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CMSC 131 Spring 2009 Jan Plane (adapted from Bonnie Dorr)
Lecture Set 2:
Starting Java
1. Java Concepts
2. Java Programming Basics
3. User output
4. Variables and types
5. Expressions
6. User input
CMSC 131 Spring 2009 Jan Plane (adapted from Bonnie Dorr)
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This Course: Intro to Procedural
Programming using Java
Why Java?
Popular modern language
Used in web, business, telecom applications
Developed in 1990s, incorporates many features
from earlier languages
Object-orientation
Garbage collection
Portability of object code
CMSC 131 Spring 2009 Jan Plane (adapted from Bonnie Dorr)
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Portability of Object Code?
Object code is 2GL (assembly) / 1GL
(machine code)
Last time we said that 2GL / 1GL is
architecture-specific
How can Java have portable object code?
Answer: Java Virtual Machine (JVM)
CMSC 131 Spring 2009 Jan Plane (adapted from Bonnie Dorr)
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Java Virtual Machine
Java includes definition of Java bytecode = “fake” machine
code for Java
Java compilers produce Java bytecode
To run Java bytecode, must have bytecode interpreter (“Java
Virtual Machine”) on client machine
.java
Java compiler
.class
client
client
JVM
JVM
source code object code
CMSC 131 Spring 2009 Jan Plane (adapted from Bonnie Dorr)
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Facts about JVMs
For efficiency, JVMs often compile bytecode
into native machine code
There are also “native” Java compilers (these
compile Java directly to machine code)
CMSC 131 Spring 2009 Jan Plane (adapted from Bonnie Dorr)
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Method Headers
main is a method = “operation”
Operations require operands = data to work on
Operations return new data (result)
Header gives information on form of operands, result for
methods
For main:
Operand is collection of Strings
Result is “void” (= unimportant)
More later on “public”, “static”
Every program must have exactly one “main”
method (where execution begins)
CMSC 131 Spring 2009 Jan Plane (adapted from Bonnie Dorr)
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Holding and calculating values
variables
declaration
initialization
assignment
value use
mathematical expressions
calculated to take on a value
based on values of literals and variables
CMSC 131 Spring 2009 Jan Plane (adapted from Bonnie Dorr)
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Java Program Organization
Methods
Where most computation takes place
Each method has a name, a list of arguments enclosed in
(…), and body (collection of statements ) in {…}
public static void main( String[ ] args ) { … (contents of the main method go here) … }
Variables
Storage locations that program can operate on
Variables can store data of different forms (integers, for
example)
int secondsPerMinute = 60; int minutesPerLecture = 50;
CMSC 131 Spring 2009 Jan Plane (adapted from Bonnie Dorr)
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Java Program Organization
Statements: Many different types
Declarations – specify variable types (and optionally initialize)
int x, y, z; // three integer variables String s = “Howdy”; // a character string variable boolean isValid = true; // a boolean (true/false) variable
Assignments – assign variables new values
x = 13;
Method invocation – call other methods
System.out.println( “Print this message“ );
Control flow – determine the order of statement execution.
(These include if-then-else , while , do-while , for. More later.)
Built-in Operators: For manipulating values (+, -, *, /, etc.)
i.e. String Concatenation for output
CMSC 131 Spring 2009 Jan Plane (adapted from Bonnie Dorr)
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Built-in (Primitive) Types
Type name Size (bytes)
Integers
byte 1
short 2
int 4
long 8
Reals
float 4
double 8
Other
char 2
boolean 1
CMSC 131 Spring 2009 Jan Plane (adapted from Bonnie Dorr)
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String Type
Elements of String type are sequences of
characters
“abc” “Call me Ishmael” etc.
String type is not built-in
We will use it a lot
Useful operation:
concatenation (+)
“abc” + “def” is equivalent to “abcdef”
CMSC 131 Spring 2009 Jan Plane (adapted from Bonnie Dorr)
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Writing Programs in Java
1. EXPRESSIONS: computations that carry a value
2. OPERATORS: symbols like +, *, -, etc.
3. Statements end with a semicolon
4. Types of statements:
a) DECLARATION (where a variable is created)
b) ASSIGNMENT (where a variable is given a value)
c) METHOD INVOCATIONS (where another method is called)
d) others - later
5. You can put blank lines in almost anytime you want
1. except not in the middle of an identifier or a keyword
2. and except not in a set of quotation marks
6. Proper indenting helps readability
CMSC 131 Spring 2009 Jan Plane (adapted from Bonnie Dorr)
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Primitive-Type
Literals/Constants
Constants are also called literals
Integer types: byte short optional sign and digits (0-9): 12 -1 +234 0 1234567 int long Same as above, but followed by ‘L’ or ‘l’: -1394382953L
Floating-point types : double Two allowable forms:
Decimal notation : 3.14159 -234.421 0.0042 -43.
