Lecture3.ppt

Lecture 3Java Basics

Lecture3.ppt

Keywordsabstract continue for new switch

assert default goto package synchronized

boolean do if private this

break double implements protected throw

byte else import public throws

case enum instanceof return transient

catch extends int short try

char final interface static void

class finally long strictfp volatile

const float native super while

** http://java.sun.com/docs/books/tutorial/java/nutsandbolts/_keywords.html

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Identifiers

• Can’t be keywords• Case-sensitive• Begin with and consist of:

– Letters (a... Z)– Numbers (0…9)– Underscore (_)– Dollar sign ($)

Same as C++

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Primitive Types

• boolean 8 bits• char 16 bits • byte 8 bits • short 16 bits • int 32 bits • long 64 bits• float 32 bits • double 64 bits

Guaranteed to occupy same number of bits regardless of platform

boolean – zero and non-zero DO NOT equate to true/false respectively

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Literals• boolean

– true or false– zero and non-zero do NOT equate to true/false

• int– 14 796 2147361

• long– ends with “L”

• 65L 23412396432L

• float– ends with “f” or “F”

• 6.23f 5.96F 1e-32f

• double– 2.0146e123 3.1415926

• char– Contained in single quotes

• ‘b’ ‘&’ ‘*’– Escape sequences (similar to c++)

• ‘\”’ ‘\n’ ‘\t’ ‘\’’

Lecture3.ppt

String Literals

• Enclosed in double quotes (“)– “USNA” “This is a string literal” “A”

• Concatenating Strings– “United” + “ States” + “ Naval” + “ Academy”

• Like C++, String is class not a primitive data type

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Strings

• Java defines the String class to handle strings• A String is a collection of characters treated as a single unit• It is not an array of char variables• Multiple String constructors

String s1 = new String(“Hello World”);String s2 = “Hello World”;

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String Example

public class Strings { public static void main (String[] args) { String m = "was a Roman"; String c = "Cicero " + m; System.out.println(c); String s = "Java is hot!"; s = 'L' + s.substring(1); System.out.println(s); }}

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Basic Operators• Arithmetic Operations in Java

– Addition, subtraction, multiplication, division, modulus: (+, -, *, /, %)• Equality and Relational Operators

– Equality, not equals, greater than, greater than or equal, less than, less than or equal: (==, !=, >, >=, <, <=)

• Logical Operators– Binary operators

• logical AND: && (two boolean operands w/ short circuit)• logical AND: & (two boolean operands no short circuit)• bitwise AND: & (two integer operands)• logical OR: || (two boolean operands w/ short circuit)• logical OR: | (two boolean operands no short circuit)• bitwise OR: | (two integer operands)• logical exclusive OR: ^ (two boolean operands)• bitwise exclusive OR: ^ (two integer operands)

• Unary operator• logical NOT: ! (single boolean operand)• bitwise NOT: ~ (single integer operand)

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Assignment Operators

+= c += 7 c = c + 7 10 to c

-= d -=4 d = d – 4 1 to d

*= e *= 5 e = e * 5 20 to e

/= f /= 3 f = f / 3 2 to f

%= g %= 9 g = g % 9 3 to g

int c = 3, d = 5, e = 4, f = 6, g = 12;

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Increment and Decrement Operators

++ preincrement ++a Increment a by 1 then use the new value

++ postincrement a++ Use the current value of a then increment by 1

-- predecrement --b Decrement b by 1 then use the new value

-- postdecrement b-- Use the current value of b then decrement by 1

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Casting

• Automatic promotion works in Java, but not demotion…need explicit cast to demote:

double d = 3.2;

double e = (double)i/(double)j;d = j; int j = 4;

int I = (int)d;

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Control Structures

• Sequential execution is the default (like C++)• Control structures can be used to alter this sequential flow of execution

(same as C++)– Selection structures – if/switch statements– Repetition structures – while/for/do-while statements

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Selection Structures• Generally the same as C++• The if selection structure

– Only difference with C++ is that the condition must be a Boolean expression evaluating to true or false

– Zero/non-zero can NOT be used for Java conditions– The following is illegal

int flag = 10;if (flag) { //illegal condition System.out.println("Flag was non-zero");}

• The if/else selection structure– Same as C++ with the above restriction on the condition portion

• The conditional operator (?:)– Same as C++, but must be a Boolean expression System.out.println(grade >= 60 ? "Passed" : "Failed");

• The switch statement– Identical to C++ switch statement

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Repetition Structures

• The while repetition structure– Same as C++ with similar restriction to if statement regarding the condition

portion (i.e. must evaluate to either true or false)• The do/while structure

– Same as C++, but condition must be a true or false expression• The break and continue Statements

– Same basic meaning as C++– break

• Can only occur within the body of a loop, or a switch statement• Execution exits the innermost enclosing loop or switch block and continues with the

next statement following the block– continue

• Can only occur within the body of a loop• Terminates the current iteration of the loop without exiting the loop body

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Console I/O• Three standard streams in Java included as part of java.lang

– System.in – InputStream defaults from the keyboard– System.out – PrintStream defaults to the screen– System.err – PrintStream defaults to the screen

• Scanner– A simple text scanner which can parse primitive types and strings– A Scanner breaks its input into tokens using a delimiter pattern, which by

default matches whitespace– The resulting tokens may then be converted into values of different types

using the various next methods– Resides in the java.util package

• Wrap a Scanner around an InputStream, i.e., System.in to read console input (keyboard) by Scanner input = new Scanner(System.in);

• A Scanner can also be wrapped around a File object to read from a text file…more on this later.

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Keyboard input with Scanner

• Instantiate a ScannerScanner myScanner = new Scanner(System.in);

• Read an entire line of textString input = myScanner.nextLine();

• Read an individual token, i.e., intint i = scanner.nextInt();

• What if next input isn’t an int?

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Scanner Exampleimport java.util.*;public class ScannerDemo { public static void main(String[] args) { int age; String name; Scanner myScanner = new Scanner(System.in); System.out.println("Enter first and last name: "); name = myScanner.nextLine(); System.out.println("How old are you? "); age = myScanner.nextInt(); System.out.println(name + '\t' + age); }}

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Scanner Exampleimport java.util.*;public class ScannerDemo { public static void main(String[] args) { int age; String first; String last; Scanner myScanner = new Scanner(System.in); System.out.println("Enter first and last name: "); first = myScanner.next(); last = myScanner.next(); System.out.println("How old are you? "); age = myScanner.nextInt(); System.out.println(last +", " + first + '\t' + age); }}

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Input Errors

• What happens if the user doesn’t enter an integer when asked for the age?

• There are a couple of ways to handle it– Look ahead to see if the user entered an integer before we read itor– Read the input and handle the resulting error

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Look Ahead

if (myScanner.hasNextInt()) { age = myScanner.nextInt(); } else { age = 30; String junk = myScanner.next(); }

• Scanner provide the ability to look at the next token in the input stream before we actually read it into our program– hasNextInt()– hasNextDouble()– hasNext()– etc…

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Input Exceptions (errors)

• What happens when we try to read an integer (myScanner.nextInt()) and the user enters something different?

• Java “throws” and exception, i.e., issues and error message.• We can “catch” the errors and handle them, thereby preventing the program from

crashing

try{ age = myScanner.nextInt();}catch(InputMismatchException e){ age = 30;}

The InputMismatchException is part of the java.util library so we must import java.util.InputMismatchException or java.util.* in order to catch the exception.