01 java language

8/11/2019 01 Java Language

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PHN 1:

NGN NG JAVA

BI 1: LM QUEN VI NGN NG JAVA


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Java History

James Gosling and Sun Microsystems

Oak

Java, May 20, 1995, Sun World

HotJava The first Java-enabled Web browser

JDK Evolutions

J2SE, J2ME, and J2EE

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JDK Editions

Java Standard Edition (J2SE) J2SE can be used to develop client-side standalone applications

or applets.

Java Enterprise Edition (J2EE) J2EE can be used to develop server-side applications such as

Java servlets and Java ServerPages.

Java Micro Edition (J2ME). J2ME can be used to develop applications for mobile devices

such as cell phones.

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Java IDE Tools

Text Pad

Net Bean

EClipse

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Your first Java program!

public class Hello {

public static void main(String[] args) {

System.out.println("Hello, world!");

}

}

What does this code output(print to the user) when yourun(execute) it?


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Compiling a program

Before you run a program, you must compileit.

compiler:Translates a computer program

written in one language (i.e., Java) to another

language (i.e., byte code)

Compile (javac) Execute (java)

outputsource code

(Hello.java)

byte code

(Hello.class)


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The Java Virtual Machine

(JVM, or VM)

The Java Virtual Machine executes byte code

Use the java command to execute it

It only understands byte code (.class files)

The VM makes Java a bit different from older

programming languages (C, C++)

Its an extra step; compilers for other languagesdirectly produce machine code

Its slower

But it allows the same byte code to run on any

machine with a VM


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Program execution

The outputis printed to the console.

Some editors pop up the console as another window.


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Another Java program

public class Hello2 {


System.out.println("Hello, world!");

System.out.println();

System.out.println("This program produces");

System.out.println("four lines of output");

}

}


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Syntax

syntax:The set of legal structures and commands that

can be used.

Examples: Every basic statement ends with a semi-colon.

The contents of a class occur between curly braces.


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Syntax Errors

syntax error:A problem in the structure of a program.

1 public class Hello {

2 pooblic static void main(String[] args) {

3 System.owt.println("Hello, world!")4 }

5 }

2 errors found:File:Hello.java [line: 2]

Error:Hello.java:2: expected

File:Hello.java [line: 3]

Error:Hello.java:3: ';' expected

compiler output:


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More on syntax errors

Java is case-sensitive

Helloand helloare not the same

1 Public class Hello {

2 public static void main(String[] args) {

3 System.out.println("Hello, world!");

4 }

5 }

1 error found:

File:Hello.java [line: 1]

Error:Hello.java:1: class, interface, or enum

expected

compiler output:


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System.out.println

System.out.println: A statement to print a line

of output to the console.

pronounced print-linn

Two ways to use System.out.println:

System.out.println("");

Prints the given message as a line of text to the console.

System.out.println();

Prints a blank line to the console.


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Strings

string:A sequence of text characters.

Start and end with quotation mark characters

Examples:

"hello"

"This is a string"

"This, too, is a string. It can be very long!"


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Details about strings

A string may not span across multiple lines."This is not

a legal string."

A string may not contain a character.

The character is okay.

"This is not a "legal" string either."

"This is 'okay' though."

This begs the question


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Escape sequences

A string can represent certain special characters by preceding themwith a backslash \(this is called an escape sequence). \t tab character

\n newline character

\" quotation mark character

Example:System.out.println("Hello!\nHow are \"you\"?");

Output:

Hello!

How are "you"?

This begs another question


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Comments

comment: A note written in the source code to make the code easier tounderstand.

Comments are not executed when your program runs.

Most Java editors show your comments with a special color.

Comment, general syntax:

/* */

or,

//

Examples:

/* A comment goes here. *//* It can even span

multiple lines. */

// This is a one-line comment.


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Comments: Where do you go?

at the top of each file (also called a "comment header"), naming

the author and explaining what the program does

at the start of every method, describing its behavior

inside methods, to explain complex pieces of code


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Comments: Why?

Comments provide important documentation.

Later programs will span hundreds or thousands of lines, split into

many classes and methods.

Comments provide a simple description of what each class, method,

etc. is doing.

