java'lang and java'util

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java.lang and java.util
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Overview

• java.lang and java.util are packages that contain
  some important classes
• no need to import java.lang (free)
• Inside these packages are
• Wrappers
• Math
• Strings
• Collections

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The Wrapper Classes

• A wrapper is a class (complex type) designed to
  hold a primitive
• Primitives are passed by value and classes are
  pass by reference
• Wrappers provide functionality for converting
  data types (parseInt)

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The Wrapper Classes

• Boolean - hold a boolean
• Byte - holds a byte
• Character - holds a char
• Double - holds a double
• Float - holds a float
• Integer - holds an int
• Long - holds a long
• Short - holds a short

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Using Wrapper Classes

• Usually have 2 constructors
• one with primitive data type
• one with a String passed
• Character class has one constructor
• To extract value, use the Xvalue( ), where X is
  the name of the primitive
• public byte byteValue( )
• Wrappers override equals( )

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Math Class

• Has several static methods
• abs( ) returns absolute value of a number (ignore
  sign)
• ceil( ) returns the ceiling of a number (round up
  no matter what!)
• floor( ) returns the floor of a number (round
  down no matter what)
• max( ) takes in two numbers and returns the
  greater of the two

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Math Class

• Static methods continued
• min( ) returns the minimum of two numbers
• random( ) returns a value between 0.0d and 1.0d
• round( ) finds the closest integer to that number
  (.5 and above)
• sqrt( ) returns the square root of a positive
  number
• sin( ), cos( ), tan( ) are trig functions

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String Operators

• You can use and operators on Strings
• Example
• String s Jeff rox
• s the world
• Another example
• s 5

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Immutability of Strings

• Whenever you create a String, it is read only
  (you cant change it!)
• Before we saw the and operators!?
• These operators actually create a new String!

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Pools of Strings

• There are 2 ways to create Strings in Java
• Using new
• Using literals
• Each Java class has a String pool for literals
• Eliminates duplicate copies
• So if we said
• String s1 Jeff String s2 Jeff
• String s3 Jeff String s4 Jeff
• We would only open up enough space for 1 String,
  and s1, s2, s3, and s4 would point to it!

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Pools continued

• Look at this code
• String s1 Jeff
• String s2 Jeff
• if (s1 s2) System.out.println (Yes!)
• s1 and s2 point to same String in pool

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Pool Quiz

• String s1, s2
• s1 Jeff
• s2 new String (Jeff)
• if (s1 s2)
• System.out.println ( )
• if (s1.equals(s2) )
• System.out.println (equals( ))
• What would print?

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String MethodsCheck API for more

• charAt( ) - character at a position
• concat( ) - adds to end of String
• endsWith( ) - boolean using a String
• equals( ) - char by char
• equalsIgnoreCase( )
• indexOf( ) - uses characters or substrings

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String MethodsContinued

• lastIndexOf( ) - last occurrence
• length( ) - size of the String
• replace( ) - replaces occurrences
• startsWith( ) - returns boolean
• substring( ) - returns a substring
• toLowerCase( )
• toUpperCase( )

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String Methods

• trim( ) - trims leading and trailing white space
  (returns, tabs, etc)
• valueOf( ) - used for returning the String
  version of primitive data types

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The String Buffer Class

• Like a String, but you can change it
• Whenever you want to manipulate the contents of a
  String, you should use a String buffer
• Has methods like
• append( )
• insert( )
• reverse( )
• setCharAt( )

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Collections

• These give us the ability to store complex data
  types
• Example the Vector class
• Have multiple ways we can organize this
• can restrict to only one occurrence
• having keys to look up data
• speed etc...

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For Basic Collections

• Collection - unordered with no restrictions. Can
  have multiple occurences
• Lists - ordered (meaning indexed, not sorted) and
  can have multiple occurences.
• Sets - Do not allow duplicate values
• Maps - Have to explain this one

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Map/Hash

• Use data that youre about to store must have a
  unique key
• Have a hash function to do this
• Example
• Jeff may generate 10566
• bob may generate 2152

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Ways that these Collections can Store Data

• Arrays - seen these
• Linked Lists - having one node point to another
  (draw this on board)
• Trees - having one node point to multiple nodes
• Hash Tables - generating values from keys

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Linked ListsFrom Java I
5
17
43
98
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TreesFrom Java I
51
27
70
13
29
64
87
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