Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download System.out.println("Set the timer.")
Document related concepts
no text concepts found
Transcript
Java . . .
Announcements
Course web page at:
http://www.cmc.edu/pages/faculty/alee/cs51/
Survey form: See Announcements on the course web
Problem Set 1 (PS 1) will be ready tomorrow (due 9/11)
Tutoring starts next Monday
Use of computers in class
Only for this class
Avoid distraction for others
Reading assignment for this week: Chapter 1 of Reges
Take a break around 10:15am
Acknowledgement: some slide contents are from Pearson Education
2
What to do today
Basic Java programs with println statements
The first Java program!
Java programming environment
Simple strings
Static methods
Procedural decomposition
Data and expressions (Section 2.1)
3
Some modern languages
Procedural languages: programs are a series of commands
Pascal (1970): designed for education
C (1972): low-level operating systems and device drivers
Functional programming: functions map inputs to outputs
Lisp (1958) / Scheme (1975)
ML (1973)
Haskell (1990)
Object-oriented languages: programs use interacting "objects"
Smalltalk (1980): first major object-oriented language
C++ (1985): "object-oriented" improvements to C
successful in industry; used to build major OSes such as Windows
Java (1995): designed for embedded systems, web apps/servers
Runs on many platforms (Windows, Mac, Linux, cell phones...)
4
The Java coding process
1. Write a program
create/edit source code
2. Compile it
• compile: Translate a program from one language to another
byte code: The Java compiler converts your code into a format named
byte code that runs on many computer types
3. Run (execute) it
output: The messages printed to the user by a program
source code
output
byte code
compile
run
5
A Java program (see Hello.java)
public class Hello {
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");
}
}
Its output:
Hello, world!
This program produces
four lines of output
Console with jGRASP:
Control flow of a Java program:
Starts with the first line of main
Ends with the last line of main
6
Java IDE’s
IDE – Integrated Development Environment
Combines text editor, compiler, interpreter, and debugger into one GUI-
based environment
Several good IDE’s for Java
jGRASP
Eclipse
Netbeans
BlueJ
We will use jGRASP and Eclipse (You may use either)
Installing Java, jGRASP (or Eclipse)
Install Java first
Install jGRASP (or Eclipse) second
7
Bugs when things go wrong
Compile-time errors:
Compiler discovers a language grammar error. Typo?
Error messages are often inaccurate as to the line number where the
error occurs. Have to look “around” the reported line number.
Syntax errors are “normal”
Runtime errors:
Program crashes when it runs. Divide by zero?
Java tries to help with detecting some runtime errors
Logic errors:
Program runs and does not crash but there are errors in the program’s
logic or algorithm
Can be hard to find especially in medium to large programs
Test, test, test!
8
Structure of a Java program
class: a program
public class name {
public static void main(String[] args) {
statement;
statement;
method: a named group
...
of statements
statement;
}
}
statement: a command to be executed
ends with a ;
Every executable Java program consists of a class,
that contains a method named main,
that contains the statements (commands) to be executed.
9
System.out.println
A statement that prints a line of output on the console
sometimes called a "println statement" for short
Two ways to use System.out.println :
• System.out.println("text");
Prints the given message as output
• System.out.println();
Prints a blank line of output
10
Names and identifiers
You must give your program a name
public class Calendar {
Naming convention: capitalize each word (e.g., MyClassName)
Your program's file must match the class name exactly
(Calendar.java)
includes capitalization (Java is "case-sensitive")
identifier: A name given to an item in your program
must start with a letter or _ or $
subsequent characters can be any of those or a number
legal:
illegal:
_myName TheThing UP_TO_23
u+me
76ers
side-kick
$cling$
Ph.D's
11
Keywords
keyword: An identifier that you cannot use because it already has a
reserved meaning in Java
abstract
boolean
break
byte
case
catch
char
class
const
continue
default
do
double
else
extends
final
finally
float
for
goto
if
implements
import
instanceof
int
interface
long
native
new
package
private
protected
public
return
short
static
strictfp
super
switch
synchronized
this
throw
throws
transient
try
void
volatile
while
E.g., You may not use char or while for the name of a class
12
Syntax
syntax: The set of legal structures and commands that can be used
in a particular language
Every basic Java statement ends with a semicolon ;
The contents of a class or method occur between { and }
syntax error (compiler error): A problem in the structure of a
program that causes the compiler to fail.
