Primitive Types Strings and Console IO n Variables

Primitive Types, Strings, and Console I/O n Variables and Expressions n The Class String n Keyboard and Screen I/O n Documentation and Style Reading => Section 1. 2 1. 1 ©Silberschatz, Korth and Sudarshan

Variables and Values n Variables are memory locations that store data such as numbers and letters. n The data stored by a variable is called its value. Ø The value is stored in the memory location. n A variables value can be changed. 1. 2 ©Silberschatz, Korth and Sudarshan

Variables and Values public class Egg. Basket Output: { If you have public static void main (String[] args) 6 eggs per basket and { int number. Of. Baskets, eggs. Per. Basket, total. Eggs; 10 baskets, then the total number of eggs is 60 number. Of. Baskets = 10; eggs. Per. Basket = 6; total. Eggs = number. Of. Baskets * eggs. Per. Basket; System. out. println ("If you have"); System. out. println (eggs. Per. Basket + " eggs per basket and"); System. out. println (number. Of. Baskets + " baskets, then"); System. out. println ("the total number of eggs is " + total. Eggs); } } 1. 3 ©Silberschatz, Korth and Sudarshan

Variables and Values n Variables: number. Of. Baskets eggs. Per. Basket total. Eggs n Assigning values: number. Of. Baskets = 10; eggs. Per. Basket = 6; total. Eggs = number. Of. Baskets * eggs. Per. Basket; n A variable must be declared before it is used. n When you declare a variable, you provide its name and type. int number. Of. Baskets, eggs. Per. Basket, total. Eggs; n A variable’s type determines what kinds of values it can hold, e. g. , integers, real numbers, characters. 1. 4 ©Silberschatz, Korth and Sudarshan

Syntax and Examples n Variable declaration syntax: type variable_1, variable_2, …; n Examples: int style. Choice, number. Of. Checks; double balance, interest. Rate; char joint. Or. Individual; n 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 */ : } 1. 5 ©Silberschatz, Korth and Sudarshan

Types in Java n In most programming languages, a type implies several things: Ø A set of values Ø A (hardware) representation for those values Ø A set of operations on those values n Example - type int: Ø Values – integers in the range -2147483648 to 2147483647 Ø Representation – 4 bytes, binary, “two’s complement” Ø Operations – addition (+), subtraction (-), multiplication (*), division (/), etc. n Two kinds of types: Ø Primitive types Ø Class types 1. 6 ©Silberschatz, Korth and Sudarshan

Types in Java n A primitive type: Ø values are “simple, ” non-decomposable values such as an individual number or individual character Ø int, double, char n A class type: Ø Ø values are “complex” objects a class of objects has both data and methods “Think WHIRLED peas. ” is a value of class type String ‘November 10, 1989’ is a value of class type date (non-Java) 1. 7 ©Silberschatz, Korth and Sudarshan

Java Identifiers n An identifier: Ø a name given to something in a program Ø name of a variable, name of a method, name of a class, etc. Ø created by the programmer, generally n Identifiers may contain only: Ø Ø Ø 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. n Identifiers may not contain any spaces, dots (. ), asterisks (*), or other characters not mentioned above: 7 -11 netscape. com util. * 1. 8 (not allowed) ©Silberschatz, Korth and Sudarshan

Java Identifiers, cont. n Java is case sensitive i. e. , stuff, Stuff, and STUFF are different identifiers. n 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 finally float for goto if implements import instanceof interface long native new null package private protected public return short static strictfp super switch synchronized this throws transient true try void volatile while n Keywords cannot be used as identifiers n Identifiers can be arbitrarily long. 1. 9 ©Silberschatz, Korth and Sudarshan

Naming Conventions n Choose names that are helpful, or rather readable, such as count or speed, but not c or s. n Class types: Ø begin with an uppercase letter (e. g. String). n Primitive types: Ø begin with a lowercase letter (e. g. int). n Variables of both class and primitive types: Ø begin with a lowercase letters (e. g. my. Name, my. Balance). Ø multiword names are “punctuated” using uppercase letters. 1. 10 ©Silberschatz, Korth and Sudarshan

Primitive Types n Four integer types: Ø Ø byte short int (most common) long n Two floating-point types: Ø float Ø double (most common) n One character type: Ø char n One boolean type: Ø boolean 1. 11 ©Silberschatz, Korth and Sudarshan

Primitive Types, cont. 1. 12 ©Silberschatz, Korth and Sudarshan

Examples of Primitive Values n Integer values: 0 -1 365 12000 n Floating-point values: 0. 99 -22. 8 3. 14159 5. 0 n Character values: `a` `A` `#` ` ` n Boolean values: true false 1. 13 ©Silberschatz, Korth and Sudarshan

Motivation for Primitive Types n Why are there several different integer types? Ø storage space Ø operator efficiency n More generally, why are there different types at all? Ø reflects how people understand different kinds of data, e. g. , letter vs. numeric grades. Ø helps prevent programmer errors. 1. 14 ©Silberschatz, Korth and Sudarshan

