Intro to Java

1
Intro to Java

• Or, CS1 in one week or less!
• (part 1)

2
Java Classes

• In Java, everything is encapsulated
• no such thing as a non-member function
• Java program consists of a series of class
  descriptions
• A class is a blueprint for an object
• member fields (aka attributes, variables) store
  data
• methods (functions) specify operations

3
Class interactions

• A function call in Java is called a message
• One object interacts with another by sending and
  receiving messages
• sender is the client
• receiver is the server

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Java Code Example
import java.lang. public class
ProgEx1 public static void main(String
args) System.out.println (Obligatory first
example)
Outer set of brackets enclose the definition of
class ProgEx1
Inner set enclose the definition of method main( )
5
Editing compiling Java code

• Save file with a .java extension
• can use any text editor (e.g. NotePad)
• An IDE provides richer editing tools can use
  one specifically for Java, or use generic or
  different language editor (e.g. dev)
• Set up environment
• Javas \bin directory needs to be in your PATH
• Javas \lib directory needs to be in your
  CLASSPATH

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Compiling executing Java code

• To compile example from previous slide, type
  javac ProgEx1.java - if compilation is
  successful, will create ProgEx1.class
• To run the compiled code, type java ProgEx1

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Code organization

• Java source code files have .java suffix public
  classes must be in a file with the same name
• Compiled code has .class suffix
• No header files class methods are defined within
  the class definition, and import statements bring
  in classes from the Java API

8
Code organization

• A program consists of one or more classes in one
  or more files
• Can organize source code files into packages,
  which can be imported (analogous to, but
  different from, include and libraries in C)

9
Code organization

• Files can use classes from another file in 2
  different ways
• import class from another package - can then use
  name of class without prefix
• can use class directly (without import) by
  prefixing class name with package name

10
The big picture Java API

• The Java API, or Application Programming
  Interface, is analogous to the standard libraries
  of C
• The statement import java.lang. makes a package
  from the API (java.lang) visible to the class
  description that follows it
• This line requests that the code in java.lang be
  treated as part of this program

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When to use import statements

• A classs full name consists of its package name
  followed by its class name examples from the
  Java library include
• java.util.ArrayList (ArrayList is a class in the
  java.util package)
• javax.swing.JOptionPane (JOptionPane is a class
  in the javax.swing package)
• We use import statements to minimize the
  necessity of using a classs full name

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When to use import statements

• For example, to use the ArrayList class in a
  program without its prefix, include the
  statement
• import java.util.ArrayList // or
• import java.util.
• Second example above imports all classes from the
  java.util package
• Classes from the java.lang packages (including
  String and Math) can be used without prefix or
  import statement (so import java.lang. is
  really unnecessary and rarely used)

13
Closer look at Java code
Class definition consists of header public
class ProgEx1
And body // data fields methods
Note there is no semicolon at the end of the
class definition, as there would be in C
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Method definition header

• public static void main (String args)
• public and static are modifiers
• public is an access modifier, meaning this method
  can be seen by other objects - other options
  include private and protected
• static is a lifetime modifier static means only
  one instance of this method exists, and it is
  shared by all objects of this class - since it is
  independent of class instances, it exists even if
  there are none

15
Method definition header

• public static void main (String args)
• void is the return type, main is the method name
• the argument list is in parentheses the standard
  argument list for main is shown
• allows for command-line arguments
• args is an array of strings can access
  individual arguments using notation
• number of arguments is stored in attribute
  args.length

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Method definition body

• System.out.println(Obligatory first program)
• System is the name of one of the classes defined
  in the Java API (in package java.lang)
• out is a static variable defined in System
  outside their classes, static variables can be
  accessed by preceding them with class name
• println is a method associated with outs data
  type analogous to operator ltlt, with out
  analogous to cout

17
More on println method

• Prints one line of output to screen, then moves
  cursor to next line
• C equivalent would be something like
• stdcout ltlt Obligatory first example ltlt endl
• There is also a print method - works like
  println, but doesnt print the end of line
  character

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Typical structure of Java application

• Consists of collection of classes
• One class has main() method
• actually, any and all classes can have a main(),
  but only one per program is actually executed
• can include main() in any class for testing
  purposes

19
More typical example
public class Greeter public Greeter (String
aName) name aName public String
sayHello() return Hello, name
! private String name
Class features
Constructor creates new instances of class
Method(s) algorithm(s) applied to class objects
Field(s) objects data member(s)
20
Constructor in Java