Examples:
Missing semicolon
Too many or too few { } braces
Illegal identifier for class name
Class and file names do not match
...
13
Syntax error example
1
2
3
4
5
public class Hello {
pooblic static void main(String[] args) {
System.owt.println("Hello, world!")_
}
}
Compiler output:
Hello.java:2: <identifier> expected
pooblic static void main(String[] args) {
^
Hello.java:3: ';' expected
}
^
2 errors
The compiler shows the line number where it found the error
The error messages can be tough to understand!
14
Strings and escape sequences
15
Strings
string: A sequence of characters to be printed
Starts and ends with a " quote character
The quotes do not appear in the output
Examples:
"hello"
"This is a string.
It's very long!"
Restrictions:
May not span multiple lines
"This is not
a legal String."
May not contain a " character
"This is not a "legal" String either."
16
Escape sequences
escape sequence: A special sequence of characters used to
represent certain special characters in a string
\t
\n
\"
\\
tab character
new line character
quotation mark character
backslash character
Example:
System.out.println("\\hello\nhow\tare \"you\"?\\\\");
Output:
\hello
how
are "you"?\\
17
Questions
What is the output of the following println statements?
System.out.println("\ta\tb\tc");
System.out.println("\\\\");
System.out.println("'");
System.out.println("\"\"\"");
System.out.println("C:\nin\the downward spiral");
Write a println statement to produce this output:
/ \ // \\ /// \\\
18
Answers
Output of each println statement:
a
\\
'
"""
C:
in
b
c
he downward spiral
println statement to produce the line of output:
System.out.println("/ \\ // \\\\ /// \\\\\\");
19
Questions
What println statements will generate this output?
This program prints a
quote from the Gettysburg Address.
"Four score and seven years ago,
our 'fore fathers' brought forth on
this continent a new nation."
What println statements will generate this output?
A "quoted" String is
'much' better if you learn
the rules of "escape sequences."
Also, "" represents an empty String.
Don't forget: use \" instead of " !
'' is not the same as "
20
Answers
println statements to generate the output:
System.out.println("This program prints a");
System.out.println("quote from the Gettysburg Address.");
System.out.println();
System.out.println("\"Four score and seven years ago,");
System.out.println("our 'fore fathers' brought forth on");
System.out.println("this continent a new nation.\"");
println statements to generate the output:
System.out.println("A \"quoted\" String is");
System.out.println("'much' better if you learn");
System.out.println("the rules of \"escape sequences.\"");
System.out.println();
System.out.println("Also, \"\" represents an empty String.");
System.out.println("Don't forget: use \\\" instead of \" !");
System.out.println("'' is not the same as \"");
21
Comments
comment: A note written in source code by the
programmer to describe or clarify the code.
Comments are not executed when your program runs.
Syntax:
// comment text, on one line
or,
/* comment text; may span multiple lines */
Examples:
// This is a one-line comment.
/* This is a very long
multi-line comment. */
22
Using comments
Where to place comments:
at the top of each file (a "comment header")
at the start of every method (seen later)
to explain complex pieces of code
Comments are useful for:
Understanding larger, more complex programs.
Multiple programmers working together, who must understand
each other's code.
23
Comments example
/* Suzy Student, CS 101, Fall 2019
This program prints lyrics about ... something. */
public class BaWitDaBa {
public static void main(String[] args) {
// first verse
System.out.println("Bawitdaba");
System.out.println("da bang a dang diggy diggy");
System.out.println();
// second verse
System.out.println("diggy said the boogy");
System.out.println("said up jump the boogy");
}
}
24
Static methods
reading: 1.4
self-check: 16-25
exercises: #5-10
Algorithms
algorithm: A list of steps for solving a problem.
Example algorithm: "Bake sugar cookies"
Mix the dry ingredients.
Cream the butter and sugar.
Beat in the eggs.
Stir in the dry ingredients.
Set the oven temperature.
Set the timer.
Place the cookies into the oven.
Allow the cookies to bake.
Mix ingredients for frosting.
Spread frosting and sprinkles onto the cookies.
...
26
Problems with algorithms
lack of structure: Many tiny steps; tough to remember.
redundancy: Consider making a double batch...