Assignment Statements n An assignment statement is used to assign a value to a variable: int answer; answer = 42; n The “equal sign” is called the assignment operator. n In the above example, the variable named answer is assigned a value of 42, or more simply, answer is assigned 42. 1. 15 ©Silberschatz, Korth and Sudarshan

Assignment Statements, cont. n Assignment syntax: variable = expression ; where expression can be Ø a literal or constant (such as a number), Ø another variable, or Ø an expression which combines variables and literals using operators 1. 16 ©Silberschatz, Korth and Sudarshan

Assignment Examples n Examples: int amount; int score, number. Of. Cards, handicap; int eggs. Per. Basket; char first. Initial; : amount = 3; first. Initial = ‘W’; score = number. Of. Cards + handicap; eggs. Per. Basket = eggs. Per. Basket - 2; => last line looks weird in mathematics, why? 1. 17 ©Silberschatz, Korth and Sudarshan

Assignment Evaluation n The expression on the right-hand side of the assignment operator (=) is evaluated first. n The result is used to set the value of the variable on the left-hand side of the assignment operator. score = number. Of. Cards + handicap; eggs. Per. Basket = eggs. Per. Basket - 2; 1. 18 ©Silberschatz, Korth and Sudarshan

Simple Screen Output n The following outputs the String literal “The count is “ followed by the current value of the variable count. System. out. println(“The count is “ + count); n “+” means concatenation if at least one argument is a string 1. 19 ©Silberschatz, Korth and Sudarshan

Simple Input n Sometimes data is needed and obtained from the user at run time. n Simple keyboard input requires: import java. util. *; or import java. util. Scanner; at the beginning of the file. 1. 20 ©Silberschatz, Korth and Sudarshan

Simple Input, cont. n A “Scanner” object must be initialized before inputting data: Scanner keyboard = new Scanner(System. in); n To input data: eggs. Per. Basket = keyboard. next. Int(); which reads one int value from the keyboard and assigns it to the variable eggs. Per. Basket. 1. 21 ©Silberschatz, Korth and Sudarshan

Simple Input, cont. n class Egg. Basket 2 1. 22 ©Silberschatz, Korth and Sudarshan

Command-Line Arguments n Frequently input is provided to a program at the command-line. public class Use. Argument { public static void main(String[] args) { System. out. print(“Hi, ”); System. out. print(args[0]); System. out. println(“. How are you? ”); } } n Sample interaction: % javac Use. Argument. java % java Use. Argument Alice Hi, Alice. How are you? % java Use. Argument Bob Hi, Bob. How are you? 1. 23 ©Silberschatz, Korth and Sudarshan

Command-Line Arguments n Frequently multiple values are provided at the command-line. public class Use 3 Arguments { 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]); } } n Sample interaction: % javac Use 3 Arguments. java % java Use 3 Arguments dog cat cow The first word is dog, the second is cat, and the third is cow 1. 24 ©Silberschatz, Korth and Sudarshan

Command-Line Arguments n Command-line arguments can be numeric. public class Int. Ops { public static void main(String[] args) { int a = Integer. parse. Int(args[0]); // Notice the variable declaration int b = Integer. parse. Int(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); } } n Sample interaction: % javac Int. Ops. java % java Int. Ops 1234 99 1234 + 99 = 1333 1234 * 99 = 122166 1234 / 99 = 12 1234 % 99 = 46 1. 25 ©Silberschatz, Korth and Sudarshan

Literals n Values such as 2, 3. 7, or ’y’ are called constants or literals. n Integer literals can be preceded by a + or - sign, but cannot contain commas. n There are two distinct properties that every integer literal has: Ø format – either decimal, hexadecimal or octal, and Ø type – either long or int n Both the format and the type of an integer literal can be determined by…looking at it! 1. 26 ©Silberschatz, Korth and Sudarshan

Integer Literal Formats n Decimal: Ø 0, 10, 37, 643, 829, etc. Ø first digit must NOT be zero, for any non-zero integer n Hexadecimal: Ø consists of the leading characters 0 x or 0 X followed by one or more hexadecimal digits, i. e. , 0 through F (lower and upper case equivalent) Ø each letter may be either upper case or lower case Ø 0 x. A (decimal 10), 0 x 0 (decimal 0), 0 x. FF (decimal 255), 0 xf 0 (decimal 240), 0 x 34 A 2, 0 Xff decimal (255), etc. n Octal: Ø Consists of the leading digit 0 followed by one or more octal digits, i. e. , 0 through 7 Ø 010 (decimal 8), 023 (decimal 19), 037 (decimal 31) 1. 27 ©Silberschatz, Korth and Sudarshan

Integer Literal Formats n What is the format? Ø Ø Ø Ø Ø 0 2 0372 0 x. Dada. Cafe 1996 0 x 00 FF 0 A 3 C 0945 450 x. A 1. 28 ©Silberschatz, Korth and Sudarshan

Integer Literal Formats n The format used to specify an integer literal has no impact on its corresponding internal value. n 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 = 0 x 10; System. out. println("Value: " + x); 1. 29 ©Silberschatz, Korth and Sudarshan

