Computer Science A 15: 17/4

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Computer Science A 15 17/4

CS A
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Today

• Overview of java
• How to decribe programming languages
• Main message
• The core of java is small
• Even a small language can be tricky

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Lexical rules

• Lexical analysis split the text into words
• Rule split at the longest legal sequence
• Spaces are almost always ignored
• You can have spaces but not line breaks in text
  strings
• x????x or x??x
• int?x int?intx

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A Java Program

• public class A
• public static void main(String args)
• System.out.println(Hello)
• or
• public?class?A??public?static?void?
• main?(?String? ?args?)??System?.?out?
• .?println?(?Hello?)?? ?

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Java Language Specification

• A detailed definition of the the programming
  language. It describes all language constructions
  in java, syntax, semantics og pragmatics
• http//java.sun.com/docs/books/jls/index.html
• In the following I present an extract of
  approximately 2/3 of Java the parts you are
  likely to use. I present the syntax in EBNF

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EBNF

• Extended Backus Naur Form
• Meta language used to describe programming
  languages
• text repeat text 0 or more times
• text include text 0 or 1 time
• ( text1 text 2 ) either use text1 or text2
• Use nonterminals to allow recursive descriptions
• Exp is a nonterminal are describe balanced
  expressions
• Exp Exp

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Identifiers and numbers

• Identifier Letter digit letter _
• VarName Identifier
• TypeName Name
• ClassName Name
• Number digit digit l L
• Char character
• String character
• character \r \n \b \f \t
  \ \ \\ .

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Simple expressions

• Primary ( Expression )
• this Arguments
• super Arguments
• new Identifier Expression
  Expression
• new Identifier Arguments
• Identifier . Identifier Arguments
• Number
• String
• Char
• Arguments ( Expression , Expression
  )

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Expressions

• Expression Expression1 AssignmentOperator
  Expression1
• AssignmentOperator -
  / ltlt gtgt
  gtgtgt
• Expression1 Expression2 ? Expression
  Expression1
• Expression2 Expression3 Infixop Expression3
  
• Expression3 instanceof Type
• Infixop
  ! lt gt lt gt ltlt gtgt
  gtgtgt - /
• Expression3 PrefixOp Expression3
• ( Type ) Expression3
• Primary Selector PostfixOp
• Selector . Identifier Expression
• PrefixOp -- !
  -
• PostfixOp --

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Types, etc.

• Type
• Identifier . Identifier BracketsOpt
• BasicType
• BasicType byte short char int
  long float double boolean
• Catches CatchClause CatchClause
• CatchClause catch ( FormalParameter )
  Block
• ForInit Expression , Expression
• final Type VariableDeclarator ,
  VariableDeclarator
• ForUpdate Expression , Expression
• SwitchBlockStatementGroups SwitchLabel
  Statement
• SwitchLabel case Expression default
  

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Statements

• Statement Block
• if (Expression ) Statement else
  Statement
• for ( ForInit Expression
  ForUpdate ) Statement
• for ( Type Identifier Expression )
  Statement
• while (Expression ) Statement
• do Statement while (Expression )
• try Block ( Catches Catches finally
  Block )
• switch (Expression ) SwitchBlockStatement
  Groups
• synchronized (Expression ) Block
• return Expression
• throw Expression
• break Identifier
• continue Identifier
• Expression
• Identifier Statement

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Methods consist of statements

• Block BlockStatement
• BlockStatement
• final Type VariableDeclarator ,
  VariableDeclarator
• Identifier Statement
• VariableDeclarator
• Identifier BracketsOpt
  VariableInitializer
• BracketsOpt
• FormalParameters
• ( FormalParameter , FormalParameter
  )
• FormalParameter Type Identifier BracketsOpt

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A class consists of methods

• ClassBody ClassBodyDeclaration
• ClassBodyDeclaration
• static Block
• Modifier MemberDecl
• MemberDecl
• void Type Identifier FormalParameters
  BracketsOpt
• throws IdentifierList ( Block )
• Type Identifier BracketsOpt
  VariableInitializer
• VariableInitializer
• VariableInitializer ,
  VariableInitializer ,
• Expression

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A program consists of classes

• CompilationUnit
• package QualifiedIdentifier
• ImportDeclaration ClassDeclaration
• ImportDeclaration import Identifier .
  Identifier .
• ClassDeclaration
• Modifier class Identifier extends
  Type
• implements TypeList ClassBody
• Modifier public protected private
  static abstract final native
  synchronized transient volatile
  strictfp

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Syntax tree

• Expression x x

expression
expression1
expression2
expression3
expression3
infixop

expression3
primary
prefixop
postfixop

identifier
primary

x
identifier
x
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Example

• class Point
• int x int y
• Point(int x,int y)
• this.xx this.yy
• public String toString()
• return "("x","y")"
• class Point3D extends Point
• int z
• Point3D(int x,int y,int z)
• super(x,y) this.zz
• public String toString()
• return "("x","y","z")"

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Example

• Point anew Point(2,4)
• System.out.println(a.toString())
• Point3D bnew Point3D(7,9,13)
• System.out.println(b.toString())
• A Point3D is-a Point object and may be used as
  a Point object

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Basics

• class A
• int f()return 2
• class B extends A
• int f()return 3
• ..
• A a1new A() int ia1.f() // ok, i2
• A a2new B() int ia2.f() // ok, i3
• B b1new A() // not ok
• B b2new B() int ib2.f() // ok, i3

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Cast an instanceof

• class A ..
• class B extends A ..void g()..
• A a new B() // ok
• a.g() // not ok
• If(a instanceof B)
• B b (B) a // cast
• b.g() // ok

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Access control

• Java has four levels of controlling access to
  fields, methods, and classes
• public access
• Can be accessed by methods of all classes
• private access
• Can be accessed only by the methods of their own
  class
• protected access
• Access from subclasses
• package access
• The default, when no access modifier is given
• Can be accessed by all classes in the same
  package
• Good default for classes, but extremely
  unfortunate for field

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Subtyping and generics

• Tst1 t1new Tst2()
• t1.f(1) t1.f(1.0) // Double 1.0 Double
  1.0
• Tst2 t2new Tst2()
• t2.f(1) t2.f(1.0) // Int 1.0 Double
  1.0
• class Tst1
• void f(double d)System.out.println("double
  "d)
• class Tst2 extends Tst1
• void f(double d)System.out.println("Double
  "d)
• void f(int i)System.out.println("Int "i)

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Avoid finally

• int i0
• try
• iInteger.parseInt(
• JOptionPane.showInputDialog("input"))
• catch(NumberFormatException e)
• System.out.println("bad")
• iInteger.parseInt(
• JOptionPane.showInputDialog("input"))
• finally
• System.out.println("typed "i)
• System.out.println("typed "i)

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More features

• Arrays and ArrayList
• Inheritance
• Methods, recursion
• Nested loops
• Libraries util, io, awt, swing