Introduction to Java Part 4

1
Introduction to Java Part 4

• Informatik III

2
Variables
3
Instance Variables

• Instance variables are defined in the class and
  are available for all methods
• this. allows to access instance variables
• Example
• public class Name
• public String
  firstName
• public String
  lastName

4
Local variables

• Formal parameters of methods, and variables
  declared inside the bodies of methods, are local
  variables.
• These are a third kind of variable in Java they
  are neither instance variables or class
  variables.
• Only available in the defining block which is
  indicated via

5
Class Variables

• Besides instance variables, a class may contain
  global variables that are not associated with
  any instance. These variables are the same for
  all classes / instances.
• A class variable (also called a static variable)
  is flagged by the static modifier in its
  declaration
• class Potato
• public String name
• static public int num 0
• //
  Class variablenumber of potatoes.

6
Modifiers
7
Access Modifiers
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Access Modifiers (for methods, fields / classes)

• For this training all instance variables should
  be private and all classes and methods should be
  public

public class Math private int number
public int getNumber() return number

9
Addional Modifiers for fields

• Additional possible field Modifiers are
• staticthis is a class variable see earlier.
• finalthis field cannot be modified after it is
  initialized.
• transientthe value of this field will not be
  included in a serialized representation of an
  instance.
• volatileany cached copy of the field maintained
  by an individual thread will be reconciled with
  the master copy every time the field is accessed

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Additional modifiers of Classes

• Additional class modifiers are
• defaultthe class may be used by code in current
  package
• abstractthe class contains abstract methods
  without implementation (abstract classes will
  have subclasses that define implementation of
  methods).
• finalthis class cannot have a subclass
• strictfpall intermediate results in all float or
  double expressions appearing in the class have
  strict IEEE 754 exponents.
• staticdefines that the class is static, e.g.
  class can be accessed without an instance

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Additional modifiers of Methods

• Additional method modifiers are
• abstractthe method has no implementation
  heredeclaration has a semicolon in place of a
  body.
• staticthis is a class method see earlier.
• finalthis method cannot be overriden see later.
• synchronizedother synchronized methods are
  locked out while this method is executing see
  later.
• nativethe implementation of this method is given
  in a platform-dependent language. Declaration
  has a semicolon in place of a body.
• strictfpintermediate results in all float or
  double expressions appearing in the body have
  strict IEEE 754 exponents.

12
Java Language Basics
13
Java Keywords

• null, true, and false are literals with special
  meaning

14
Java Expression
15
Expressions

• For assignment statements you use , ,
• int i 5 // i has the value of 5
• i 1 // equals i i 1
• Arithmetic uses the familiar - /
• Use and to add or substract one value
• i // adds one to i
• i-- // substract one from i
• Java has boolean operators !
• Java has comparisons lt lt ! gt gt

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Boolean expression

• A boolean expression is a logical expression
  which is either true or false
• (5 gt 4) -gt false
• (5 5) -gt true
• means and and means or
• (5gt4) (4gt5) -gt true
• (5gt4) (4gt5) -gt false
• Primitives Variables can use while Reference
  Types usually uses equals
• String s hello s.equals(hello) -gt true
  s.equals(Hello) -gt false

17
Java Control Structure
18
Control statements - Overview

• if (boolean expression) statement
• if (boolean expression) statementelse
  statement(s)
• while (boolean expression) statement(s)
• do statement(s) while (boolean expression)
• for (int i 0 i lt max i) statements

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If Statements I

• If allows the conditional execution of
  statements, e.g. if (person.getAgegt 18)
  drinkABeer
• Conditional execution of statements
• if (some Boolean expression)
• statements to be executed if true

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If Statements - II

• If allows an else statements which is executed if
  the if statement is not truee.
• if (some Boolean expression)
• statements to be executed if true
• else
• statements to be executed if false
• You can also nest several if statements
• if (some Boolean expression) . . .
• else if (another Boolean expression) .
  . .
• else . . .

21
while Loop Constructs

• while loop
• while (boolean expression)
• Stuff to do
• Example

public void doStuff() int i 0
int j 0 while(i lt 10)
j j 1 i

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The for loop

• for (declaration1 booleanExpression
  expressionList2)
• Statements to do
• The declaration declaration1 is executed at start
  of the loop, expressionList2 is evaluated after
  every iteration, and the loop terminates when
  booleanExpression is false.
• Typical example
• for (int i 0 i lt 10 i)
• j j 1

23
Switch

• Java also introduces the try statement
• Will evaluate n and go to the corresponding case
  statement

switch (n) case 0 m n - 1 break case
1 m n 1 case 3 m m n break
default m -n break
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break and continue

• Unlabeled break statement immediately exits the
  enclosing switch, while, do or for construct
• while (true)
• if (booleanExpression) break
  
• The continue statement skips to the next
  iteration of the enclosing while, do or for. The
  statements after continue are not executed anymore