Static, main, math, Strings

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Static, main, math, Strings
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What helps me learn Java the most

1. Going to the Mac lab
2. Writing Java on the Board
3. Doing the Homework
4. Watching the Instructor code in BlueJ
5. Doing the Programming Assignments
6. clickers

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Review reading javadoc and methods

• java.lang.Math
• static double sqrt(double a)           Returns
  the correctly rounded positive square root of a
  double value.
• All methods in java.lang.Math are static

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• To call sqrt, do the following
• int x Math.sqrt(4)
• int x sqrt(4)
• Math.sqrt(4)
• int x new Math( ) x.Math.sqrt(4)
• int x new Math( ) x.sqrt(4)

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static

• static means "class level", not "object level"
• public int x // note public just for demo
• // instance variables should be private
• public static int y

exists at class level before any objects are
created
object is created on object bench
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Using static variables/methodsFor each line ok?
A. YES B. NO

• class MyClass public int x
• public
  static int y
• MyClass c new MyClass( ) // ok?
• c.x 4 // ok?
• c.y 8 // ok?
• MyClass d new MyClass( ) //ok?
• // c.x is A. 0 B. 4 C. neither D. undefined
• // c.y is A. 0 B. 8 C. neither D. undefined

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More on using static

• class MyClass public int x
• public
  static int y
• MyClass c new MyClass( )
• c.y 8 // ok? A. YES B. NO
• MyClass.y 8 // ok? A. YES B. NO
• MyClass.x 4 // ok? A. YES B. NO
• For static variables/names, use name of class,
  not object name. can use either
• For object variables, use name of object, not
  class name. canNOT use either

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Why have static?

• If instance variables are not needed
• Math.PI, Math.sqrt, JOptionPane.showMessageDialog
• when in doubt, do NOT use static

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static methods

• static methods must use static everything since
  they exist even if an object doesn't
• SO
• static methods can only call static methods
• static methods cannot modify instance variables

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main

• main is the first method that is called when a
  program is run
• main must be static in Java because it is created
  before an object is created
• because main is static, all methods it calls must
  be static
• moral main should create an object, and that
  object should do the work of the program

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program with main

• public class MyClass
• // do stuff with methods and constructors
• public static void main(String args)
• new MyClass( ) // call constructor

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String args

• You can specify arguments at the command line

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• public static void main(String args)
• for (String mystring args)
• // Note NO references to the array in foreach
• System.out.println("You entered "
  mystring)
• / the same loop with a for would be
• for (int i0iltargs.lengthi)
• System.out.println("You entered " argsi)
• /

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Wrapper Classes
Wrapper Type Primitive Type
Boolean boolean
Integer int
Double double
Character char

• Sometimes, primitives need to act like objects
  and the reverse
• Each primitive has a "wrapper" class that makes
  it look like an object

primitives are all lowercase
Wrappers are instances of Classes, so are capital
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this refers to the classor class's instance
variable

• public class Member
• private String name
• private String motto
• privateltMembergt friends new
  ArrayListltMembergt ( )
• public void addMyBestFriend( )
• // want to say
• // friends.add(me)
• friends.add(this) // will add the current
  Member as a friend

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More practice using javadoc String(expand S.o.p
to System.out.println)

•  int compareTo(String anotherString)
            Compares two strings
  lexicographically.
• Returns the value 0 if the argument string is
  equal to this string a value less than 0 if this
  string is lexicographically less than the string
  argument and a value greater than 0 if this
  string is lexicographically greater than the
  string argument.
• Call to compareTo
• String s1, s2 if (s1 lt s2) S.o.p("s1 lt s2")
• String s1, s2 if (s1.compareTo(s2) ) S.o.p ("s1
  lt s2")
• String s1, s2 if (compareTo(s1, s2)) S.o.p("s1 lt
  s2")
• String s1, s2 if (s1.compareTo(s2) lt 0)
  S.o.p("s1 lt s2")
• String s1, s2 int x 0
• if (x s1.compareTo(s2) )
  S.o.p("s1 lt s2")