Mix the dry ingredients.
Cream the butter and sugar.
Beat in the eggs.
Stir in the dry ingredients.
Set the oven temperature.
Set the timer.
Place the first batch of cookies into the oven.
Allow the cookies to bake.
Set the oven temperature.
Set the timer.
Place the second batch of cookies into the oven.
Allow the cookies to bake.
Mix ingredients for frosting.
...
27
Structured algorithms
structured algorithm: Split into coherent tasks.
1 Make the cookie batter.
Mix the dry ingredients.
Cream the butter and sugar.
Beat in the eggs.
Stir in the dry ingredients.
2 Bake the cookies.
Set the oven temperature.
Set the timer.
Place the cookies into the oven.
Allow the cookies to bake.
3 Add frosting and sprinkles.
Mix the ingredients for the frosting.
Spread frosting and sprinkles onto the cookies.
...
28
Removing redundancy
A well-structured algorithm can describe repeated tasks
with less redundancy.
1 Make the cookie batter.
Mix the dry ingredients.
...
2a Bake the cookies (first batch).
Set the oven temperature.
Set the timer.
...
2b Bake the cookies (second batch).
3 Decorate the cookies.
...
29
A program with redundancy
public class BakeCookies {
public static void main(String[] args) {
System.out.println("Mix the dry ingredients.");
System.out.println("Cream the butter and sugar.");
System.out.println("Beat in the eggs.");
System.out.println("Stir in the dry ingredients.");
System.out.println("Set the oven temperature.");
System.out.println("Set the timer.");
System.out.println("Place a batch of cookies into the oven.");
System.out.println("Allow the cookies to bake.");
System.out.println("Set the oven temperature.");
System.out.println("Set the timer.");
System.out.println("Place a batch of cookies into the oven.");
System.out.println("Allow the cookies to bake.");
System.out.println("Mix ingredients for frosting.");
System.out.println("Spread frosting and sprinkles.");
}
}
30
Static methods
static method: A named group of statements.
denotes the structure of a program
eliminates redundancy by code reuse
procedural decomposition:
dividing a problem into methods
Writing a static method is like
class
n
n
n
n
n
adding a new command to Java.
n
n
n
method A
statement
statement
statement
method B
statement
statement
method C
statement
statement
statement
31
Using static methods
1. Design the algorithm.
Look at the structure, and see which commands are repeated.
Decide what are the important overall tasks.
2. Declare (write down) the methods.
Arrange statements into groups and give each group a name.
3. Call (run) the methods.
The program's main method executes the other methods to
perform the overall task.
32
Design of an algorithm
// This program displays a delicious recipe for baking cookies.
public class BakeCookies2 {
public static void main(String[] args) {
// Step 1: Make the cake batter.
System.out.println("Mix the dry ingredients.");
System.out.println("Cream the butter and sugar.");
System.out.println("Beat in the eggs.");
System.out.println("Stir in the dry ingredients.");
// Step 2a: Bake cookies (first batch).
System.out.println("Set the oven temperature.");
System.out.println("Set the timer.");
System.out.println("Place a batch of cookies into the oven.");
System.out.println("Allow the cookies to bake.");
// Step 2b: Bake cookies (second batch).
System.out.println("Set the oven temperature.");
System.out.println("Set the timer.");
System.out.println("Place a batch of cookies into the oven.");
System.out.println("Allow the cookies to bake.");
}
}
// Step 3: Decorate the cookies.
System.out.println("Mix ingredients for frosting.");
System.out.println("Spread frosting and sprinkles.");
33
1. Declaring a method
Gives your method a name so it can be executed
Syntax:
public static void name() {
statement;
statement;
...
statement;
}
Example:
public static void printWarning() {
System.out.println("This product causes cancer");
System.out.println("in lab rats and humans.");
}
34
2. Calling a method
Executes the method's code
Syntax:
name();
You can call the same method many times if you like.
Example:
printWarning();
Output:
This product causes cancer
in lab rats and humans.
