13 x 11 java lecture 2 cs 1311 beginning java (sort of boring but oh so necessary syntax stuff) 13 x...
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Java Lecture 2
CS 1311
Beginning Java
(Sort of Boring
but Oh So Necessary Syntax Stuff)
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Ever have one of those days...
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Books
• Thinking in Java -- Bruce Eckel– Free on the web [search for Eckel]– Also available in paper
• Java in a Nutshell -- David Flanagan (O'Reilly)– Mostly reference– Actually 3 books
• Kurt: • Just Java 2 (4th Edition) Peter van der Linden
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Online
• http://www.javasoft.com
• Tutorial is excellent
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Beginning Java
• Identifiers• Comments• Keywords• Data types• Expressions and operators• Operator precedence• The assignment statement• Organizing code• Selection statements• Iteration statements
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Structure
• Java "programs" consist of files containing classes.
• The classes will consist of variable declarations and modules which will be called methods (and constructors)
• All of this is written in a language quite different in appearance from Scheme.
• If Scheme is your first programming experience then things may seem backwards at first
• If you have prior experience with other languages it may seem familiar but watch out!
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Identifiers
• Used to name things– Variables– Methods– Classes
• CaSe SeNsItIvE• Rules
– Underscore or letter– 0 or more underscores, letters, numbers– Any length!
Identifiers• Style
– Class names capitalized– Variables and method names start with lower case letter.
If multi-word capitalize all but first.– Constants in all caps
• Examples
class Widget
class LinkedList
Variables: x, n pressure, isEmpty,
hasMore, steamTemp
Constants: FEETPERMILE, HOURSPERWEEK
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Comments
// until end of line
/* This is a multiline comment
it continues for
* as many lines as I want
* these stars mean nothing!
*
and now for the end -->>> */
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Careful
/*
Comments about this section
yada - yada - yada
*/
x = 3;// Here is some
y = 4; // info about
z = 5; // these lines
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Careful
/*
Comments about this section
yada - yada - yada
// (note) */
x = 3;// Here is some
y = 4; // info about
z = 5; // these lines
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Careful
/*
Comments about this section
yada - yada - yada
x = 3;// Here is some
y = 4; // info about
z = 5; // these lines
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Keywords
abstract default if private throwboolean do implements protected throwsbreak double import public
transientbyte else instanceof return trycase extends int short voidcatch final interface static
volatilechar finally long super whileclass float native switchconst* for new synchronizedcontinue goto* package this
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Data types
• What is data typing?• Why is it used?• What's good about it?• What's bad about it?• Java is considered a strongly typed language.
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Java Primitive Data Types
• boolean -- true/false• byte• short• int• long• float• double• char -- typically used for printable characters
Whole Numbers
Fractional Numbers (with decimal digits)
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Java Primitive Data Types
• boolean -- true/false• byte• short• int• long• float• double• char -- typically used for printable characters
Whole Numbers
Fractional Numbers (with decimal digits)
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Primitive Data Type RangesType Size Min Default
boolean false
Max
1 false* true*
char '\u0000' (null)16
byte (byte) 08 -128 127
short (short) 016 -32,768 32,767
int 032 -2,147,483,648 2,147,483,647
long 0L64 -9,223,372,036,854,775,808 9,223,372,036,854,775,807
float 0.0F32 Approx ±3.4E+38 with 7 significant digits
double 0.0D64 Approx ±1.7E+308 with 15 significant digits
void
* Not truly min and max.
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Declarations
int i;
int a, b, c;
double d = 42.0;
float f = 38.0; /* ERROR */
float f = 38.0F;
char ch1;
char ch2 = 's';
boolean isFull = true;
boolean isFull;
isFull = true;Equivalent?
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Constants
public static final int FEETPERMILE = 5280;
public static final String MYGTNUM="gt1234x";
public static final boolean DEBUG = true;
public static final int JAN = 1;
public static final int FEB = 2;
public static final int MAR = 3;
etc.
if(DEBUG)
{
/* Do Something */
}
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Mysterious Strings
• Strings are objects• They are different from every other language!!!• Don't use strings to figure out how Java works
– They have all kinds of special rules
String someStr = "This is a string";
someStr = "Strings can be " + "concatenated.";
someStr = "If they won't fit on one line " +
"you need to do it like this";
someStr = "They can be printed";
System.out.println(someStr);
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References
• Variables which hold a "reference" or "pointer" to an object.