Integer Literal Type n An integer literal is of type long if it is suffixed with an letter L or l; otherwise it is of type int. Ø note that capital L is preferred n Integer literals of type long: Ø Ø Ø Ø Ø 2 L 0372 L 0 x. Dada. Cafe. L 1996 L 0 x 00 FFL 0 l 0777 L 0 x 10000 l 2147483648 L 0 x. C 0 B 0 L 1. 30 ©Silberschatz, Korth and Sudarshan

Integer Literal Type n See the program: Ø http: //www. cs. fit. edu/~pbernhar/teaching/cse 1001/literals 1. 31 ©Silberschatz, Korth and Sudarshan

Floating Point Literals n Just like with integer literals… n Floating point literals can also be preceded by a + or - sign, but cannot contain commas. n There are two distinct properties that every floating point literal has: Ø format – either fully expanded notation or scientific notation Ø type – either float or double n Both the format and the type of a floating point literal can be determined by…looking at it! 1. 32 ©Silberschatz, Korth and Sudarshan

Floating Point Literal Formats n Floating-point constants can be written: Ø with digits after a decimal point, as in 3. 5 or Ø using scientific notation n Examples: Ø 865000000. 0 can be written as 8. 65 e 8 Ø 0. 000483 can be written as 4. 83 e-4 n The number in front of the “e” does not need to contain a decimal point, e. g. 4 e-4 1. 33 ©Silberschatz, Korth and Sudarshan

Floating Point Literal Type n An floating point literal is of type float if it is suffixed with an letter F or f; otherwise it is of type double. n Floating point literals of type float: Ø Ø 2. 5 F 0. 0 f 8. 65 e 8 f 4 e-4 F Ø 3 f Ø +35. 4 f Ø -16 F 1. 34 ©Silberschatz, Korth and Sudarshan

Assignment Compatibilities n 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. 0 f; x x y z y = = = y; z; z; 3. 6; 25; // // // illegal (3. 6 is of type double) legal, but interesting… 1. 35 ©Silberschatz, Korth and Sudarshan

Assignment Compatibilities n Sometimes automatic conversions between types do take place: short s; int x; s = 83; x = s; double. Variable; int. Variable = 7; double. Variable = int. Variable; 1. 36 ©Silberschatz, Korth and Sudarshan

Assignment Compatibilities, cont. n A value 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. n Makes sense intuitively because, for example, any legal byte value is a legal short value. n On the other hand, many legal short values are not legal byte values. 1. 37 ©Silberschatz, Korth and Sudarshan

Assignment Compatibilities, cont. n Example – all of the following are legal, and will compile: byte b; short s; int i; long l; float f; double d; b = 10; s = b; i = b; l = i; f = l; // This one is interesting, why? d = f; 1. 38 ©Silberschatz, Korth and Sudarshan

Assignment Compatibilities, cont. n 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; 1. 39 ©Silberschatz, Korth and Sudarshan

Type Casting n A type cast creates a value in a new type from an original type. n A type cast can be used to force an assignment when otherwise it would be illegal (thereby over-riding the compiler, in a sense). n Example: double distance; distance = 9. 0; int points; points = (int)distance; n The above would be illegal without (int). 1. 40 ©Silberschatz, Korth and Sudarshan

Type Casting, cont. n The value of (int)distance is 9, but the value of distance, both before and after the cast, is 9. 0. n The type of distance does NOT change and remains double. n What happens if distance contains 9. 7? Ø Any nonzero value to the right of the decimal point is truncated (as oppossed to rounded). n Remember the rule – “cast with care, ” because the results can be unpredictable. 1. 41 ©Silberschatz, Korth and Sudarshan

Characters as Integers n Like everything else, each character is represented by a binary sequence. n The binary sequence corresponding to a character is a positive integer. n Which integer corresponds to each character is dictated by a standardized character encoding. Ø each character is assigned a unique integer code Ø the codes are different for upper and lower case letters, e. g. , 97 may be the integer value for ‘a’ and 65 for ‘A’ Ø some characters are printable, others are not n Why should different computers use the same code? n ASCII and Unicode are common character codes 1. 42 ©Silberschatz, Korth and Sudarshan

Unicode Character Set n Most programming languages use the ASCII character encoding. Ø American Standard Code for Information Interchange (ASCII) Ø (only) encodes characters from the north American keyboard Ø uses one byte of storage n Java uses the Unicode character encoding which includes ASCII. n The Unicode character set: Ø uses two bytes of storage Ø includes characters from many different alphabets other than English (in contrast to ASCII) Ø codes characters from the North American keyboard the same way that ASCII does 1. 43 ©Silberschatz, Korth and Sudarshan

ASCII/Unicode 32 48 0 … 33 34 35 36 37 38 39 40 41 ! “ # $ % & ‘ ( ) 57 9 … 65 … A 90 Z 1. 44 … 97 a … 122 z ©Silberschatz, Korth and Sudarshan

