basic syntax: data & expressions chapter 2 programming tip: timtowdi (pronounced “tim...
TRANSCRIPT
Basic Syntax: Data & Expressions
Chapter 2
Programming Tip:
TIMTOWDI (pronounced “Tim Toady”)There’s more than one way to do it-Perl motto
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
Reviewpublic class CokeMachine{
private price;
public CokeMachine(){ price = 300}
public int getPrice{ return Price;}
}
;
()
int
p
What is wrong here?(there are five
errors!)
From Applets to Applications• Applets: GUIs that are easily incorporated into
web pages– Special init methods instead of constructor– Special paint method for drawing to screen
(using Graphics object)
• Applications: stand-alone -- console-based or GUI– Code starts executing from main method– Initialization done in constructor– Call constructor in main method to run
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//********************************************************************// Lincoln.java Author: Lewis/Loftus//// Demonstrates the basic structure of a Java application.//********************************************************************
public class Lincoln{ //----------------------------------------------------------------- // Prints a presidential quote. //----------------------------------------------------------------- public static void main (String[] args) { System.out.println ("A quote by Abraham Lincoln:");
System.out.println ("Whatever you are, be a good one."); }}
Recall Lincoln.java…
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//********************************************************************// Lincoln.java Author: Lewis/Loftus//// Demonstrates the basic structure of a Java application.//********************************************************************
public class Lincoln{ //----------------------------------------------------------------- // Prints a presidential quote. //----------------------------------------------------------------- public static void main (String[] args) { System.out.println ("A quote by Abraham Lincoln:");
System.out.println ("Whatever you are, be a good one."); }}
Output
A quote by Abraham Lincoln:Whatever you are, be a good one.
Java Program Structure
public class MyProgram
{
}
// comments about the class
class header
class body
Comments can be placed almost anywhere
(for human consumption)
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Java Program Structure
public class MyProgram
{
}
// comments about the class
public static void main (String[] args)
{
}
// comments about the method
method headermethod body
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Will explain later in course
Data and Expressions• Let's explore some other fundamental programming
concepts
• Chapter 2 focuses on:– character strings– primitive data– the declaration and use of variables– expressions and operator precedence– data conversions– accepting input from the user– Java applets– introduction to graphics
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Outline
Character Strings
Variables and Assignment
Primitive Data Types
Expressions
Data Conversion
Interactive Programs
Graphics
Applets
Drawing Shapes
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Character Strings• string literal - represented by double quotes around text
• Examples:
"This is a string literal.""123 Main Street""X"
• Every character string is an object in Java, defined by the String class
• Every string literal represents a String object
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The println Method• In the Lincoln program from Chapter 1, we invoked the println method to print a character string
• The System.out object represents a destination (the monitor screen) to which we can send output
System.out.println ("Whatever you are, be a good one.");
object methodname
information provided to the method(parameters)
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The print Method• Also provided by System.out.print
• Similar to println, except does not advance to next line
• Anything printed after a print statement will appear on the same line
• See Countdown.java
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//********************************************************************// Countdown.java Author: Lewis/Loftus//// Demonstrates the difference between print and println.//********************************************************************
public class Countdown{ //----------------------------------------------------------------- // Prints two lines of output representing a rocket countdown. //----------------------------------------------------------------- public static void main (String[] args) { System.out.print ("Three... "); System.out.print ("Two... "); System.out.print ("One... "); System.out.print ("Zero... "); System.out.println ("Liftoff!"); // appears on first output line System.out.println ("Houston, we have a problem."); }} Output
Three... Two... One... Zero... Liftoff!Houston, we have a problem.
String Concatenation• The string concatenation operator (+) is used to
append one string to the end of another
"Peanut butter " + "and jelly"
• It can also be used to append a number to a string
• A string literal cannot be broken across two lines in a program– (White space in general doesn’t affect execution, except
newlines in string literals and single-line comments)
• See Facts.java
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//********************************************************************// Facts.java Author: Lewis/Loftus//// Demonstrates the use of the string concatenation operator and the// automatic conversion of an integer to a string.//********************************************************************
public class Facts{ //----------------------------------------------------------------- // Prints various facts. //----------------------------------------------------------------- public static void main (String[] args) { // Strings can be concatenated into one long string System.out.println ("We present the following facts for your " + "extracurricular edification:");
System.out.println ();
// A string can contain numeric digits System.out.println ("Letters in the Hawaiian alphabet: 12");
continue
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continue
// A numeric value can be concatenated to a string System.out.println ("Dialing code for Antarctica: " + 672);
System.out.println ("Year in which Leonardo da Vinci invented " + "the parachute: " + 1515);
System.out.println ("Speed of ketchup: " + 40 + " km per year"); }}
OutputWe present the following facts for your extracurricular edification:
Letters in the Hawaiian alphabet: 12Dialing code for Antarctica: 672Year in which Leonardo da Vinci invented the parachute: 1515Speed of ketchup: 40 km per year
String Concatenation• But the + operator is also used for arithmetic addition!