Scientific notation : (use E or e for base 10 exponent) 3.145E5 = 3.145 x 10^5 = 314500.
1834.23e-6 = 1834.23 x 10-6^ = 0. float Same as double, but followed by ‘f’ or ‘F’: 3.14159F -43.2f
Note : By default, integer constants are int , unless ‘L’/‘l’ is used to indicate they are long. Floating constants are double , unless ‘F’/‘f’ is used to indicate they are float.
Avoid this lowercase L. It looks too much like the digit ‘1’
CMSC 131 Spring 2009 Jan Plane (adapted from Bonnie Dorr)
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Character and String
Constants
Char constants : Single character in single quotes (‘…’) including: Letters and digits : ‘A’, ‘B’, ‘C’, …, ‘a’, ‘b’, ‘c’, …, ‘0’, ‘1’, …, ‘9’ Punctuation symbols : ‘*’, ‘#’, ‘@’, ‘$’ (except ‘ and backslash ‘\’) Escape sequences : (see below) String constants : 0 or more characters in double quotes (“…”) Escape sequences : Allows inclusion of special characters: \” double quote \n new-line character (start a new line) \’ single quote \t tab character \ backslash Examples : char x = ’\’’; → (x contains a single quote) String s1= ”\”Hi there!\””; → s1 contains ”Hi there!” String s2= ”C:\WINDOWS”; → s2 contains C:\WINDOWS
CMSC 131 Spring 2009 Jan Plane (adapted from Bonnie Dorr)
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Common Numeric Operators
Arithmetic operators : Unary negation: -x Addition/subtraction: x+y x-y
Multiplication/division: x*y x/y Division between integer types truncates to integer: 23/4 → 5 x%y returns the remainder of x divided by y: 23%4 → 3 Division with real types yields a real result: 23.0/4.0 → 5.
Comparison operators : Equality/inequality: x == y x != y
Less than/greater than: x < y x > y Less than or equal/greater than or equal: x <= y x >= y
These comparison operators return a boolean value: true or false.
CMSC 131 Spring 2009 Jan Plane (adapted from Bonnie Dorr)
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Common String Operators
String Concatenation : The ‘+’ operator concatenates (joins) two strings. “Go” + “Terps” →→→→ “GoTerps”
When a string is concatenated with another type, the other type is first evaluated and converted into its string representation.
(84) + “degrees”* →→→→ “32degrees” (1 + 2) + “ 5 ” →→→→ “ 35 ”
String Comparison : Strings have special comparison functions. s.equals(t) : returns true if s and t have the same characters. s.compareTo(t) : compares strings lexicographically (dictionary order) result < 0 if s precedes t result == 0 if s is equal to t result > 0 if s follows t
“dilbert”.compareTo( “dogbert” ) →→→→ -1 (which is < 0)
Note: Concatenation does not add any space
Both functions are case-sensitive.
CMSC 131 Spring 2009 Jan Plane (adapted from Bonnie Dorr)
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User Input in Java
We've done output (System.out); what about
input?
Java 5.0 includes the Scanner class feature
Can use Scanner to create “scanner objects”
Scanner objects convert user input into data
To use Scannner need to import a library:
import java.util.Scanner;
CMSC 131 Spring 2009 Jan Plane (adapted from Bonnie Dorr)
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Scanner Class Details
To create a scanner object: new Scanner( input_source ); Input source can be keyboard (System.in), files, etc. Object must be assigned to a variable (e.g. sc) Operations nextBoolean() nextByte() nextDouble() nextFloat() nextInt() nextLong() nextShort()
next() Returns sequence of characters up to next whitespace (space, carriage return, tab, etc.)
nextLine() Returns sequence of characters up to next carriage return
Returns value of indicated type (reports error if type mismatch