• Can be thought of as the address of an object in memory
• Normally don't manipulate (like in C)– So no need for address of/dereferencing operators
<Type i.e. name of some class> <identifier>;
String name;
Queue myQueue;
CokeMachine cc;
• More later...
Can also initialize in same statement
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Expressions
• Unlike Scheme, expressions in Java don't have to return a value.
• Java uses infix notation so(and (< 25 (+ (* x x) (* y y))) (> x 0))
• becomes(x * x + y * y < 25) && (x > 0)
• Note: && means "AND" not the same as &
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Operators
Arithmetic: + - * / %
Relational: > < == <= >= !=
Logical: && || !
Assignment: =
+= -= *= /= &= |=
^= %= <<= >>= >>>=
Conditional: ? :
Bit manipulation: & | ~ ^ << >> >>> Funky (Inc/Dec): ++ --
SchemePredicates
SchemePredicates
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Operators
Arithmetic: + - * / %
Relational: > < == <= >= !=
Logical: && || !
Assignment: =
+= -= *= /= &= |=
^= %= <<= >>= >>>=
Conditional: ? :
Bit manipulation: & | ~ ^ << >> >>> Funky (Inc/Dec): ++ --
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Operator Precedence
1 + 2 * 3• When in doubt use parentheses• (Don't tell Gosling)
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The following table shows the precedence assigned to the operators. The operators in this table are listed in precedence order: the higher in the table an operator appears, the higher its precedence. Operators with higher precedence are evaluated before operators with a relatively lower precedence. Operators on the same line have equal precedence.
postfix operators [] . (params) expr++ expr-- unary operators ++expr --expr +expr -expr ~ ! creation or cast new (type)expr multiplicative * / % additive + - shift << >> >>> relational < > <= >= instanceof equality == != bitwise AND & bitwise exclusive OR ^ bitwise inclusive OR | logical AND && logical OR || conditional ? : assignment = += -= *= /= %= &= ^= |= <<= >>= >>>=
When operators of equal precedence appear in the same expression, a rule must govern which is evaluated first. All binary operators
except for the assignment operators are evaluated in left-to-right order. Assignment
operators are evaluated right to left.
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The assignment statementx = 1 + 1;
• Evaluate expression on right side• Place result in variable on left side• Left side must be capable of storing a value
3 = x; // Bad idea!
• End assignment statements with a ;• Legal?
a = b = c = 0; • Good idea?• = not ==
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The Classic Duality
• Programming languages have always wrestled with the difference between assigning a value and the equality relational operator
• Programming languages have always wrestled with the difference between assigning a value and the equality relational operator
• Equality (Boolean Result)– BASIC A = B– Pascal A = B– FORTRAN A .EQ. B– C A == B– Pseudocode A = B– Scheme (= A B)– Java A == B
• Equality (Boolean Result)– BASIC A = B– Pascal A = B– FORTRAN A .EQ. B– C A == B– Pseudocode A = B– Scheme (= A B)– Java A == B
• Assignment– BASIC LET A = B– Pascal A := B– FORTRAN A = B– C A = B– Pseudocode A B– Scheme (set! A B)– Java A = B
• Assignment– BASIC LET A = B– Pascal A := B– FORTRAN A = B– C A = B– Pseudocode A B– Scheme (set! A B)– Java A = B
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Common Errors
• Misuse of ++ or --
• Confusing = and ==
• Using
& instead of &&
| instead of ||
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Warning!
• Just because a language will let you do something doesn't mean it's a good idea
_ += _-- - ++_;
/* Clear? */
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Organizing code
• Block statements– Surround statements with curly braces
{
int i;
x = 1;
y = 2;
i = x + y;
}– Can be used in place of a single statement– Note: variables declared in a block have scope only in
the block!
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Organizing code
• Null statement
– Just a lone semicolon ;
– Not necessarily your friend!