• Operation depends on the type of the information
– “hello” + “ ” + “world” string concatenation
– 5 + 7 addition
• The + operator is evaluated left to right, but parentheses can be used to force the order/evaluation
• See Addition.java
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Copyright © 2012 Pearson Education, Inc.
//********************************************************************// Addition.java Author: Lewis/Loftus//// Demonstrates the difference between the addition and string// concatenation operators.//********************************************************************
public class Addition{ //----------------------------------------------------------------- // Concatenates and adds two numbers and prints the results. //----------------------------------------------------------------- public static void main (String[] args) { System.out.println ("24 and 45 concatenated: " + 24 + 45);
System.out.println ("24 and 45 added: " + (24 + 45)); }}
Output
24 and 45 concatenated: 244524 and 45 added: 69
Quick Check
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What output is produced by the following?
System.out.println ("X: " + 25);System.out.println ("Y: " + (15 + 50));System.out.println ("Z: " + 300 + 50);
System.out.println ("hello" + 5 + 7);System.out.println (5 + 7 + "hello");
Escape Sequences• What if we wanted to print the quote character?
• Compiler interprets second quote as end of string:
System.out.println ("I said "Hello" to you.");
• escape sequence: series of characters representing a special character
• An escape sequence begins with a backslash character (\)
System.out.println ("I said \"Hello\" to you.");
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Escape Sequences
• Some Java escape sequences:
Escape Sequence
\b\t\n\r\"\'\\
Meaning
backspacetabnewlinecarriage returndouble quotesingle quotebackslash
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• See Roses.java
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//********************************************************************// Roses.java Author: Lewis/Loftus//// Demonstrates the use of escape sequences.//********************************************************************
public class Roses{ //----------------------------------------------------------------- // Prints a poem (of sorts) on multiple lines. //----------------------------------------------------------------- public static void main (String[] args) { System.out.println ("Roses are red,\n\tViolets are blue,\n" + "Sugar is sweet,\n\tBut I have \"commitment issues\",\n\t" + "So I'd rather just be friends\n\tAt this point in our " + "relationship."); }}
Output
Roses are red,Violets are blue,
Sugar is sweet,But I have "commitment issues",So I'd rather just be friendsAt this point in our relationship.
Quick Check
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Write a single println statement that produces the following output:
"Thank you all for coming to my hometonight," he said mysteriously.
Outline
Character Strings
Variables and Assignment
Primitive Data Types
Expressions
Data Conversion
Interactive Programs
Graphics
Applets
Drawing Shapes
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Variables• A variable is a name for a location in memory that
holds a value
• A variable declaration specifies the variable's name and the type of information that it will hold
int total;
int count, temp, result;
Multiple variables can be created in one declaration
data type variable name
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Variable Initialization
• A variable can be given an initial value in the declaration
int sum = 0;int base = 32, max = 149;
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• When a variable is referenced in a program, its current value is used
• See PianoKeys.java
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//********************************************************************// PianoKeys.java Author: Lewis/Loftus//// Demonstrates the declaration, initialization, and use of an// integer variable.//********************************************************************
public class PianoKeys{ //----------------------------------------------------------------- // Prints the number of keys on a piano. //----------------------------------------------------------------- public static void main (String[] args) { int keys = 88; System.out.println ("A piano has " + keys + " keys."); }}
Output
A piano has 88 keys.
Assignment• An assignment statement changes the value of a
variable
• The assignment operator is the = sign
total = 55;
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• The value that was in total is overwritten
• You can only assign a value to a variable that is consistent with the variable's declared type
• See Geometry.java
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//********************************************************************// Geometry.java Author: Lewis/Loftus//// Demonstrates the use of an assignment statement to change the// value stored in a variable.//********************************************************************
public class Geometry{ //----------------------------------------------------------------- // Prints the number of sides of several geometric shapes. //----------------------------------------------------------------- public static void main (String[] args) { int sides = 7; // declaration with initialization System.out.println ("A heptagon has " + sides + " sides.");
sides = 10; // assignment statement System.out.println ("A decagon has " + sides + " sides.");
sides = 12; System.out.println ("A dodecagon has " + sides + " sides."); }} Output
A heptagon has 7 sides.A decagon has 10 sides.a dodecagon has 12 sides.
Constants: a special variable• A constant is like a variable except it holds the
same value during its entire existence
– i.e., it is constant, not variable
• The compiler will issue an error if you try to change the value of a constant
• To declare a constant in Java, we use final:
final int MIN_HEIGHT = 69;
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Why use constants?• Make unclear literal values more meaningful
– Example: MAX_LOAD means more than the literal 250
• Facilitate program maintenance– If a constant is used in multiple places, its value need
only be set in one place
• Formally define that a value should not change
– Avoids inadvertent errors by other programmers (or yourself!)
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Outline
Character Strings
Variables and Assignment
Primitive Data Types
Expressions
Data Conversion
Interactive Programs
Graphics
Applets
Drawing Shapes
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What types of values can we store in variables?
Characters• A char variable stores a single character,
delimited by single quotes:
'a' 'X' '7' '$' ',' '\n'
• Example declarations:
char topGrade = 'A';char terminator = ';', separator = ' ';
• How is declaring a character literal different from declaring a String?
