1. Lecture 3 Java Basics
Lecture3.ppt

2. Keywords abstract continue for new switch assert default goto package
synchronized
boolean do if
private
this
break
double
implements
protected
throw
byte
else
import
public
throws
case
enum
instanceof
return
transient
catch
extends
int
short
try
char
final
interface
static
void
class
finally
long
strictfp
volatile
const
float
native
super
while **
Lecture3.ppt

3. Identifiers Same as C++ Can’t be keywords Case-sensitive
Begin with and consist of:
Letters (a... Z)
Numbers (0…9)
Underscore (_)
Dollar sign ($)
Same as C++
Lecture3.ppt

4. Primitive Types boolean 8 bits char 16 bits byte 8 bits short 16 bits
int 32 bits
long 64 bits
float 32 bits
double 64 bits
Guaranteed to occupy same number of bits regardless of platform
boolean – zero and non-zero DO NOT equate to true/false respectively
unicode.org
Lecture3.ppt

5. Literals boolean int long float double char true or false
zero and non-zero do NOT equate to true/false
int
long
ends with “L”
65L L
float
ends with “f” or “F”
6.23f F 1e-32f
double
2.0146e
char
Contained in single quotes
‘b’ ‘&’ ‘*’
Escape sequences (similar to c++)
‘\”’ ‘\n’ ‘\t’ ‘\’’
Lecture3.ppt

6. String Literals Enclosed in double quotes (“) Concatenating Strings
“USNA” “This is a string literal” “A”
Concatenating Strings
“United” + “ States” + “ Naval” + “ Academy”
Like C++, String is class not a primitive data type
Lecture3.ppt

7. Strings Java defines the String class to handle strings
A String is a collection of characters treated as a single unit
It is not an array of char variables
Multiple String constructors
String s1 = new String(“Hello World”);
String s2 = “Hello World”;
Lecture3.ppt

8. String Example public class Strings {
public static void main (String[] args) {
String m = "was a Roman";
String c = "Cicero " + m;
System.out.println(c);
String s = "Java is hot!";
s = 'L' + s.substring(1);
System.out.println(s); }
What happened to the String “Java is hot!”
Lecture3.ppt

9. Basic Operators Arithmetic Operations in Java
Addition, subtraction, multiplication, division, modulus: (+, -, *, /, %)
Equality and Relational Operators
Equality, not equals, greater than, greater than or equal, less than, less than or equal: (==, !=, >, >=, <, <=)
Logical Operators
Binary operators
logical AND: && (two boolean operands w/ short circuit)
logical AND: & (two boolean operands no short circuit)
bitwise AND: & (two integer operands)
logical OR: || (two boolean operands w/ short circuit)
logical OR: | (two boolean operands no short circuit)
bitwise OR: | (two integer operands)
logical exclusive OR: ^ (two boolean operands)
bitwise exclusive OR: ^ (two integer operands)
Unary operator
logical NOT: ! (single boolean operand)
bitwise NOT: ~ (single integer operand)
Lecture3.ppt

10. Assignment Operators += c += 7 c = c + 7 10 to c -= d -=4 d = d – 4
1 to d *=
e *= 5
e = e * 5
20 to e /=
f /= 3
f = f / 3
2 to f %=
g %= 9
g = g % 9
3 to g
int c = 3, d = 5, e = 4, f = 6, g = 12;
Lecture3.ppt

11. Increment and Decrement Operators++
preincrement
++a
Increment a by 1 then use the new value
postincrement
a++
Use the current value of a then increment by 1 --
predecrement
--b
Decrement b by 1 then use the new value
postdecrement
b--
Use the current value of b then decrement by 1
Lecture3.ppt

12. Casting
Automatic promotion works in Java, but not demotion…need explicit cast to demote:
double d = 3.2;
int I = (int)d;
int j = 4;
d = j;
double e = (double)i/(double)j;
Code demonstration
Lecture3.ppt

13. Control Structures Sequential execution is the default (like C++)
Control structures can be used to alter this sequential flow of execution (same as C++)
Selection structures – if/switch statements
Repetition structures – while/for/do-while statements
Lecture3.ppt