Assigning a char to an int n A value of type char can be assigned to a variable of type int to obtain its Unicode value. n Example: char answer = ’y’; System. out. println(answer); System. out. println((int)answer); >y >121 n See the program at: http: //www. cs. fit. edu/~pbernhar/teaching/cse 1001/char. Test 1. 45 ©Silberschatz, Korth and Sudarshan

Initializing Variables n 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; n Some languages automatically initialize a variable when it’s declared, other languages don’t. n Others (Java) initialize in some circumstances, but not in others. Ø Variables declared to be of a primitive type are NOT automatically initialized. Ø In other cases Java will initialize variables; this will be discussed later. 1. 46 ©Silberschatz, Korth and Sudarshan

Initializing Variables n Some languages report an error when a variable is used prior to initialization by the program. Ø Sometimes at compile time, other times at run-time. n Some languages won’t report an error, and will even let you use an uninitialized variable. Ø In such cases the initial value of the variable is arbitrary! Ø The program might appear to run correctly sometimes, but give errors on others. n Java: Ø The compiler will (try) to catch uninitialized variables. => Always make sure your variables are initialized prior to use! 1. 47 ©Silberschatz, Korth and Sudarshan

Initializing Variables, cont. n In Java, a variable can be assigned an initial value in it’s declaration. n Examples: int count = 0; char grade = ’A’; // default is an A n Syntax: type variable 1 = expression 1, variable 2 = expression 2, …; 1. 48 ©Silberschatz, Korth and Sudarshan

Arithmetic Operations n Arithmetic expressions: Ø are formed using the +, -, *, / and % operators Ø operators have operands, which are literals, variables or sub-expressions. n Expressions with two or more operators can be viewed as a series of steps, each involving only two operands. Ø The result of one step produces an operand which is used in the next step. Ø Note that Java is left-associative. n Example: int x = 0, y = 50, z = 20; double balance = 50. 25, rate = 0. 05; x = x + y + z; balance = balance + (balance * rate) 1. 49 ©Silberschatz, Korth and Sudarshan

Expression Type n An arithmetic expression can have operands of different numeric types. Ø x + (y * z) / w Ø Note that this does not contradict our rules for assignment. n Every arithmetic expression has a (resulting) type. n Given an arithmetic expression: Ø If any operand in the expression is of type double, then the expression has type double. Ø Otherwise, if any operand in the expression is of type float, then the expression has type float. Ø Otherwise, if any operand in the expression is of type long, then the expression has type long. Ø Otherwise the expression has type int. 1. 50 ©Silberschatz, Korth and Sudarshan

Expression Type, cont. n Example: int hours. Worked double pay. Rate = 40; = 8. 25; double total. Pay; Then the expression in the assignment: total. Pay = hours. Worked * pay. Rate is a double with a value of 500. 0. 1. 51 ©Silberschatz, Korth and Sudarshan

Operators with integer and floating point numbers n See the program: Ø http: //www. cs. fit. edu/~pbernhar/teaching/cse 1001/expressions Ø http: //www. cs. fit. edu/~pbernhar/teaching/cse 1001/integral. Conversion 1. 52 ©Silberschatz, Korth and Sudarshan

The Division Operator n The division operator (/) behaves as expected. n If one of the operands is a floating-point type then the result is of the same floating point type. Ø 9. 0 / 2 = 4. 5 Ø 9 / 2. 0 = 4. 5 n If both operands are integer types then the result is truncated, not rounded. Ø 9/2 = 4 Ø 99 / 100 = 0 1. 53 ©Silberschatz, Korth and Sudarshan

The mod Operator n The mod (%) operator is used with operands of integer type to obtain the remainder after integer division. n 14 divided by 4 is 3 with a remainder of 2. Ø Hence, 14 % 4 is equal to 2. n The mod operator has many uses, including determining: Ø If an integer is odd or even (x % 2 = 0) Ø If one integer is evenly divisible by another integer (a % b = 0) 1. 54 ©Silberschatz, Korth and Sudarshan

Case Study: Vending Machine Change n Program Requirements: Ø The user enters an amount between 1 cent and 99 cents. Ø The program determines a combination of coins equal to that amount. Ø For example, 55 cents can be two quarters and one nickel. => What are “requirements” anyway? n 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 1. 55 ©Silberschatz, Korth and Sudarshan

Case Study, cont. n 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. => What is an “algorithm” anyway? => What is “pseudo-code” also? n Program Variables Needed: int amount, quarters, dimes, nickels, pennies; 1. 56 ©Silberschatz, Korth and Sudarshan

Case Study, cont. n Interpreted literally, the algorithm doesn’t work properly, because the original amount is changed by the intermediate steps. Ø The original value of amount is lost. n Add an additional variable: int amount, original. Amount, quarters, dimes, nickles, pennies; and update the algorithm. 1. 57 ©Silberschatz, Korth and Sudarshan

Case Study, cont. n Algorithm (version #2): 1. 2. 3. 4. 5. 6. n Read the amount. Make a copy of the amount. Find the maximum number of quarters in the amount. Subtract the value of the quarters from the amount. Repeat the last two steps for dimes, nickels, and pennies. Print the original amount and the quantities of each coin. Typically pseudo-code is iteratively embellished or enhanced. 1. 58 ©Silberschatz, Korth and Sudarshan

