Programming

• Commonly-Used Objects


  • The Object Class
  • The String Classes
  • Wrapper Classes
  • Utility Classes
  • Collections
  • Exceptions
• The Object Class


  "The Mother of All Classes"
• The Object Class


  • All classes are subclasses of Object
  • All classes inherit these methods: clone(), equals(), finalize(), getClass(), hashCode(), notify(), notifyAll(), toString(), wait()
• toString()


  public String toString()

  returns a String representation of the object

  in the Object class, returns a concatenation of the runtime class name and the hashcode

  usually overridden to return something more appropriate
• equals()


  public boolean equals(Object obj)

  in the Object class, returns true when the parameter obj is the same instance as the current object

  usually overridden to more appropriate implementation

  Ex: for String - checks for spelling

  Ex: for BigDecimal - checks if value and scale are equal
• hashCode()


  public int hashCode()

  returns an integer value that is the object's "hashcode"

  used in "hashing" algorithms such as in HashSet and HashMap

  rarely called directly

  Best Practice: if you override equals, you must override hashcode otherwise, hashing algorithms fail
• String and StringBuffer/StringBuilder


  Java actually has TWO types of strings!

  String - Immutable - once you create it, you can never change its value.

  StringBuffer/StringBuilder - Mutable - these are the classes you should use if you need to do a lot of string manipulation.
• String


  • The JVM maintains a pool of String objects. Each newly created String is stored in the pool. The instance lingers even if no references are pointing to it.

  Great performance if there is no need to modify, since you don't incur the cost of instantiation.

  Otherwise, performance is terrible. Each "modification" actually creates a new String.

  The string pool is also the reason why you should never initialize a string with the "new" operator if the literal will do.
• String a = "SELECT";

  String b = "SELECT";

  a == b true! a and b point to the same instance
• String c = new String("SELECT");

  String d = new String("SELECT");

  c == b false! c and d point to different instances
• String Methods


  • Notice that all methods for "string manipulation" will not modify the String instance. They will only return a new String.

  Methods for searching for characters or patterns in a String: charAt, startsWith, endsWith, indexOf, lastIndexOf

  Methods for comparing Strings: equals, equalsIgnoreCase, compareTo, compareToIgnoreCase
• StringBuffer/StringBuilder


  Maintains an internal character array, contents can be modified.

  Great performance when modifications are needed.
• Primitive Wrappers


  • Number
  • Short
  • Integer
  • Byte
  • Long
  • Float
  • Double
  • BigDecimal
  • Boolean
  • Character
• Primitive Wrappers


  • Allow primitive values to participate in the world of objects.

  For example, allows primitives to be stored in "Collections" (Java data structures)

  Immutable - Just like String, once value is set, it cannot be changed.
• Creating a Primitive Wrapper


  Primitive wrappers are usually* created simply by calling the constructor and feeding the primitive value as a constructor.

  Retrieving Primitive Values: Call one of the methods that end in "___Value"
• Before Java 1.5, you can't use primitive wrappers directly in arithmetic nor comparison operators. You have to extract the primitive values first.
• Autoboxing and Unboxing in Java 1.5


  Java 1.5 automatically converts primitives to wrappers and wrappers to primitives
• equals() method


  Returns true if the object passed is of the same type and has the same value
• Primitive Wrappers as Utility Classes


  Primitive Wrappers also serve as "Utility Classes"*.

  They contain static methods and static fields that are useful for the type of primitive that each of them wraps.
• BigDecimal

  Computers cannot accurately store decimal values!

  Using floats and doubles are dangerous in financial applications!

  BigDecimal accurately stores a decimal number to a specified level of precision.

  Use the constructor that takes in a String to create precise values.

  BigDecimal has methods for arithmetic operations: add, subtract, multiply, divide, min, max, negate, pow, abs, remainder, round
• Collections will be discussed in more detail in a succeeding lecture.
• Utility Classes


  Classes that just hold useful routines.

  All methods and fields are static.

  Never instantiated.
• System


  Used to get some access the underlying operating system.

  Fields: out, err, in

  Methods: arraycopy, currentTimeMillis, exit, getProperties, getProperty, gc
• FIFO


  First In, First Out
• Priority ordering


  Some sort of ordering based on priority
• Collections will be discussed in more detail in a succeeding lecture
• Utility Classes


  Classes that hold routines for other classes
• All methods and fields in Utility Classes are static
• Utility Classes are never instantiated
• Utility Classes to be discussed


  • System
  • Math
  • Arrays
  • Collections
• System


  Used to get some access the underlying operating system
• System fields and methods


  • out
  • err
  • in
  • arraycopy
  • currentTimeMillis
  • exit
  • getProperties
  • getProperty
  • gc
• Math


  Contains methods for mathematical operations
• Math methods


  • abs
  • round
  • min
  • max
  • sin
  • cos
  • tan
  • floor
  • ceil
  • exp
  • log
  • random
• Arrays


  Contains methods for working with arrays
• Arrays methods


  • sort
  • binarySearch
  • equals
  • asList
  • toString
• Collections


  Contains methods for working with Collections
• Collections methods


  • sort
  • binarySearch
  • addAll
  • copy
  • min
  • max
  • frequency
  • unmodifiable____
• Date


  Encapsulates a long value representing a specific moment in time
• Calendar


  Represents a point in time interpreted appropriately for some locale and timezone