CDIT Java01 00_Introduction to Java

Part 1: Java as an Object-oriented

Programming Language

00. Introduction to Java

STP Curriculum – Java course

Java History

• James Gosling and Sun Microsystems

• Oak

• Java, May 20, 1995, Sun World

• Hot Java – 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 you run (execute) it?

5

Compiling a program

• Before you run a program, you must compile it.

• compiler: Translates a computer program written in one language (i.e., Java) to another language (i.e., byte code)

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Compile (javac) Execute (java)

outputsource code(Hello.java)

byte code(Hello.class)

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++)– It’s an extra step; compilers for other languages

directly produce machine code

– It’s slower

– But it allows the same byte code to run on any machine with a VM

Program execution

• The output is printed to the console.

• Some editors pop up the console as another window.

8

Another Java program

public class Hello2 {

public static void main(String[] args) {

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 }

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2 errors found:

File: Hello.java [line: 2]

Error: Hello.java:2: <identifier> expected

File: Hello.java [line: 3]

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

compiler output:

More on syntax errors

• Java is case-sensitive– Hello and hello are not the same

1 Public class Hello {

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

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

4 }

5 }

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1 error found:

File: Hello.java [line: 1]

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

expected

compiler output:

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("<message>");

– 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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Primitive data types,

expressions, and variables

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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: Isn’t every integer a real number? Why bother?

15

Other Primitive Data Types

Discrete Types

byte

short

int

long

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Continuous Types

float

double

Non-numeric Types

boolean

char

Data Type Representations

Type Representation Bits Bytes #Values

boolean true or 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

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 string and 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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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.

Declaring variables

• To create a variable, it must be declared.

• Variable declaration syntax:<type> <name>;

• Convention: Variable identifiers follow the same rules as method names.

• Examples:int x;

double myGPA;

int varName;

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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 throws

byte 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 Class or cLaSs

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

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Increment and decrement

• Incrementing and decrementing 1 is used often enough that they have a special shortcut operator!

Shorthand Equivalent longer version<variable>++; <variable> = <variable> + 1;

<variable>--; <variable> = <variable> - 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/else statements

The if statement

• if statement: A control structure that executes a block of statements only if a certain condition is true.

• General syntax:if (<test>) {

<statement(s)> ;

}

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

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

}

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if statement flow chart

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The if/else statement

• if/else statement: A control structure that executes one block of statements if a certain condition is true, and a second block of statements if it is false. We refer to each block as a branch.

• General syntax:if (<test>) {

<statement(s)> ;} else {

<statement(s)> ;}

• Example:if (gpa >= 3.0) {

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

} else {

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

}

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if/else statement flow chart

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Chained if/else statements

• Chained if/else statement: A chain of if/else that can select between many different outcomes based on several tests.

• General syntax:if (<test>) {

<statement(s)> ;} else if (<test>) {

<statement(s)> ;} else {

<statement(s)> ;}

• 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/else flow chart

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if (<test>) {

<statement(s)>;

} else if (<test>) {

<statement(s)>;

} else {

<statement(s)>;

}

Chained if/else if flow chart

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if (<test>) {

<statement(s)>;

} else if (<test>) {

<statement(s)>;

} else if (<test>) {

<statement(s)>;

}

Boolean Arithmetic

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The boolean Type

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The boolean type has two possible values:true and false

boolean variables are declared and initialized just like other primitive data types:

boolean iAmSoSmrt = false; //just like

int i = 2;

boolean minor = (age < 21); //just like

int x = y*2;

Relational expressions

• Relational expressions have

– numeric arguments and

– boolean values.

• They use one of the following six relational operators:

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

false126 <= 100less than or equal to<=

true10 > 5greater than>

false10 < 5less than<

true3.2 != 2.5does not equal!=

true1 + 1 == 2equals==

ValueExampleMeaningOperator

Evaluating Relational expressions

• Relational operators have lower precedence than math operators.

5 * 7 >= 3 + 5 * (7 - 1)

5 * 7 >= 3 + 5 * 6

35 >= 3 + 30

35 >= 33

true

• Relational operators cannot be chained (unlike math operators)

2 <= x <= 10

true <= 10

error!