35
Program with static method
public class FreshPrince {
public static void main(String[] args) {
rap();
// Calling (running) the rap method
System.out.println();
rap();
// Calling the rap method again
}
// This method prints the lyrics to my favorite song.
public static void rap() {
System.out.println("Now this is the story all about how");
System.out.println("My life got flipped turned upside-down");
}
}
Output:
Now this is the story all about how
My life got flipped turned upside-down
Now this is the story all about how
My life got flipped turned upside-down
36
Final cookie program
// This program displays a delicious recipe for baking cookies.
public class BakeCookies3 {
public static void main(String[] args) {
makeBatter();
bake();
// 1st batch
bake();
// 2nd batch
decorate();
}
// Step 1: Make the cake batter.
public static void makeBatter() {
System.out.println("Mix the dry ingredients.");
System.out.println("Cream the butter and sugar.");
System.out.println("Beat in the eggs.");
System.out.println("Stir in the dry ingredients.");
}
// Step 2: Bake a batch of cookies.
public static void bake() {
System.out.println("Set the oven temperature.");
System.out.println("Set the timer.");
System.out.println("Place a batch of cookies into the oven.");
System.out.println("Allow the cookies to bake.");
}
// Step 3: Decorate the cookies.
public static void decorate() {
System.out.println("Mix ingredients for frosting.");
System.out.println("Spread frosting and sprinkles.");
}
}
37
Program structure with multiple methods
public class ClassName {
public static void main(String[] args) {
statements
}
public static void method1() {
statements
}
. . .
public static void methodN() {
statements
}
}
// Methods may be in any order
38
Methods calling methods
public class MethodsExample {
public static void main(String[] args) {
message1();
message2();
System.out.println("Done with main.");
}
public static void message1() {
System.out.println("This is message1.");
}
public static void message2() {
System.out.println("This is message2.");
message1();
System.out.println("Done with message2.");
}
}
Output:
This
This
This
Done
Done
is message1.
is message2.
is message1.
with message2.
with main.
39
Control flow
When a method is called, the program's execution...
"jumps" into that method, executing its statements, then
"jumps" back to the point where the method was called.
public class MethodsExample {
public static void main(String[] args) {
public static void message1() {
message1();
System.out.println("This is message1.");
}
message2();
public static void message2() {
System.out.println("This is message2.");
message1();
System.out.println("Done System.out.println("Done
with main.");
with message2.");
}
}
}
...
public static void message1() {
System.out.println("This is message1.");
}
40
When to use methods
Place statements into a static method if:
The statements are related structurally, and/or
The statements are repeated.
You should not create static methods for:
An individual println statement.
Only blank lines. (Put blank printlns in main.)
Unrelated or weakly related statements.
(Consider splitting them into two smaller methods.)
41
Drawing complex figures
with static methods
reading: 1.5
(Ch. 1 Case Study: DrawFigures)
exercises: #7-9
We may skip this part in class if not enough time…
Static methods question
Write a program to print these figures using methods.
______
/
\
/
\
\
/
\______/
\
/
\______/
+--------+
______
/
/
|
\
\
\
|
/
STOP
\______/
______
/
\
/
\
+--------+
43
Development strategy
______
/
\
/
\
\
/
\______/
\
n
/
\______/
+--------+
______
/
/
|
\
First version (unstructured):
n
n
\
Create an empty program and main method.
Copy the expected output into it, surrounding
each line with System.out.println syntax.
Run it to verify the output.
\
|
/
STOP
\______/
______
/
\
/
\
+--------+
44
Program version 1
public class Figures1 {
public static void main(String[] args) {
System.out.println(" ______");
System.out.println(" /
\\");
System.out.println("/
\\");
System.out.println("\\
/");
System.out.println(" \\______/");
System.out.println();
System.out.println("\\
/");
System.out.println(" \\______/");
System.out.println("+--------+");
System.out.println();
System.out.println(" ______");
System.out.println(" /
\\");
System.out.println("/
\\");
System.out.println("| STOP |");
System.out.println("\\
/");
System.out.println(" \\______/");
System.out.println();
System.out.println(" ______");
System.out.println(" /
\\");
System.out.println("/
\\");
System.out.println("+--------+");
}
}
45
Development strategy 2
______
/
\
/
\
\
/
\______/
\
Second version (structured, with redundancy):
n
Identify the structure of the output.
/
\______/
+--------+
n
Divide the main method into static methods
based on this structure.