When multiple programmers work together, comments help one

programmer understand the other's code.


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That thing called style

What is style?

Indentation

Capitalization

Formatting / spacing

Structured code

No redundancy

Why is it important?


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Primitive data types,

expressions, and

variables


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How the computer sees the world

Internally, the computer stores everything in terms of 1s

and 0s

Example:

h 0110100

"hi" 01101000110101

104 0110100

How can the computer tell the difference between an h

and 104?


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Data types

data type: A category of data values.

Example: integer, real number, string

Data types are divided into two classes:

primitive types: Java's built-in simpledata types for

numbers, text characters, and logic.

object types: Coming soon!


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Primitive types

Java has eight primitive types. Here are two examples:

Name Description Examples

int integers 42, -3, 0, 926394

double real numbers 3.4, -2.53, 91.4e3

Numbers with a decimal point are treated as real numbers.

Question: Isnt every integer a real number? Why bother?


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Other Primitive Data Types

Discrete Typesbyte

short

intlong

Continuous Typesfloat

double

Non-numeric Types

booleanchar


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Data Type Representations

Type Representation Bits Bytes #Valuesboolean trueor false 1 N/A 2

char a or 7 or \n 16 2 216= 65,536

byte ,-2,-1,0,1,2, 8 1 28= 256

short ,-2,-1,0,1,2, 16 2 216= 65,536

int ,-2,-1,0,1,2, 4 > 4.29 million

long ,-2,-1,0,1,2, 8 > 18 quintillion

float 0.0, 10.5, -100.7 32

double 0.0, 10.5, -100.7 64


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Precision in real numbers

The computer internally represents real numbers in an

imprecise way.

Example:

System.out.println(0.1 + 0.2);

The output is 0.30000000000000004!


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Concatenation: Operating on strings

string concatenation: Using the +operator between a stringand another value to make a longer string.

Examples:

"hello" + 42 is "hello42"1 + "abc" + 2 is "1abc2"

"abc" + 1 + 2 is "abc12"

1 + 2 + "abc is "3abc"

"abc" + 9 * 3 is "abc27" (what happened here?)

"1" + 1 is "11"4 - 1 + "abc is "3abc"

"abc" + 4 - 1causes a compiler error. Why?


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Question

ints are stored in 4 bytes (32 bits)

In 32 bits, we can store at most 232different

numbers

What happens if we take the largest of these,

and add 1 to it?

ERROR!

This is known as overflow: trying to store somethingthat does not fit into the bits reserved for a data type.

Overflow errors are NOTautomatically detected!

Its the programmers responsibility to prevent these.

The actual result in this case is a negative number.


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Overflow example

int n = 2000000000;

System.out.println(n * n);

// output: -1651507200

the result of n*n is 4,000,000,000,000,000,000 which needs 64-bits:

---------- high-order bytes -------

00110111 10000010 11011010 11001110

---------- low order bytes --------

10011101 10010000 00000000 00000000

In the case of overflow, Java discards the high-order bytes, retaining

only the low-order ones

In this case, the low order bytes represent 1651507200, and since

the right most bit is a 1 the sign value is negative.


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Declaring variables

To create a variable, it must be declared.

Variable declaration syntax:;

Convention: Variable identifiers follow the samerules as method names.

Examples:int x;

double myGPA;

int varName;


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Declaring a variable sets aside a piece of memory inwhich you can store a value.

int x;int y;

Inside the computer:

x: ? y: ?

(The memory still has no value yet.)

Declaring variables


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Identifiers: Say my name!

identifier: A name given to an entity in a program such as a class or

method.

Identifiers allow us to refer to the entities.

Examples (in bold): public class Hello

public static voidmain

doublesalary

Conventions for naming in Java (which we will follow):

classes: capitalize each word (ClassName)

everything else: capitalize each word after the first(myLastName)


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Identifiers: Keywords

keyword: An identifier that you cannot use, because it already has a

reserved meaning in the Java language.

Complete list of Java keywords:abstract default if private this

boolean do implements protected throw

break double import public throwsbyte else instanceof return transient

case extends int short try

catch final interface static void

char finally long strictfp volatile

class float native super while

const for new switch

continue goto package synchronized

NB: Because Java is case-sensitive, you could technically use Classor cLaSs

as identifiers, but this is very confusing and thus strongly discouraged.