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Boolean• A boolean value represents true or false
boolean done = false;
• Can be used to represent any two states, such as a light bulb being on or off
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Outline
Character Strings
Variables and Assignment
Primitive Data Types
Expressions
Data Conversion
Interactive Programs
Graphics
Applets
Drawing Shapes
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How can we combine variables and values to calculate new values?
Expressions• An expression is a combination of one or more
operators and operands• Arithmetic expressions use arithmetic operators:
AdditionSubtractionMultiplicationDivisionRemainder
+-*/%
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• If either (or both) operands are floating point values, then the result is a floating point value
Integer Division and Remainder• What does 7 / 2 equal in Java?
3.5 / 4 / 3• If both operands to division (/) are integers, the
result is an integer (the remainder is discarded)14 / 3 equals 48 / 12 equals 0
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• The remainder operator (%), modulus, returns the remainder after dividing the first operand by the second
14 % 3 equals 28 % 12 equals 8
Quick Check
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What are the results of the following expressions?
12 / 2
12.0 / 2.0
10 / 4
10 / 4.0
4 / 10
4.0 / 10
12 % 3
10 % 3
3 % 10
= 6
= 6.0
= 2
= 2.5
= 0
= 0.4
= 0
= 1
= 3
Evaluating Expressions & Operator Precedence
• Operators can be combined into larger expressions:
result = total + count / max - offset;
• Examples:4 + 10 / 2 = (4 + 10) / 2 =
• *, /, % evaluated before +, -
• Arithmetic operators with the same precedence are evaluated from left to right
– Use parentheses to force evaluation order• Expressions can contain method calls:
result = 2 * (circle.getArea() + square.getArea());
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Quick Check
a + b + c + d + e a + b * c - d / e
a / (b + c) - d % e
a / (b * (c + (d - e)))
1 432 3 241
2 341
4 123
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In what order are the operators evaluated in the following expressions?
Assignment Revisited• The assignment operator has a lower precedence
than the arithmetic operators
First the expression on the right handside of the = operator is evaluated
Then the result is stored in thevariable on the left hand side
answer = sum / 4 + MAX * lowest;
14 3 2
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Assignment Revisited• The right and left hand sides of an assignment
statement can contain the same variable
First, one is added to theoriginal value of count
Then the result is stored back into count(overwriting the original value)
count = count + 1;
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Increment and Decrement Shorthand• The increment (++) and decrement (--) operators
use only one operand
• The statement
count++;
is functionally equivalent to
count = count + 1;
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Increment and Decrement• The increment and decrement operators can be
applied in postfix form:
count++
• or prefix form:
++count
• When used as part of a larger expression, the two forms can have different effects
– x++: use in expression, then increment
– ++x: increment, then use in expression
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Created by Emily Hill
public class IncrementOperators{ //----------------------------------------------------------------- // Compares prefix and postfix increment operators //----------------------------------------------------------------- public static void main (String[] args) { int x = 0;
System.out.println("Postfix:");System.out.println("x : " + x);System.out.println("x++: " + x++);System.out.println("x : " + x);
x = 0;
System.out.println("\nPrefix:");System.out.println("x : " + x);System.out.println("++x: " + ++x);System.out.println("x : " + x);
}}
Output
Postfix:x : 0x++: 0x : 1
Prefix:x : 0++x: 1x : 1
Assignment Operators• Often we perform an operation on a variable, and
then store the result back into that variable
• Java provides assignment operators to simplify that process
• For example, the statement
num += count;
is equivalent to
num = num + count;
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Shorthand Assignment Operators• There are many assignment operators in Java,
including the following:
Operator
+=-=*=/=%=
Example
x += yx -= yx *= yx /= yx %= y
Equivalent To
x = x + yx = x - yx = x * yx = x / yx = x % y
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• If the operands to the += operator are strings, the assignment operator performs string concatenation
Assignment Operators• The right hand side of an assignment operator can
be a complex expression
• The entire right-hand expression is evaluated first, then the result is combined with the original variable
• Therefore
result /= (total-MIN) % num;
is equivalent to
result = result / ((total-MIN) % num);
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Outline
Character Strings
Variables and Assignment
Primitive Data Types
Expressions
Data Conversion
Interactive Programs
Graphics
Applets
Drawing Shapes
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How can we convert between different types of values, like from an int to a double?
Assignment Conversion• Assignment conversion: a value of one type is
assigned to a variable of another
• Example:
int dollars = 20;double money = dollars;
• Note that the value or type of dollars did not change
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Promotion• Promotion: when operators convert their operands
• Example:
int count = 12;double sum = 490.27;result = sum / count;
• Because sum is a double, the value of count is converted to a double to perform the division
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Casting: force a conversion• Casting – powerful (& dangerous!) conversion
technique
• To cast, the type is put in parentheses in front of the value being converted:
int total = 50;float result = (float) total / 6;
• Without the cast, the fractional part of the answer would be lost
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Homework• Work on Lab 2• Work on Codingbat exercises• Read Chapter 3
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