______
/
/
|
\
\
\
|
/
STOP
\______/
______
/
\
/
\
+--------+
46
Output structure
______
/
\
/
\
\
/
\______/
\
/
\______/
+--------+
The structure of the output:
n initial "egg" figure
n second "teacup" figure
n third "stop sign" figure
n fourth "hat" figure
______
/
/
|
\
\
\
|
/
STOP
\______/
______
/
\
/
\
+--------+
This structure can be represented by methods:
n egg
n teaCup
n stopSign
n hat
47
Program version 2
public class Figures2 {
public static void main(String[] args) {
egg();
teaCup();
stopSign();
hat();
}
public static void egg() {
System.out.println(" ______");
System.out.println(" /
\\");
System.out.println("/
\\");
System.out.println("\\
/");
System.out.println(" \\______/");
System.out.println();
}
public static void teaCup() {
System.out.println("\\
/");
System.out.println(" \\______/");
System.out.println("+--------+");
System.out.println();
}
...
48
Program version 2, cont'd.
...
public static void stopSign() {
System.out.println(" ______");
System.out.println(" /
\\");
System.out.println("/
\\");
System.out.println("| STOP |");
System.out.println("\\
/");
System.out.println(" \\______/");
System.out.println();
}
}
public static void hat() {
System.out.println(" ______");
System.out.println(" /
\\");
System.out.println("/
\\");
System.out.println("+--------+");
}
49
Development strategy 3
______
/
\
/
\
\
/
\______/
\
Third version (structured, without redundancy):
n
/
\______/
+--------+
n
Identify redundancy in the output, and create
methods to eliminate as much as possible.
Add comments to the program.
______
/
/
|
\
\
\
|
/
STOP
\______/
______
/
\
/
\
+--------+
50
Output redundancy
______
/
\
/
\
\
/
\______/
\
The redundancy in the output:
/
n
\______/
+--------+
n
n
______
/
/
|
\
reused on stop sign, hat
reused on teacup, stop sign
used on teacup, hat
\
\
|
/
STOP
\______/
______
/
egg top:
egg bottom:
divider line:
\
/
\
+--------+
This redundancy can be fixed by methods:
n eggTop
n eggBottom
n line
51
Program version 3
// Suzy Student, CSE 138, Spring 2094
// Prints several figures, with methods for structure and redundancy.
public class Figures3 {
public static void main(String[] args) {
egg();
teaCup();
stopSign();
hat();
}
// Draws the top half of an an egg figure.
public static void eggTop() {
System.out.println(" ______");
System.out.println(" /
\\");
System.out.println("/
\\");
}
// Draws the bottom half of an egg figure.
public static void eggBottom() {
System.out.println("\\
/");
System.out.println(" \\______/");
}
// Draws a complete egg figure.
public static void egg() {
eggTop();
eggBottom();
System.out.println();
}
...
52
Program version 3, cont'd.
}
...
// Draws a teacup figure.
public static void teaCup() {
eggBottom();
line();
System.out.println();
}
// Draws a stop sign figure.
public static void stopSign() {
eggTop();
System.out.println("| STOP |");
eggBottom();
System.out.println();
}
// Draws a figure that looks sort of like a hat.
public static void hat() {
eggTop();
line();
}
// Draws a line of dashes.
public static void line() {
System.out.println("+--------+");
}
53
Data and expressions
reading: 2.1
self-check: 1-4
54
Data types
type: A category or set of data values.
Constrains the operations that can be performed on data
Many languages ask the programmer to specify types
Examples: integer, real number, string
Internally, computers store everything as 1s and 0s
104
à 01101000
"hi" à 01101000110101
55
Java's primitive types
primitive types: 8 simple types for numbers, text, etc.
Java also has object types, which we'll talk about later
Name
Description
Examples
int
integers
42, -3, 0, 926394
double
real numbers
3.1, -0.25, 9.4e3
char
single text characters
'a', 'X', '?', '\n'
boolean
logical values
true, false
• Why does Java distinguish integers vs. real numbers?
56
Expressions
expression: A value or operation that computes a value.
• Examples:
1 + 4 * 5
(7 + 2) * 6 / 3
42
The simplest expression is a literal value.
A complex expression can use operators and parentheses.
57
Arithmetic operators
operator: Combines multiple values or expressions.
addition
subtraction (or negation)
multiplication
division
modulus (a.k.a. remainder)
+
*
/
%
As a program runs, its expressions are evaluated.