14. Selection Structures Generally the same as C++
The if selection structure
Only difference with C++ is that the condition must be a Boolean expression evaluating to true or false
Zero/non-zero can NOT be used for Java conditions
The following is illegal
int flag = 10;
if (flag) { //illegal condition
System.out.println("Flag was non-zero"); }
The if/else selection structure
Same as C++ with the above restriction on the condition portion
The conditional operator (?:)
Same as C++, but must be a Boolean expression
System.out.println(grade >= 60 ? "Passed" : "Failed");
The switch statement
Identical to C++ switch statement
Lecture3.ppt

15. Repetition Structures
The while repetition structure
Same as C++ with similar restriction to if statement regarding the condition portion (i.e. must evaluate to either true or false)
The do/while structure
Same as C++, but condition must be a true or false expression
The break and continue Statements
Same basic meaning as C++
break
Can only occur within the body of a loop, or a switch statement
Execution exits the innermost enclosing loop or switch block and continues with the next statement following the block
continue
Can only occur within the body of a loop
Terminates the current iteration of the loop without exiting the loop body
Lecture3.ppt

16. Console I/O
Three standard streams in Java included as part of java.lang
System.in – InputStream defaults from the keyboard
System.out – PrintStream defaults to the screen
System.err – PrintStream defaults to the screen
Scanner
A simple text scanner which can parse primitive types and strings
A Scanner breaks its input into tokens using a delimiter pattern, which by default matches whitespace
The resulting tokens may then be converted into values of different types using the various next methods
Resides in the java.util package
Wrap a Scanner around an InputStream, i.e., System.in to read console input (keyboard) by Scanner input = new Scanner(System.in);
A Scanner can also be wrapped around a File object to read from a text file…more on this later.
Lecture3.ppt

17. Keyboard input with Scanner
Instantiate a Scanner
Scanner myScanner = new Scanner(System.in);
Read an entire line of text
String input = myScanner.nextLine();
Read an individual token, i.e., int
int i = scanner.nextInt();
What if next input isn’t an int?
Draw a scanner using a keyboard.
Lecture3.ppt

18. Scanner Example import java.util.*; public class ScannerDemo {
public static void main(String[] args) {
int age;
String name;
Scanner myScanner = new Scanner(System.in);
System.out.println("Enter first and last name: ");
name = myScanner.nextLine();
System.out.println("How old are you? ");
age = myScanner.nextInt();
System.out.println(name + '\t' + age); }
Run with basic code
* Don’t open more than one Scanner with System.in as it’s inputStream…will cause problems
Lecture3.ppt

19. Scanner Example import java.util.*; public class ScannerDemo {
public static void main(String[] args) {
int age;
String first;
String last;
Scanner myScanner = new Scanner(System.in);
System.out.println("Enter first and last name: ");
first = myScanner.next();
last = myScanner.next();
System.out.println("How old are you? ");
age = myScanner.nextInt();
System.out.println(last +", " + first + '\t' + age); }
Change nextLine() to next()
Lecture3.ppt

20. Input Errors
What happens if the user doesn’t enter an integer when asked for the age?
There are a couple of ways to handle it
Look ahead to see if the user entered an integer before we read it or
Read the input and handle the resulting error
Show error if age not entered as an int (double or string)
Lecture3.ppt

21. Look Ahead
Scanner provide the ability to look at the next token in the input stream before we actually read it into our program
hasNextInt()
hasNextDouble()
hasNext()
etc…
if (myScanner.hasNextInt()) {
age = myScanner.nextInt(); }
else
age = 30;
String junk = myScanner.next();
Lecture3.ppt

22. Input Exceptions (errors)
What happens when we try to read an integer (myScanner.nextInt()) and the user enters something different?
Java “throws” and exception, i.e., issues and error message.
We can “catch” the errors and handle them, thereby preventing the program from crashing
try {
age = myScanner.nextInt(); }
catch(InputMismatchException e)
age = 30;
Show error in java docs!!
The InputMismatchException is part of the java.util library so we must import java.util.InputMismatchException or java.util.* in order to catch the exception.
Lecture3.ppt