The java.lang Package Certification - Wrapper Classes

• one for each primitive type: Boolean, Byte, Character, Double, Float, Integer, Long, and Short
• Byte, Double, Float, Integer and Short extend the abstract Number class
• all are public final ie cannot be extended
• get around limitations of primitive types
• allow objects to be created from primitive types

• all the classes have two constructor forms
  • a constructor that takes the primitive type and creates an object eg Character(char), Integer(int)
  • a constructor that converts a String into an object eg Integer("1"). Throws a NumberFormatException if the String cannot be converted to a number
    

• all, except Character, have a valueOf(String s) method which is equivalent to new Type(String s)
• all have a typeValue() method which returns the value of the object as it's primitive type. These are all abstract methods defined in Number and overridden in each class
  • public byte byteValue()
  • public short shortValue()
  • public int intValue()
  • public long longValue()
  • public float floatValue()
  • public double doubleValue()

• all the classes override equals(), hashCode() and toString() in Object
  • equals() returns true if the values of the compared objects are the same
  • hashCode() returns the same hashcode for objects of the same type having the same value
  • toString() returns the string representation of the objects value

• all have a public static final TYPE field which is the Class object for that primitive type
• all have two static fields MIN_VALUE and MAX_VALUE for the minimum and maximum values that can be held by the type

Void

• there is also a wrapper class for Void which cannot be instantiated.

Character

• contains two methods for returning the numeric value of a character in the various number systems
  • public static int digit(char ch, int radix)
  • public static int getNumber(char ch)
• and one method to return the character value of a number
  • public static char forDigit(int digit, int radix)
• has two case conversion methods
  • public static char toLowerCase(char ch)
  • public static char toUpperCase(char ch)
• also contains a variety of other methods to test wether a character is of a specific type eg isLetter(), isDefined(), isSpaceChar(), etc
• getType() returns an int that defines a character's Unicode type

Integer, Short, Byte and Long

• all have parseType methods eg parseInt(), parseShort, etc that take a String and parse it into the appropriate type
• the Integer and Long classes also have the static methods toBinaryString(), toOctalString() and toHexString() which take an integer value and convert it to the appropriate String representation

Float and Double

• both classes have static fields which define POSITIVE_INFINITY, NEGATIVE_INFINITY, and NaN
• and the following methods to test a value
  • public boolean isNan()
  • public static boolean isNaN(type value)
  • public boolean isInfinite()
  • public static boolean isInfinite(type value)
• Float also has a constructor that takes a double value
• both classes have methods to convert a value into a bit pattern or vice versa
  • public static int floatToIntBits(float value)
  • public static float intBitsToFloat(int bits)
  • public static long doubleToLongBits(double value)
  • public static double longBitsToDouble(long bits)