The Wrapper Classes in Java

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103 The Wrapper Classes
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Wrapper classes were introduced with the discussion of the primitive data types (see Section 22, p 28), and also in connection with boxing and unboxing of primitive values (see Section 51, p 162) Primitive values in Java are not objects In order to manipulate these values as objects, the javalang package provides a wrapper class for each of the primitive data types All wrapper classes are final The objects of all wrapper classes that can be instantiated are immutable, ie, the value in the wrapper object cannot be changed Although the Void class is considered a wrapper class, it does not wrap any primitive value and is not instantiable (ie, has no public constructors) It just denotes
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103: THE WRAPPER CLASSES
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the Class object representing the keyword void The Void class will not be discussed further in this section In addition to the methods defined for constructing and manipulating objects of primitive values, the wrapper classes also define useful constants, fields, and conversion methods
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Converting Values Between Primitive, Wrapper, and String Types
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string
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String str;
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str = WrapperTypetoString(v); str = StringvalueOf(v); str = "" + v;
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// (6a) // (6b) // (6c)
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str = reftoString();
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// (3)
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v = WrapperTypeparseType(str); // (5)
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// (2a) ref = new WrapperType(str); ref = WrapperTypevalueOf(str); // (2b)
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primitive value
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type v; v = ref; v = reftypeValue();
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type is: boolean char byte short int long float double WrapperType is: Boolean Character Byte Short Integer Long Float Double
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object
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WrapperType ref; // (4a) // (4b) ref = v; ref = new WrapperType(v); ref = WrapperTypevalueOf(v); // (1a) // (1b) // (1c)
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Comments: (1a) Boxing (2a) Not for Character type Can throw NumberFormatException (2b) Not for Character type Can throw NumberFormatException (4a) Unboxing (5) For numeric wrapper types only Can throw NumberFormatException (6b) Not for byte and short primitive types
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Common Wrapper Class Constructors
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The Character class has only one public constructor, taking a char value as parameter The other wrapper classes all have two public one-argument constructors: one takes a primitive value and the other takes a string
WrapperType(type v) WrapperType(String str)
The type is a primitive data type The string argument is converted to a primitive value that corresponds to the WrapperType Otherwise a NumberFormatException is thrown if the string is not parsable
CHAPTER 10: FUNDAMENTAL CLASSES
Wrapping Primitive Values in Objects
Boxing is a convenient way to wrap a primitive value in an object (see (1a) in Figure 102 and Section 51, p 162)
Character Boolean Integer Double charObj1 boolObj1 intObj1 doubleObj1 = = = = '\n'; true; 2008; 314;
A constructor that takes a primitive value can be used to create wrapper objects (see (1b) in Figure 102)
Character Boolean Integer Double charObj1 boolObj1 intObj1 doubleObj1 = = = = new new new new Character('\n'); Boolean(true); Integer(2008); Double(314);
We can also use the valueOf() method that takes the primitive value to wrap as an argument (see (1c) in Figure 102)
Character Boolean Integer Double charObj1 boolObj1 intObj1 doubleObj1 = = = = CharactervalueOf('\n'); BooleanvalueOf(true); IntegervalueOf(2008); DoublevalueOf(314);
Converting Strings to Wrapper Objects
A constructor that takes a String object representing the primitive value can also be used to create wrapper objects The constructors for the numeric wrapper types throw an unchecked NumberFormatException if the String parameter does not parse to a valid number (see (2a) in Figure 102)
Boolean Boolean Integer Double Long boolObj2 boolObj3 intObj2 doubleObj2 longObj1 = = = = = new new new new new Boolean("TrUe"); Boolean("XX"); Integer("2008"); Double("314"); Long("314"); // case ignored: true // false
// NumberFormatException
Common Wrapper Class Utility Methods
Converting Strings to Wrapper Objects
Each wrapper class (except Character) defines the static method valueOf(String str) that returns the wrapper object corresponding to the primitive value represented by the String object passed as argument (see (6b) in Figure 102) This method for the numeric wrapper types also throws a NumberFormatException if the String parameter is not a valid number
static WrapperType valueOf(String str)
Boolean boolObj4 = BooleanvalueOf("false"); Integer intObj3 = IntegervalueOf("1949"); Double doubleObj3 = DoublevalueOf("-30");