©silberschatz, korth and sudarshan1 primitive types, strings, and console i/o n variables and...

©Silberschatz, Korth and Sudarshan1

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

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.

public class EggBasket

public static void main (String[] args)

int baskets, eggsPer, totalEggs;

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);

Output:

If you have

6 eggs per basket and

10 baskets, then

the total number of eggs is 60

public class EggBasket

public static void main (String[] args)

int baskets, eggsPer, totalEggs;

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);

Output:

If you have

6 eggs per basket and

10 baskets, then

the total number of eggs is 60

Variables:basketseggsPertotalEggs

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.

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 */

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

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)

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.

Java Identifiers

Legal Examples:

count name address

count1 count2 count3

myFavoriteHobby My_Favorite_Hobby

Illegal Examples:

1count _name

7-11 netscape.com util.*

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.

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.

Primitive Types

Four integer types: byte

int (most common)

Two floating-point types: float

double (most common)

One character type: char

One boolean type: boolean

Primitive Types, cont.

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

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.

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.

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

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?

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;

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.

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.

Simple Input, cont.

class EggBasket2

Command-Line ArgumentsCommand-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 AliceHi, Alice. How are you?% java UseArgument BobHi, Bob. How are you?

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 cowThe first word is dog, the second is cat, and the third is cow

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 991234 + 99 = 13331234 * 99 = 1221661234 / 99 = 121234 % 99 = 46

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!

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

010 (decimal 8), 023 (decimal 19), 037 (decimal 31)

What is the format? 0

0xDadaCafe

0x00FF00FF

0x5FGC3

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;

x = 0x10;

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

Integer Literal Type

See the program: http://www.cs.fit.edu/~pbernhar/teaching/cse1001/literals

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!

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…

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

8.65e8f

+35.4f

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 = y; // illegal x = z; // illegaly = z; // illegalz = 3.6; // illegal (3.6 is of type double)y = 25; // legal, but…why?

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;

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.

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;

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;

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

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;

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.

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!!!!

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

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.

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!

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, …;

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)

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.

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.

Operators with integerand floating point numbers

See the program: http://www.cs.fit.edu/~pbernhar/teaching/cse1001/expressions

http://www.cs.fit.edu/~pbernhar/teaching/cse1001/integralConversion

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!

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.

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.8787 cents in coins:

3 quarters1 dime0 nickels2 pennies

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;

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.

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.

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.

Example, cont.

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.

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

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?

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

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--;

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++)

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.

Sample Expressions

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”

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);

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);

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.

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

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)

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.

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;

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.

Positions in a String, cont.

Indexing Characters within a Stringusing 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

Example of Class String

String Methods

See the Java on-line documentation for more details!

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.”);

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.

Examples

Examples:

System.out.println(“abc\\def”); => abc\def

System.out.println(“new\nline”); => new

char singleQuote = ‘\’’;

System.out.println(singleQuote); => ‘

Number Constants

Decimal Binary Octal Hexidecimal0000 000000 0000 00000001 000001 0001 00010002 000010 0002 00020003 000011 0003 00030004 000100 0004 00040005 000101 0005 00050006 000110 0006 00060007 000111 0007 00070008 001000 0010 00080009 001001 0011 00090010 001010 0012 000A0011 001011 0013 000B0012 001100 0014 000C0013 001101 0015 000D0014 001110 0016 000E0015 001111 0017 000F0016 010000 0020 00100017 010001 0021 00110018 010010 0022 00120019 010011 0023 00130020 010100 0024 00140021 010101 0025 00150022 010110 0026 00160023 010111 0027 00170024 011000 0030 00180025 011001 0031 00190026 011010 0032 001A0027 011011 0033 001B0028 011100 0034 001C0029 011101 0035 001D0030 011110 0036 001E0031 011111 0037 001F0032 100000 0040 0020

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dbms korth