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Errors in coding

ERROR:Declaring two variables with the same

name

Example:int x;

int x; // ERROR: x already exists

ERROR:Reading a variables value before it has

been assigned

Example:int x;

System.out.println(x); // ERROR: x has no value


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Increment and decrement

Incrementingand decrementing1is used often enough that they have aspecial shortcut operator!

Shor thand Equivalent longer vers ion

++; = + 1;

--; = - 1;

Examples:

int x = 2;

x++; // x = x + 1;

// x now stores 3

double gpa = 2.5;

gpa++; // gpa = gpa + 1;

// gpa now stores 3.5


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if/elsestatements


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The ifstatement

ifstatement: A control structurethat executes a block ofstatements only if a certain condition is true.

General syntax:

if () { ;

}

Example (with grade inflation):if (gpa >= 2.0) {

System.out.println("You get an A!");

}


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ifstatement flow chart


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The if/elsestatement

if/elsestatement: A control structure that executes one block ofstatements if a certain condition is true, and a second block ofstatements if it is false. We refer to each block as a branch.

General syntax:

if () { ;

} else {

;

}

Example:if (gpa >= 3.0) {

System.out.println("Welcome to Temple!");

} else {

System.out.println("Try applying to Penn.");

}


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if/elsestatement flow chart


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Chained if/elsestatements

Chained if/elsestatement: A chain of if/elsethat can selectbetween many different outcomes based on several tests.

General syntax:if () {

;

} else if () {

;} else {

;

}

Example:if (number > 0) {

System.out.println("Positive");

} else if (number < 0) {System.out.println("Negative");

} else {System.out.println("Zero");

}


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Chained if/elseflow chart

if () {

;

} else if () {

;

} else {

;

}


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Chained if/elseifflow chart

if () {

;

} else if () {

;

} else if () {

;

}


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Boolean Arithmetic


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ThebooleanType

The booleantype has two possible values:true and false

booleanvariables are declared and initializedjust like other primitive data types:

boolean iAmSoSmrt = false; //just like int i = 2;

boolean minor = (age < 21); //just like int x = y*2;


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Relational expressions

Relational expressionshave

numeric arguments and

boolean values.

They use one of the following six relational operators:

true5.0 >= 5.0greater than or equal to>=

false126

false10 < 5less than= 3 + 5 * (7 - 1)

5 * 7>= 3 + 5 * 6

35 >= 3 + 30

35 >= 33

true

Relational operators cannot be chained (unlike math

operators)2


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Logical operators

Logical operatorshave

boolean arguments and

boolean values

!(7 > 0)

(2 == 3) || (-1 < 5)

(9 != 6) && (2 < 3)

Example

not

or

and

Description

!

||

true&&

ResultOperator


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What is the result of each of the following expressions?

int x = 42;

int y = 17;

int z = 25;

y < x && y


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Class Constants

A class constant is a variable

whose scopeis the entire class, and

whose valuecan never change after it has

been initialized.

To give it the right scope, simply declare it right

inside the class:

public class MyClass {

public static finalint myConstant = 4;

}


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The forloop


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Looping via the forloop

forloop: A block of Java code that executes a group of statementsrepeatedly until a given test fails.

General syntax:for (; ; ) {

;;

...

;

}

Example:for (int i = 1; i


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Control Structure

The forloop is a con tro l st ructurea syntactic

structure that controlsthe execution of other

statements.

Example:

Shampoo hair. Rinse. Repeat.


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forloop over range of ints

We'll write forloops over integers in a given range. The declares a loop counter variable that is used in the test,

update, and body of the loop.

for (int = 1;


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forloop flow diagramfor (; ; ) {

;

;...

;

}


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Loop walkthrough

Code:for (int i = 1; i


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Loop example

Code:System.out.println("+----+");

for (int i = 1; i


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Varying the forloop

The initial and final values for the loop counter variable can be arbitraryexpressions:

Example:for (int i = -3; i


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Varying the forloop

The update can be a --(or any other operator). Caution: This requires changing the test from =.