Case Study, cont. n How do we determine the number of quarters in an amount? Ø There are 2 quarters in 55 cents, but there also 2 quarters in 65 cents. Ø That’s because 55 / 25 = 2 and 65 / 25 = 2. n How do we determine the remaining amount? Ø Using the mod operator: 55 % 25 = 5 and 65 % 25 = 15 n Similarly for dimes, nickels and pennies Ø Pennies are simply amount % 5. 1. 59 ©Silberschatz, Korth and Sudarshan

Case Study, cont. 1. 60 ©Silberschatz, Korth and Sudarshan

Case Study—testing the implementation n The program should be tested with several different amounts. n Test with values that give zero values for each possible coin denomination. n Test with amounts close to: Ø “Extreme” or “boundary condition” values such as 0, 1, 98 and 99. Ø Coin denominations such as 24, 25, and 26. 1. 61 ©Silberschatz, Korth and Sudarshan

Increment (and Decrement) Operators n Used to increase (or decrease) the value of a variable by 1. Ø count = count +1 Ø count = count -1 n Easy to use, important to recognize. n The increment operator: Ø count++ Ø ++count n The decrement operator: Ø count-Ø --count 1. 62 ©Silberschatz, Korth and Sudarshan

Increment (and Decrement) Operators n “Mostly” equivalent operations: count++; ++count; count = count + 1; count--; --count; count = count - 1; 1. 63 ©Silberschatz, Korth and Sudarshan

Increment (and Decrement) Operators in Expressions n After executing: int m = 4; int result = 3 * (++m); result has a value of 15 and m has a value of 5 n After executing: int m = 4; int result = 3 * (m++); result has a value of 12 and m has a value of 5 1. 64 ©Silberschatz, Korth and Sudarshan

Increment and Decrement Operator, Cont. n Common code: int n = 3; int m = 4; int result; n 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--; 1. 65 ©Silberschatz, Korth and Sudarshan

Precedence Rules—Binary Operators n An expression (without parenthesis) is evaluated according to the (basic) rules of precedence: Ø Highest precedence – unary operators +, -, ++, -- and ! Ø Intermediate precedence – binary arithmetic operators *, / and % Ø Lowest precedence – binary arithmetic operators + and n Binary operators in Java are “left-associative: ” Ø When binary operators have equal precedence the operator on the left has higher precedence than the operator(s) on the right. n Unary operators in Java are “right-associative: ” Ø When unary operators have equal precedence the operator on the right has higher precedence Ø 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++) 1. 66 ©Silberschatz, Korth and Sudarshan

Use Parentheses n Parentheses can be used to over-ride the rules of precedence: (cost + tax) * discount n Parenthesis can also be used to clarify code, even when precedence is not over-ridden: balance + (interest. Rate * balance) n Spaces also clarify code: balance + interest. Rate*balance but spaces do not dictate precedence. 1. 67 ©Silberschatz, Korth and Sudarshan

Sample Expressions 1. 68 ©Silberschatz, Korth and Sudarshan

The Class String n As we already have seen, a String is a sequence of characters. n 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” 1. 69 ©Silberschatz, Korth and Sudarshan

Declaring and Printing Strings n Variables of type String can be declared and Initialized: String greeting; greeting = “Hello!”; n Equivalent to the above: String greeting = “Hello!”; String greeting = new String(“Hello!”); n Printing: System. out. println(greeting); 1. 70 ©Silberschatz, Korth and Sudarshan

Concatenation of Strings n 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); n Any number of strings can be concatenated using the + operator. 1. 71 ©Silberschatz, Korth and Sudarshan

Concatenating Strings and Integers n Strings can be concatenated with other types: String solution; solution = “The temperature is “ + 72; System. out. println (solution); n Output: The temperature is 72 1. 72 ©Silberschatz, Korth and Sudarshan

Positions in a String n Each character in a String has its own position. n Positions are numbered starting at 0. Ø ‘J’ in “Java is fun. ” is in position 0 Ø ‘f’ in “Java is fun. ” is in position 8 n The position of a character is also referred to as its index. 1. 73 ©Silberschatz, Korth and Sudarshan

Positions in a String, cont. 1. 74 ©Silberschatz, Korth and Sudarshan

Classes n Recall that Java has primitive types and class types: Ø primitive types Ø class types n Primitive types have: Ø simple, “atomic, ” non-decomposable values Ø Operations (built-in) n Class types have: Ø complex values, with structure Ø Methods (some built-in, others user-defined) 1. 75 ©Silberschatz, Korth and Sudarshan

Objects, Methods, and Data n A primitive type is use to create variables. n A class type is used to produce “objects. ” n An object is an entity that: Ø stores data Ø can take actions defined by methods Ø basically a complex variable. n The data and methods applicable to an object are defined by its class. 1. 76 ©Silberschatz, Korth and Sudarshan