1 + 1 evaluates to 2
System.out.println(3 * 4); prints 12
How would we print the text 3 * 4 ?
58
Integer division with /
When we divide integers, the quotient is also an integer.
14 / 4 is 3, not 3.5
3
4 ) 14
12
2
4
10 ) 45
40
5
52
27 ) 1425
135
75
54
21
More examples:
32 / 5
is 6
84 / 10
is 8
156 / 100 is 1
Dividing by 0 causes an error when your program runs.
59
Integer remainder with %
The % operator computes the remainder from integer division.
14 % 4
218 % 5
is 2
is 3
3
4 ) 14
12
2
43
5 ) 218
20
18
15
3
What is the result?
45 % 6
2 % 2
8 % 20
11 % 0
Applications of % operator:
Obtain last digit of a number:
230857 % 10 is 7
Obtain last 4 digits:
658236489 % 10000 is 6489
See whether a number is odd:
7 % 2 is 1, 42 % 2 is 0
60
Precedence
precedence: Order in which operators are evaluated.
Generally operators evaluate left-to-right.
1 - 2 - 3 is (1 - 2) - 3 which is -4
But */% have a higher level of precedence than +-
1 + 3 * 4
is 13
6 + 8 / 2 * 3
6 +
4
* 3
6 +
12
is 18
Parentheses can force a certain order of evaluation:
(1 + 3) * 4
is 16
Spacing does not affect order of evaluation
1+3 * 4-2
is 11
61
Precedence examples
1 * 2 + 3 * 5 % 4
\_/
|
2
+ 3 * 5 % 4
\_/
|
2
+ 15
% 4
\___/
|
2
+
3
\________/
|
5
n
n
n
n
n
1 + 8 % 3 * 2 - 9
\_/
|
1 +
2
* 2 - 9
\___/
|
1 +
4
- 9
\______/
|
5
- 9
\_________/
|
-4
62
Precedence questions
What values result from the following expressions?
9 / 5
695 % 20
7 + 6 * 5
7 * 6 + 5
248 % 100 / 5
6 * 3 - 9 / 4
(5 - 7) * 4
6 + (18 % (17 - 12))
63
Real numbers (type double)
Examples:
6.022 ,
-42.0 ,
2.143e17
Placing .0 or . after an integer makes it a double.
The operators +-*/%() all still work with double.
/ produces an exact answer: 15.0 / 2.0 is 7.5
Precedence is the same: () before */% before +-
64
Real number example
2.0 * 2.4 + 2.25 * 4.0 / 2.0
\___/
|
4.8
+ 2.25 * 4.0 / 2.0
\___/
|
4.8
+
9.0
/ 2.0
\_____/
|
4.8
+
4.5
\____________/
|
9.3
65
Mixing types
When int and double are mixed, the result is a double.
4.2 * 3 is 12.6
The conversion is per-operator, affecting only its operands.
7 / 3 * 1.2 + 3 / 2
\_/
|
2
* 1.2 + 3 / 2
\___/
|
2.4
+ 3 / 2
\_/
|
2.4
+
1
\________/
|
3.4
3 / 2 is 1 above, not 1.5.
2.0 + 10 / 3 * 2.5 - 6 / 4
\___/
|
2.0 +
3
* 2.5 - 6 / 4
\_____/
|
2.0 +
7.5
- 6 / 4
\_/
|
2.0 +
7.5
1
\_________/
|
9.5
1
\______________/
|
8.5
66
String concatenation
string concatenation: Using + between a string and
another value to make a longer string.
"hello" + 42
1 + "abc" + 2
"abc" + 1 + 2
1 + 2 + "abc"
"abc" + 9 * 3
"1" + 1
4 - 1 + "abc"
is
is
is
is
is
is
is
"hello42"
"1abc2"
"abc12"
"3abc"
"abc27"
"11"
"3abc"
Use + to print a string and an expression's value together.
System.out.println("Grade: " + (95.1 + 71.9) / 2);
• Output: Grade: 83.5
67
Next week’s lab
Suggestion: try these before the first lab next week
DOS lab if you use Windows
Unix lab if you use Mac OS X
Install Java compiler
Install jGRASP
68