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Selection statements
• Used to control flow of program• If• If with Else• Complex if constructions
• Switch
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if
if(<boolean_value>)
<Executed if boolean is true>
<Executed in any case>
if(x > 0)
System.out.println("X greater than zero!");
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if
if(y > 0);
System.out.println("Always gets printed!");
if(z > 0 && isEmpty)
{
x = y;
System.out.println("Hello");
}
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if/elseif(<boolean_value>)
<Executed if boolean is true>
else
<Executed if boolean is true>
<Executed in all cases>
if(a == b)
{
System.out.println("Always use curlies");
}
else
{
System.out.println("What do you think?");
}
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if/else -- Another styleif(a == b) {
System.out.println("Always use curlies");
} else {
System.out.println("What do you think?");
}
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Complex
if(a == b) {
System.out.println("A equals B");
} else {
if(c == d) {
System.out.println("C equals D");
} else {
if(e == f) {
System.out.println("E equals F");
} else {
System.out.println("None");
}
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Complex
if(a == b)
System.out.println("A equals B");
else if(c == d)
System.out.println("C equals D");
else if(e == f)
System.out.println("E equals F");
else
System.out.println("None");
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Complexif(a == b){
System.out.println("A equals B");}else if(c == d){
System.out.println("C equals D");}else if(e == f){
System.out.println("E equals F");}else{
System.out.println("None");}
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Complex
if(a == b)
System.out.println("A equals B");
else if(c == d)
System.out.println("C equals D");
else if(e == f)
System.out.println("E equals F");
else
System.out.println("None");
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Complex
if(a == b)
System.out.println("A equals B");
else if(c == d)
System.out.println("C equals D");
else if(e == f)
System.out.println("E equals F");
else
System.out.println("None");
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Complex
if(a == b)
System.out.println("A equals B");
else if(c == d)
System.out.println("C equals D");
else if(e == f)
System.out.println("E equals F");
else
System.out.println("None");
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Nestingif(a == b) {
if(c == d) {
/* do something here */
} else {
/* do something else */
}
} else {
if(x == y) {
/* what to do? */
} else {
/* another option */
}
}
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Nestingif(a == b) {
if(c == d) {
/* do something here */
} else {
/* do something else */
}
} else {
if(x == y) {
/* what to do? */
} else {
/* another option */
}
}
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Switch
• Used when there is a single value to test– basically an int
• and multiple "cases"
• Common error: forgetting break
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Switchswitch (month) {
case APR:
case JUN:
case SEP:
case NOV:
numDays = 30;
break;
case FEB:
if(year % 4 == 0)
numdays = 29;
else
numdays = 28;
break;
default:
numdays = 31;
} Any potential problems?
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Iteration statements
• while loop– Tests at beginning of loop– Executed 0 or more times
• do while loop– Tests at end of loop– Executed at least once
• for loop– Equivalent to while loop– Very popular!
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While Loop
while(<boolean_Value>)
<statement>
Example:
i = 0;
while(i < 10)
i++;
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While Loop
int i = 1;
int total = 0;
while(i <= 10)
{
total += i;
i++;
}Note: No ;
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Do While Loop
do
<statement>
while(<boolean>);
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Do While Loop
int choice;
do
{
choice = menu();
switch(choice)
{
/* details omitted */
}
} while(choice != 0);Note
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For Loop
for(<init>; <testExpr>; <increment>)
<statement>
<init> and <increment> can be multiple statements separated by commas
<init>, <testExpr> and <increment> are all optional!
for(;;) // Legal statment
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For Loopfor(i=0; i < MAX; i++)
System.out.println(i);
for(i=0; i < MAX; i++)
{
System.out.println(i);
}
System.out.println(i);
for(int i=0; i <= MAX; i++)
{
System.out.println(i);
}
System.out.println(i);
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For Loopfor(i=0, total=0; i < MAX; i++)
System.out.println(i);
for(i=0; i < MAX && something; i++);
{
System.out.println(i);
}
for(int i=0, j=10; i <= MAX; i++,j--)
{
System.out.println("Sum = " + i + j);
}
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while -- for equivalence
total = 0;
i = 0
while(i < MAX)
{
/* Work */
total += i;
i++i++;
}
total = 0;
for(i = 0; i < MAX; i++i++)
{
/* Work *//* Work */
total += i;total += i;
}
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Questions?
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