The Class String n String is a class type n The length() method returns an int, which is the number of characters in a particular String object. int how. Many = solution. length(); n 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; 1. 77 ©Silberschatz, Korth and Sudarshan

Indexing Characters within a String using Methods n char. At(position) Ø returns the char at the specified position n Example: String greeting = "Hi, there!"; char ch 1 = greeting. char. At(0); char ch 2 = greeting. char. At(2); // Stores ‘H’ in ch 1 // Stores ‘, ’ in ch 2 n substring(start, end) Ø returns the string from start up to, but not including, end String my. Word; my. Word = greeting. substring(4, 7) // Stores the in my. Word 1. 78 ©Silberschatz, Korth and Sudarshan

Example of Class String 1. 79 ©Silberschatz, Korth and Sudarshan

String Methods n See the Java on-line documentation for more details! 1. 80 ©Silberschatz, Korth and Sudarshan

Escape Characters n How would you print the following? “Java” refers to a language. n The following don’t work: System. out. println(“Java refers to a language. ”); System. out. println(““Java” refers to a language. ”); n 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. ”); 1. 81 ©Silberschatz, Korth and Sudarshan

Escape Characters n “Escape sequences” are used to print problematic characters. n Each escape sequence is a single character even though it is written with two symbols. 1. 82 ©Silberschatz, Korth and Sudarshan

Examples n Examples: System. out. println(“abc\def”); => System. out. println(“newnline”); abcdef => new line char single. Quote = ‘’’; System. out. println(single. Quote); => 1. 83 ‘ ©Silberschatz, Korth and Sudarshan

Keyboard and Screen I/O: Outline n Screen Output n Keyboard Input 1. 84 ©Silberschatz, Korth and Sudarshan

Screen Output n We’ve seen several examples of screen output already. n System. out is an object that is part of Java. n println() is one of the methods available to the System. out object. 1. 85 ©Silberschatz, Korth and Sudarshan

Screen Output, cont. n The concatenation operator (+) is useful when everything does not fit on one line. : System. out. println(“When everything “ + “does not fit on one line, use the” + “ concatenation operator (’+’)”); => Do not break the line except immediately before or after the concatenation operator (+). 1. 86 ©Silberschatz, Korth and Sudarshan

Screen Output, cont. n Alternatively, use print() System. out. print(“When everything “); System. out. print(“does not fit on “); System. out. print(“one line, use the “); System. out. print(“”print” ”); System. out. println(“statement”); ending with a println(). 1. 87 ©Silberschatz, Korth and Sudarshan

Screen Output, cont. n Syntax: System. out. println(output_1 + output_2 +. . . + output_n); n Example: System. out. println (1967 + “ “ + “Oldsmobile” + “ “ + 442); 1967 Oldsmobile 442 1. 88 ©Silberschatz, Korth and Sudarshan

printf (or format) Method for Output Formatting n Heavily influenced by C n output. Stream. printf(format. String, args…) Ø System. out. printf(…) Ø smiley. Out. Stream. printf(…) n format. String specifies how to format args n System. out. printf(“%s %d %f%n”, name, id, gpa); Ø System. out. println(name + “ “ + id + “ “ + gpa); n Useful for “right justified” numbers n Numbers in println and print are “left justified” 1. 89 ©Silberschatz, Korth and Sudarshan

Formatting String n % width conversion n width specifies how many slots are available for output n If width > number of characters, spaces are printed first before the characters— “right justified” n printf(“%5 d”, count) Ø Count - 32901: - 2004: 3 2 9 0 1 - 22: - 6747280: all digits are is ignored 2 printed, 0 0 width 4 2 1. 90 2 ©Silberschatz, Korth and Sudarshan

Conversion Characters Conversion Argument Description d integer Decimal integer f floating point Decimal float s general (String, Boolean, …) String n New line c character Character (unicode) e floating point Decimal scientific notation o integer Octal integer x integer Hexadecimal integer % % 1. 91 (%% to output %) ©Silberschatz, Korth and Sudarshan

Imprecision in Floating-Point Numbers n Floating-point numbers often are only approximations since they are stored with a finite number of bits. n Hence 1. 0/3. 0 is slightly less than 1/3. n 1. 0/3. 0 + 1. 0/3. 0 could be less than 1. n See the program: Ø http: //www. cs. fit. edu/~pbernhar/teaching/cse 1001/float. Test 1. 92 ©Silberschatz, Korth and Sudarshan

Floating-point Precision n width. precision conversion n printf(“%5. 2 f”, PI) n printf(“%7. 4 f”, PI) 3 3 . . 1 1 4 4 1. 93 1 5 ©Silberschatz, Korth and Sudarshan

Left Justified n Spaces are added (padded) on the right n Minus (-) sign before the width n …printf(“%-7 s %-4 d”, name, age) n Why are there 4 spaces after “John” instead of 3? J o h n 1. 94 2 0 ©Silberschatz, Korth and Sudarshan

Example n http: //www. cs. fit. edu/~pkc/classes/cse 1001/Printf. java 1. 95 ©Silberschatz, Korth and Sudarshan

