = “The temperature is “ + 72; System.out.println (solution); Output: The temperature is 72 73 ©Silberschatz, Korth and Sudarshan Classes Recall that Java has primitive types and class types: primitive types class types Primitive types have: simple, “atomic,” non-decomposable values Operations (built-in) Class types have: complex values, with structure Methods (some built-in, others user-defined) 74 ©Silberschatz, Korth and Sudarshan Objects, Methods, and Data A primitive type is use to create variables. A class type is used to produce “objects.” An object is an entity that: stores data can take actions defined by methods basically a complex variable. The data and methods applicable to an object are defined by its class. 75 ©Silberschatz, Korth and Sudarshan The Class String String is a class type The length() method returns an int, which is the number of characters in a particular String object. String solution = “dog”; int howMany = solution.length(); You can use a call to method length() anywhere an int can be used. int X = 10; int count = solution.length(); System.out.println(solution.length()); X = X * solution.length() + 3; 76 ©Silberschatz, Korth and Sudarshan Positions in a String Each character in a String has its own position. Positions are numbered starting at 0. ‘J’ in “Java is fun.” is in position 0 ‘f’ in “Java is fun.” is in position 8 The position of a character is also referred to as its index. 77 ©Silberschatz, Korth and Sudarshan Positions in a String, cont. 78 ©Silberschatz, Korth and Sudarshan Indexing Characters within a String using Methods charAt(position) returns the char at the specified position Example: String greeting = "Hi, there!"; char ch1 = greeting.charAt(0); // Stores ‘H’ in ch1 char ch2 = greeting.charAt(2); // Stores ‘,’ in ch2 substring(start, end) returns the string from start up to, but not including, end String myWord; myWord = greeting.substring(4,7) // Stores the in myWord 79 ©Silberschatz, Korth and Sudarshan Example of Class String 80 ©Silberschatz, Korth and Sudarshan String Methods See the Java on-line documentation for more details! 81 ©Silberschatz, Korth and Sudarshan Escape Characters How would you print the following? “Java” refers to a language. The following don’t work: System.out.println(“Java refers to a language.”); System.out.println(““Java” refers to a language.”); The compiler needs to be told that the quotation marks (“) do not signal the start or end of a string, but instead are to be printed. System.out.println(“\”Java\” refers to a language.”); 82 ©Silberschatz, Korth and Sudarshan Escape Characters “Escape sequences” are used to print “problematic” characters. Each escape sequence is a single character even though it is written with two symbols. 83 ©Silberschatz, Korth and Sudarshan Examples Examples: System.out.println(“abc\\def”); => abc\def System.out.println(“new\nline”); => new line char singleQuote = ‘\’’; System.out.println(singleQuote); => 84 ‘ ©Silberschatz, Korth and Sudarshan Number Constants Decimal 0000 0001 0002 0003 0004 0005 0006 0007 0008 0009 0010 0011 0012 0013 0014 0015 0016 0017 0018 0019 0020 0021 0022 0023 0024 0025 0026 0027 0028 0029 0030 0031 0032 Binary 000000 000001 000010 000011 000100 000101 000110 000111 001000 001001 001010 001011 001100 001101 001110 001111 010000 010001 010010 010011 010100 010101 010110 010111 011000 011001 011010 011011 011100 011101 011110 011111 100000 Octal 0000 0001 0002 0003 0004 0005 0006 0007 0010 0011 0012 0013 0014 0015 0016 0017 0020 0021 0022 0023 0024 0025 0026 0027 0030 0031 0032 0033 0034 0035 0036 0037 0040 Hexidecimal 0000 0001 0002 0003 0004 0005 0006 0007 0008 0009 000A 000B 000C 000D 000E 000F 0010 0011 0012 0013 0014 0015 0016 0017 0018 0019 001A 001B 001C 001D 001E 001F 0020 85 ©Silberschatz, Korth and Sudarshan