• Invoked using operator new (unlike C, where
  declaration automatically invokes constructor)
• new returns a reference to a newly-created
  object, which is an instance of the class
• can use a variable to store the reference

21
Test program for Greeter class
public class GreeterTest public static void
main(String args) Greeter worldGreeter
new Greeter(world) // invokes constructor,
stores resulting object reference String
greeting worldGreeter.sayHello() System.out.p
rintln(greeting)
22
Java data types and their C counterparts

• Java
• byte (signed, 8 bits)
• short (signed, 16 bits)
• int (signed, 32 bits)
• long (signed, 64 bits)
• boolean (true/false)
• char (16 bits, Unicode)
• float (32 bits)
• double (64 bits)
• void
• String

• C
• char (sort of)
• int, short
• long, int
• long
• bool
• char (sort of - 8 bit ASCII)
• float
• double
• void
• string

Note string type is not primitive, built-in type
in either language
23
Variable declaration in Java

• Variables can be declared classwide or local to a
  method
• Identifiers follow similar rules as C
• name must start with a letter
• in Unicode, the symbol is considered a
  letter, so names can start with this
• Primitive type variables similar to variables in
  C

24
Variable declaration in Java

• Object array variables work differently
• they are references (think pointers) to the
  data-type-sized memory they can be associated
  with
• have to allocate memory before you can assign a
  value to them - this is done using the new
  operator with the constructor, as seen previously

25
Declaring manipulating simple variables

• Looks exactly like C - examples
• int x, y
• x 10
• y x 4
• Unless declared static, data variables are
  instance members
• tied in with individual instances of class or
  method
• every instance of the class has its own copy

26
Characters in Java

• Use 16-bit Unicode instead of 8-bit ASCII much
  richer character set (see www.unicode.org)
• Escape sequences similar to those in C e.g.
  \n is newline character, \t is horizontal
  tab, \b is backspace, \\ is backslash
  character
• Can use \uxxxx where x is a hex digit to denote
  an arbitrary Unicode character

27
Arithmetic operators in Java

• Same as C
• , /, , , -
• same meanings, same precedence, same
  associativity
• In Java, is defined for floating-point operands
• computes r a - (b q) where integer qlta/b and
  with the same sign
• for example, 6.9 1.2 .9 because .9 6.9 -
  (1.2 5)

28
Mixed-type expressions

• Similar to C
• lower types are promoted, result is highest
  type of operand in expression
• explicit cast is required to demote a data type
• Java also retains the combination
  arithmetic/assignment operators (, etc.) and
  pre and post increment and decrement (, --)

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Type conversions

• If no information loss is involved (converting
  short to int, for example), no explicit cast
  required
• Can convert char to int
• Can convert all integer types to float or double
  (even though information loss may be possible)
• Conversion of any floating point type to any
  integer type requires explicit cast
• Cant convert booleans to numbers

30
Javas Math class

• Implements several useful mathematical methods
• All methods are static, so they dont operate on
  objects, and must be invoked by referencing the
  class name
• Examples
• Math.sqrt(x) returns square root of double
  argument x
• Math.pow(x,y) returns xy (where x, y and result
  are all type double)

31
Relational/Logical Expressions

• Same relational operators as C
• gt, lt, gt, lt, ! and
• Same logical operators
• !
• Both and use short-circuit evaluation
• Java also has the ternary operator

32
Control Structures

• Syntax for while, for, do/while, if, if/else and
  switch are identical to the same structures in
  C
• As in C, brackets are required around blocks of
  code that contain more than one statement

33
Java vs. C 1

• Java classes look different because
• methods are defined inside class definitions (not
  in separate implementation file)
• everything is a class - including the driver
  class, which contains main( )
• Java programs can contain multiple instances of
  main() (but only one will execute)

34
Java vs. C 2

• Java has no pointers (at least none you can
  manipulate directly)
• You cant overload operators in Java
• You dont have to worry about manual garbage
  collection in Java
• There are no such things as structs, unions,
  enums, typedefs, friend functions, or templates

35
Java vs. C 3

• There is no default type for Java methods (unlike
  C, where functions are assumed to return int
  unless otherwise specified)
• Explicit casts are required to convert from int
  to char
• Modulus works on floating-point numbers
• Java doesnt have keyword const - can use final
  to declare constant data values
• final float PI 3.14159

36
Intro to Java

• Or, CS1 in one week or less!
• (part 1)