Keyboard Input n Starting from Java 5. 0 Ø Java has reasonable facilities for handling keyboard input. n Scanner class in the java. util package Ø A package is a library of classes. 1. 96 ©Silberschatz, Korth and Sudarshan

Using the Scanner Class n Near the beginning of your program, insert import java. util. * n Create an object of the Scanner class Scanner keyboard = new Scanner(System. in) n Read data (an int or a double, for example) int n 1 = keyboard. next. Int(); double d 1 = keyboard. next. Double(); 1. 97 ©Silberschatz, Korth and Sudarshan

Some Scanner Class Methods n Syntax: Int_Variable = Object_Name. next. Int(); Double_Variable = Object_Name. next. Double(); Float_Variable = Object_Name. next. Float(); String_Variable = Object_Name. next. Line(); Boolean_Variable = Object_Name. next. Boolean(); next. Byte(), next. Short(), next. Long() 1. 98 ©Silberschatz, Korth and Sudarshan

Some Scanner Class Methods, cont. n Examples: int count = keyboard. next. Int(); double distance = keyboard. next. Double(); String word = keyboard. next(); String whole. Line = keyboard. next. Line(); n Remember to prompt the user for input: System. out. print(“Enter an integer: “); 1. 99 ©Silberschatz, Korth and Sudarshan

Keyboard Input Demonstration n class Scanner. Demo 1. 100 ©Silberschatz, Korth and Sudarshan

next. Line()Method Caution n The next. Line() method reads the remainder of the current line, even if it is empty. 1. 101 ©Silberschatz, Korth and Sudarshan

next. Line()Method Caution, cont. n Example: int n; String s 1, s 2; n = keyboard. next. Int(); s 1 = keyboard. next. Line(); s 2 = keyboard. next. Line(); 5440 or bust n is set to 5440 but s 1 is set to the empty string. 1. 102 ©Silberschatz, Korth and Sudarshan

The Empty String n String with zero characters String s 3 = “”; n Good for String initialization 1. 103 ©Silberschatz, Korth and Sudarshan

Other Input Delimiters, cont. n class Delimiters. Demo 1. 104 ©Silberschatz, Korth and Sudarshan

Documentation and Style: Outline n Meaningful Names n Self-Documentation and Comments n Indentation n Named Constants n www. cs. fit. edu/~pkc/classes/cse 1001/First. Program. One. Line. java n Grading Ø 10% on documentation and comments Ø 10% on style (variable naming, indentation) 1. 105 ©Silberschatz, Korth and Sudarshan

Documentation and Style n Most programs are modified over time to respond to new requirements. n Programs which are easy to read and understand are easy to modify. n Even if it will be used only once, you have to read it in order to debug it. 1. 106 ©Silberschatz, Korth and Sudarshan

Meaningful Names for Variables n A variable’s name should suggest its use. n Observe conventions in choosing names for variables. Ø Use only letters and digits. Ø Use more than one character. Ø “Punctuate” using uppercase letters at word boundaries (e. g. tax. Rate). Ø Start variables with lowercase letters. Ø Start class names with uppercase letters. 1. 107 ©Silberschatz, Korth and Sudarshan

Documentation and Comments n The best programs are self-documenting. Ø clean style Ø well-chosen names n Comments are written into a program as needed explain the program. Ø They are useful to the programmer, but they are ignored by the compiler. 1. 108 ©Silberschatz, Korth and Sudarshan

When to Use Comments n Begin each program file with an explanatory comment Ø Ø what the program does the name of the author contact information for the author date of the last modification. n Provide only those comments which the expected reader of the program file will need in order to understand it. 1. 109 ©Silberschatz, Korth and Sudarshan

Comments Example n class Circle. Calculation 1. 110 ©Silberschatz, Korth and Sudarshan

/** Name: <name> E-mail address: <email> Course: cse 1001 Section: <? > Assignment: <HW? > Date: <date> Template for CSE 1001 Description: <description> */ import java. util. *; public class <Same. Class. And. File. Names> { public static void main(String[] args) { // input from the keyboard Scanner keyboard = new Scanner(System. in); } } 1. 111 ©Silberschatz, Korth and Sudarshan

Comments n A program can usually be broken into segments/blocks based on the algorithm, e. g. in HW 2: Ø Prompt the user for input Ø Input from the keyboard Ø Calculation Ø Output to the screen n Blank line between two segments n A description (comment) before each segment 1. 112 ©Silberschatz, Korth and Sudarshan

Pseudocode and Comments n Solving a problem 1. 2. 3. n Tip: 1. 2. n Devise an algorithm (steps to solve the problem) Write the algorithm in pseudocode (semi English, semi Java) English part of pseudocode becomes comments in your program type the English part of pesudocode as comments into your program first write the detailed Java instructions to satisfy/implement the pesudocode Advantages: 1. 2. 3. Each line of pseudocode helps you focus on a small task Pseudocode tells you what steps you want to achieve No need to add comments later on 1. 113 ©Silberschatz, Korth and Sudarshan