System.out.println("T-minus");

for (int i = 3; i >=1; i--) {

System.out.println(i);

}System.out.println("Blastoff!");

Output:T-minus

3

21

Blastoff!


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Errors in coding

ERROR: Loops that never execute.

for (int i = 10; i < 5; i++) {

System.out.println("How many times do I print?");

}

ERROR: Loop tests that never fail.

A loop that never terminates is called an infinite loop.

for (int i = 10; i >= 1; i++) {

System.out.println("Runaway Java program!!!");

}


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Nested forloops

nested loop: Loops placed inside one another. Caution:Make sure the inner loop's counter variable has a different name!

for (int i = 1; i


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Nested loops example



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Exercise: Nested loops

What nested forloops produce the following output?

....1

...2

..3

.4

5

Key idea: outer "vertical" loop for each of the lines

inner "horizontal" loop(s) for the patterns within each line

inner loop (repeated characters on each line)

outer loop (loops 5 times because there are 5 lines)


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Nested loops

First, write the outer loop from 1to the number of lines desired.

for (int line = 1; line


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Nested loops

Make a table:

....1

...2

..3

.4

5

Answer:for (int line = 1; line


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Errors in coding

ERROR: Using the wrong loop counter variable.

What is the output of the following piece of code?for (int i = 1; i


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whileloops


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Definite loops

definite loop: A loop that executes a known number oftimes.

The forloops we have seen so far are definite loops.

We often use language like "Repeat these statements Ntimes."

"For each of these 10 things, "

Examples: Print "hello" 10 times.

Find all the prime numbers up to an integer n.


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Indefinite loops

indefinite loop: A loop where it is not obvious in advancehow many times it will execute.

We often use language like

"Keep looping as long asor whilethis condition is still true."

"Don't stop repeating untilthe following happens."

Examples:

Print random numbers until a prime number is printed.

Continue looping while the user has not typed "n" to quit.


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whileloop flow chart


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Example

Finds and prints a number's first factor other than 1:

Scanner console = new Scanner(System.in);

System.out.print("Type a number: ");

int number = console.nextInt();

int factor = 2;while (number % factor != 0) {

factor++;

}

System.out.println("First factor: " + factor);

Sample run:Type a number: 91

First factor: 7


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forvs.while

Any forloop of the following form:

for (; ; ) {

;

}

is equivalent to a whileloop of the following form:

;

while () {;

;

}


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forvs.while: Example

What whileloop is equivalent to the following forloop?for (int i = 1; i


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Exercise: digitSum

Write a class named DigitSumthat reads an integer fromthe user and prints the sum of the digits of that number.

You may assume that the number is non-negative.

Example:Enter a nonnegative number: 29107

prints 2+9+1+0+7 or 19

Hint: Use the %operator to extract the last digit of anumber. If we do this repeatedly, when should we stop?

S


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Solution: digitSum

import java.util.Scanner;public class DigitSum {

public static void main(String [] args) {

Scanner keyboard = new Scanner(System.in);

int n = keyboard.nextInt();

int sum = 0;

while (n > 0) {

sum += n % 10; // add last digit to sum

n = n / 10; // remove last digit

}System.out.println(sum = + sum);

}

}


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Random numbers

Th l


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The Randomclass

Objects of the Randomclass generatepseudo-randomnumbers.

Class Randomis found in the java.utilpackage.

import java.util.*;

The methods of a Randomobject

returns a random real number in the range [0.0, 1.0)nextDouble()

returns a random integer in the range [0, max)in other words, from 0 to one less than max

nextInt(max)

returns a random integernextInt()

DescriptionMethod name

G


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Generating random numbers

Random rand = new Random();int randomNum = rand.nextInt(10);

// randomNum has a random value between 0 and 9

What if we wanted a number from 1 to 10?int randomNum = rand.nextInt(10) + 1;

What if we wanted a number from minto max(i.e. an

arbitrary range)?

int randomNum = rand.nextInt() +

where equals (- + 1)

R d i


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Random questions

Given the following declaration, how would you get:Random rand = new Random();

A random number between 0 and 100 inclusive?