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Logical operators

• Logical operators have

– boolean arguments and

– boolean values

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!(7 > 0)

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

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

Example

not

or

and

Description

!

||

true&&

ResultOperator

Boolean expressions

• What is the result of each of the following expressions?int x = 42;

int y = 17;

int z = 25;

– y < x && y <= z

– x % 2 == y % 2 || x % 2 == z % 2

– x <= y + z && x >= y + z

– !(x < y && x < z)

– (x + y) % 2 == 0 || !((z - y) % 2 == 0)

Answers: true, false, true, true, false

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Class Constants

A class constant is a variable

– whose scope is the entire class, and

– whose value can 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 final int myConstant = 4;

}

The final keyword means its value can’t be changed.

The for loop

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Looping via the for loop

• for loop: A block of Java code that executes a group of statements repeatedly until a given test fails.

• General syntax:for (<initialization>; <test>; <update>) {

<statement>;

<statement>;

...

<statement>;

}

• Example:for (int i = 1; i <= 30; i++) {

System.out.println("I will not throw...");

}

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Control Structure

• The for loop is a control structure—a syntactic

structure that controls the execution of other statements.

• Example:

– “Shampoo hair. Rinse. Repeat.”

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for loop over range of ints

• We'll write for loops over integers in a given range.

– The <initialization> declares a loop counter variable that is used in the test, update, and body of the loop.

for (int <name> = 1; <name> <= <value>; <name>++) {

• Example:for (int i = 1; i <= 4; i++) {

System.out.println(i + " squared is " + (i * i));

}

Output:1 squared is 1

2 squared is 4

3 squared is 9

4 squared is 16

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for loop flow diagram

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for (<init>; <test>; <update>) {

<statement>;

<statement>;

...

<statement>;

}

Loop walkthrough

• Code:for (int i = 1; i <= 3; i++) {

System.out.println(i + " squared is " + (i * i));

}

Output:1 squared is 1

2 squared is 4

3 squared is 9

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i:

Loop example

• Code:System.out.println("+----+");

for (int i = 1; i <= 3; i++) {

System.out.println("\\ /");

System.out.println("/ \\");

}

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

Output:+----+

\ /

/ \

\ /

/ \

\ /

/ \

+----+

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Varying the for loop

• The initial and final values for the loop counter variable can be arbitrary expressions:

• Example:for (int i = -3; i <= 2; i++) {

System.out.println(i);

}

Output:-3

-2-10

12

• Example:for (int i = 1 + 3 * 4; i <= 5248 % 100; i++) {

System.out.println(i + " squared is " + (i * i));

}

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Varying the for loop

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

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

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

System.out.println(i);

}

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

Output:T-minus

3

2

1

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 for loops

• 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 <= 3; i++) {System.out.println("i = " + i);for (int j = 1; j <= 2; j++) {

System.out.println(" j = " + j);}

}

Output:i = 1j = 1j = 2

i = 2j = 1j = 2

i = 3j = 1j = 2

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Nested loops example

• Code:for (int i = 1; i <= 5; i++) {

for (int j = 1; j <= 10; j++) {

System.out.print((i * j) + " ");

}

System.out.println(); // to end the line

}

Output:1 2 3 4 5 6 7 8 9 10

2 4 6 8 10 12 14 16 18 20

3 6 9 12 15 18 21 24 27 30

4 8 12 16 20 24 28 32 36 40

5 10 15 20 25 30 35 40 45 50

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Nested loops example

• Code:for (int i = 1; i <= 6; i++) {

for (int j = 1; j <= 10; j++) {

System.out.print("*");

}

System.out.println();

}

Output:**********

**********

**********

**********

**********

**********

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Nested loops example

• Code:for (int i = 1; i <= 6; i++) {

for (int j = 1; j <= i; j++) {

System.out.print("*");

}

System.out.println();

}

Output:*

**

***

****

*****

******

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Nested loops example

• Code:for (int i = 1; i <= 6; i++) {

for (int j = 1; j <= i; j++) {

System.out.print(i);

}

System.out.println();

}

Output:1

22

333

4444

55555

666666

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Nested loops example

• Code:for (int i = 1; i <= 5; i++) {

for (int j = 1; j <= (5 - i); j++) {

System.out.print(" ");

}

for (int k = 1; k <= i; k++) {

System.out.print(i);

}

System.out.println();

}

Output:1

22

333

4444

55555

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Exercise: Nested loops

• What nested for loops 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

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inner loop (repeated characters on each line)

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

Nested loops

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

for (int line = 1; line <= 5; line++) {

...