Comments n A comment can begin with //. Ø Everything after these symbols and to the end of the line is treated as a comment and is ignored by the compiler. double radius; //in centimeters 1. 114 ©Silberschatz, Korth and Sudarshan

Comments, cont. n A comment can begin with /* and end with */ Ø Everything between these symbols is treated as a comment and is ignored by the compiler. /* the simplex method is used to calculate the answer*/ 1. 115 ©Silberschatz, Korth and Sudarshan

Comments, cont. n A javadoc comment, begins with /** and ends with */. Ø It can be extracted automatically from Java software. /** method change requires the number of coins to be nonnegative */ 1. 116 ©Silberschatz, Korth and Sudarshan

Indentation n Indentation should communicate nesting clearly. n A good choice is four spaces for each level of indentation. n Indentation should be consistent. n Indentation should be used for second and subsequent lines of statements which do not fit on a single line. 1. 117 ©Silberschatz, Korth and Sudarshan

Indentation, cont. n Indentation does not change the behavior of the program. n Improper indentation can miscommunicate the behavior of the program. 1. 118 ©Silberschatz, Korth and Sudarshan

Named Constants n To avoid confusion, always name constants (and variables). circumference = PI * radius; is clearer than circumference = 3. 14159 * 6. 023; n Place constants near the beginning of the program. 1. 119 ©Silberschatz, Korth and Sudarshan

Named Constants, cont. n The value of a constant cannot be changed once it is initialized public static final double INTEREST_RATE = 6. 65; n Consider the interest rate is used many times in the program: Ø What if you type 6. 65 in some places, but 6. 56 in others? Ø What if the interested rate has changed to 7. 3? Ø Is balance * INTEREST_RATE easier to read than balance * 6. 65 ? 1. 120 ©Silberschatz, Korth and Sudarshan

Declaring Constants n syntax public static final Type Name = Constant; n examples public static final double PI = 3. 14159; public static final String MOTTO = “The customer is always right. ”; Ø By convention, uppercase letters are used for constants. 1. 121 ©Silberschatz, Korth and Sudarshan

Named Constants n class Circle. Calculation 2 1. 122 ©Silberschatz, Korth and Sudarshan

Summary n You have become familiar with Java primitive types (numbers, n n characters, etc. ). You have learned about assignment statements and expressions. You have learned about stings. You have become familiar with the basics of classes, methods, and objects. You have learned about simple keyboard input and screen output. 1. 123 ©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 0000001 000010 000011 000100 000101 000110 000111 001000 001001 001010 001011 001100 001101 001110 001111 010000 010001 010010 010011 010100 01010110 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 000 A 000 B 000 C 000 D 000 E 000 F 0010 0011 0012 0013 0014 0015 0016 0017 0018 0019 001 A 001 B 001 C 001 D 001 E 001 F 0020 1. 124 ©Silberschatz, Korth and Sudarshan

Binary Representation n Assume an 8 -bit type n 5 as an integer Ø 00000101 n ‘ 5’ as a character Ø 00110101 (53 decimal, ASCII) n 5. 0 as a floating point number Ø How? Ø What about 5. 5? 1. 125 ©Silberschatz, Korth and Sudarshan

Binary Real Numbers … n 23 22 21 20 . 2 -1 … 5. 5 Ø 101. 1 n 5. 25 Ø 101. 01 n 5. 125 Ø 101. 001 n 5. 75 Ø 101. 11 1. 126 ©Silberschatz, Korth and Sudarshan

8 bits only 25 n 24 23 22 21 20 2 -1 2 -2 5. 5 Ø 101. 1 -> 000101 10 n 5. 25 Ø 101. 01 -> 000101 01 n 5. 125 Ø 101. 001 -> ? ? n With only 2 places after the point, the precision is. 25 n What if the point is allowed to move around? 1. 127 ©Silberschatz, Korth and Sudarshan

Floating-point Numbers n Decimal Ø 54. 3 Ø 5. 43 x 101 n [scientific notation] Binary Ø 101. 001 Ø 10. 1001 x 21 Ø 1. 01001 x 22 [more correctly: 10. 1001 x 101] [more correctly: 1. 01001 x 1010] Ø What can we say about the most significant bit? 1. 128 ©Silberschatz, Korth and Sudarshan

Floating-point Numbers n General form: sign 1. mantissa x 2 exponent Ø the most significant digit is right before the dot - Always 1 [no need to represent it] Ø (more details are not discussed here: mantissa has no sign, but sign is embedded in exponent…) n 1. 01001 x 22 Ø Sign: positive (0) Ø Mantissa: 01001 Ø Exponent: 10 (decimal 2) - [IEEE standard: “biased exponent” • exponent + 2 num. Bits-1 - 1 • Example: 2 + 22 -1 – 1 = 3 => 11 in binary] 1. 129 ©Silberschatz, Korth and Sudarshan

Java Floating-point Numbers sign n exponent mantissa Sign: Ø 1 bit [0 is positive] n Mantissa: Ø 23 bits in float Ø 52 bits in double n Exponent: Ø 8 bits in float Ø 11 bits in double 1. 130 ©Silberschatz, Korth and Sudarshan