R d l ti


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Random solutions

Given the following declaration, how would you get:Random rand = new Random();


int random1 = rand.nextInt(101);


int random1 = rand.nextInt(100) + 1;


int random1 = rand.nextInt(14) + 4;

E i Di lli


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Exercise: Die-rolling

Write a program that simulates the rolling of two six-sideddice until their combined result comes up as 7.

Sample run:

Roll: 2 + 4 = 6Roll: 3 + 5 = 8

Roll: 5 + 6 = 11

Roll: 1 + 1 = 2

Roll: 4 + 3 = 7

You won after 5 tries!

S l ti Di lli


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Solution: Die-rolling

import java.util.*;

public class Roll {


Random rand = new Random();

int sum = 0;

int tries = 0;

while (sum != 7) {

int roll1 = rand.nextInt(6) + 1;

int roll2 = rand.nextInt(6) + 1;

sum = roll1 + roll2;

System.out.println("Roll: " + roll1 + " + " + roll2 + " = " + sum);

tries++;

}

System.out.println("You won after " + tries + " tries!");

}

}


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Indefinite loop

variations

V i t 1 d / hil


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Variant 1: do/while

do/whileloop: A control structure that executes statementsrepeatedly while a condition is true, testing the condition at the endof each repetition.

do/whileloop, general syntax:do {

;

} while ();

Example:// roll until we get a number other than 3

Random rand = new Random();int die;do {

die = rand.nextInt();} while (die == 3);

d / hil loop flow chart


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do/whileloop flow chart

How does this differ fromthe whileloop?

The controlledwillalways execute the first

time, regardless ofwhether the istrue or false.

V i t 2 "F " l


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Variant 2: "Forever" loops

Loops that go on forever

while (true) {

;

}

If it goes on forever, how do you stop?

b king the cycle


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breaking the cycle

breakstatement: Immediately exits a loop (for, while,do/while).

Example:while (true) {

;

if () {

break;

}

;

}

Why is the breakstatement in an ifstatement?


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Scannerobjects

Interactive programs


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Interactive programs

We have written programs that print console output.

It is also possible to read inputfrom the console.

The user types the input into the console.

The program uses the input to do something.

Such a program is called an interactive program.

Scanner


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Scanner

Constructing a Scannerobject to read the console:Scanner = new Scanner(System.in);

Example:


Scanner methods


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Scannermethods

Some methods of Scanner:

Each of these methods pauses your program untilthe user types input and presses Enter.

Then the value typed is returnedto your program.

reads and returns user input as a doublenextDouble()

reads and returns user input as an intnextInt()

reads and returns user input as a Stringnext()

DescriptionMethod

Using a Scanner object


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Using a Scannerobject

Example:

System.out.print("How old are you? "); // promptint age = console.nextInt();System.out.println("You'll be 40 in " + (40 -age)

+ " years.");

prompt: A message printed to the user, telling them

what input to type.

Input tokens


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Input tokens

token: A unit of user input, as read by the Scanner. Tokens are separated by whitespace (spaces, tabs, new lines).

How many tokens appear on the following line of input?

23 John Smith 42.0 "Hello world"

When the token doesn't match the type the Scannertries to read,

the program crashes. Example:System.out.print("What is your age? ");

int age = console.nextInt();

Sample Run:What is your age? Timmy

InputMismatchException:

at java.util.Scanner.throwFor(Unknown Source)

at java.util.Scanner.next(Unknown Source)

at java.util.Scanner.nextInt(Unknown Source)

...


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A complete program


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A complete program

import java.util.*; // so that I can use Scanner

public class ReadSomeInput {



System.out.print("What is your first name? ");

String name = console.next();

System.out.print("And how old are you? ");

int age = console.nextInt();

System.out.println(name + " is " + age + ". That's quite old!");

}

}

Sample Run:What is your first name?Marty

How old are you? 12

Marty is 12. That's quite old!

Another complete program


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Another complete program

import java.util.*; // so that I can use Scanner

public class Average {



System.out.print("Please type three numbers: ");

int num1 = console.nextInt();



double average = (num1 + num2 + num3) / 3.0;

System.out.println("The average is " + average);

}

}

Sample Run:Please type three numbers: 8 6 13

The average is 9.0

Notice that the S can read multiple values from one line

01 java language

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