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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
215/09/2014
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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.
315/09/2014
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Java IDE Tools
Text Pad
Net Bean
EClipse
415/09/2014
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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 {
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 }
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
Code:for (int i = 1; i
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Nested loops example
Code:for (int i = 1; i
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Nested loops example
Code:for (int i = 1; i
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Nested loops example
Code:for (int i = 1; i
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Nested loops example
Code:for (int i = 1; i
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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?
A random number between 1 and 100 inclusive?
A random number between 4 and 17 inclusive?
R d l ti
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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;
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 {
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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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 console = new Scanner(System.in);
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 {
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!
Another complete program
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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 S can read multiple values from one line