}

• Notice that each line has the following pattern:– some number of dots (0 dots on the last line)

– a number

....1

...2

..3

.4

5

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Nested loops

• Make a table:

....1

...2

..3

.4

5

• Answer:for (int line = 1; line <= 5; line++) {

for (int j = 1; j <= (line * -1 + 5); j++) {

System.out.print(".");

}

System.out.println(line);

}

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0

1

2

3

4

line * -1 + 5

414

0

2

3

4

# of dots

5

3

2

1

value displayed

5

3

2

1

line

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 <= 10; i++) {

for (int j = 1; i <= 5; j++) {

System.out.print(j);

}

System.out.println();

}

– What is the output of the following piece of code?for (int i = 1; i <= 10; i++) {

for (int j = 1; j <= 5; i++) {

System.out.print(j);

}

System.out.println();

}

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while loops

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Definite loops

• definite loop: A loop that executes a known number of times.– The for loops we have seen so far are definite loops.

• We often use language like

– "Repeat these statements N times."

– "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 advance how many times it will execute.

• We often use language like

– "Keep looping as long as or while this condition is still true."

– "Don't stop repeating until the 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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while loop

• while loop: A control structure that repeatedly performs a test and executes a group of statements if the test evaluates to true.

• while loop, general syntax:while (<test>) {

<statement(s)>;}

• Example:int number = 1;

while (number <= 200) {

System.out.print(number + " ");

number *= 2;

}

Output:1 2 4 8 16 32 64 128

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while loop 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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for vs. while

• Any for loop of the following form:

for (<initialization>; <test>; <update>) {<statement(s)>;

}

is equivalent to a while loop of the following form:

<initialization>;while (<test>) {

<statement(s)>;<update>;

}

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for vs. while: Example

• What while loop is equivalent to the following for loop?

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

System.out.println(i + " squared = " + (i * i));

}

Solution:

int i = 1;

while (i <= 10) {

System.out.println(i + " squared = " + (i * i));

i++;

}

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Exercise: digitSum

• Write a class named DigitSum that reads an integer from

the 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 a

number. If we do this repeatedly, when should we stop?

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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

69

The Random class

• Objects of the Random class generate pseudo-random

numbers.– Class Random is found in the java.util package.

import java.util.*;

• The methods of a Random object

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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

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 min to max (i.e. an arbitrary range)?int randomNum = rand.nextInt(<size of the range>) +

<min>

where <size of the range> equals (<max> - <min> + 1)

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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?

– A random number between 1 and 100 inclusive?

– A random number between 4 and 17 inclusive?

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Random solutions

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

– A random number between 0 and 100 inclusive?int random1 = rand.nextInt(101);

– A random number between 1 and 100 inclusive?int random1 = rand.nextInt(100) + 1;

– A random number between 4 and 17 inclusive?int random1 = rand.nextInt(14) + 4;

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Exercise: Die-rolling

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

Sample run:Roll: 2 + 4 = 6

Roll: 3 + 5 = 8

Roll: 5 + 6 = 11

Roll: 1 + 1 = 2

Roll: 4 + 3 = 7

You won after 5 tries!

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Solution: Die-rolling

import java.util.*;

public class Roll {

public static void main(String[] args) {

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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Scanner objects

76

Interactive programs

• We have written programs that print console output.

• It is also possible to read input from 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.

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Scanner

• Constructing a Scanner object to read the console:

Scanner <name> = new Scanner(System.in);

• Example:

Scanner console = new Scanner(System.in);

78

Scanner methods

• Some methods of Scanner:

• Each of these methods pauses your program until the user types input and presses Enter.– Then the value typed is returned to your program.

79

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

• Example:

System.out.print("How old are you? "); // prompt

int 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.

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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 Scanner tries 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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Importing classes

• Java class libraries: A large set of Java classes available for you to use.– Classes are grouped into packages.

– To use the classes from a package, you must include an import declaration at the top of your program.

– Scanner is in a package named java.util

• Import declaration, general syntax:import <package name>.*;

• To use Scanner, put this at the start of your program:import java.util.*;

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A complete program

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

public class ReadSomeInput {

public static void main(String[] args) {

Scanner console = new Scanner(System.in);

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!

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Another complete program

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

public class Average {

public static void main(String[] args) {

Scanner console = new Scanner(System.in);

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

int num1 = console.nextInt();

int num2 = console.nextInt();

int num3 = 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 Scanner can read multiple values from one line.

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