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The values( ) and valueOf( ) Methods
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All enumerations automatically contain two predefined methods: values( ) and valueOf( ) Their general forms are shown here: public static enum-type[ ] values( ) public static enum-type valueOf(String str) The values( ) method returns an array that contains a list of the enumeration constants The valueOf( ) method returns the enumeration constant whose value corresponds to the string passed in str In both cases, enum-type is the type of the enumeration For example, in the case of the Apple enumeration shown earlier, the return type of ApplevalueOf( Winesap ) is Winesap The following program demonstrates the values( ) and valueOf( ) methods:
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// Use the built-in enumeration methods // An enumeration of apple varieties enum Apple { Jonathan, GoldenDel, RedDel, Winesap, Cortland } class EnumDemo2 { public static void main(String args[]) { Apple ap; Systemoutprintln("Here are all Apple constants:"); // use values() Apple allapples[] = Applevalues(); for(Apple a : allapples) Systemoutprintln(a); Systemoutprintln(); // use valueOf() ap = ApplevalueOf("Winesap"); Systemoutprintln("ap contains " + ap); } }
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The output from the program is shown here: Here are all Apple constants: Jonathan GoldenDel RedDel Winesap Cortland ap contains Winesap
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Enumerations, Autoboxing, and Annotations (Metadata)
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Notice that this program uses a for-each style for loop to cycle through the array of constants obtained by calling values( ) For the sake of illustration, the variable allapples was created and assigned a reference to the enumeration array However, this step is not necessary because the for could have been written as shown here, eliminating the need for the allapples variable:
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for(Apple a : Applevalues()) Systemoutprintln(a);
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Now, notice how the value corresponding to the name Winesap was obtained by calling valueOf( )
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ap = ApplevalueOf("Winesap");
As explained, valueOf( ) returns the enumeration value associated with the name of the constant represented as a string
NOTE C/C++ programmers will notice that Java makes it much easier to translate between the
human-readable form of an enumeration constant and its binary value than do these other languages This is a significant advantage to Java s approach to enumerations
Java Enumerations Are Class Types
As explained, a Java enumeration is a class type Although you don t instantiate an enum using new, it otherwise has much the same capabilities as other classes The fact that enum defines a class gives powers to the Java enumeration that enumerations in other languages simply do not have For example, you can give them constructors, add instance variables and methods, and even implement interfaces It is important to understand that each enumeration constant is an object of its enumeration type Thus, when you define a constructor for an enum, the constructor is called when each enumeration constant is created Also, each enumeration constant has its own copy of any instance variables defined by the enumeration For example, consider the following version of Apple:
// Use an enum constructor, instance variable, and method enum Apple { Jonathan(10), GoldenDel(9), RedDel(12), Winesap(15), Cortland(8); private int price; // price of each apple // Constructor Apple(int p) { price = p; } int getPrice() { return price; } } class EnumDemo3 { public static void main(String args[]) { Apple ap;
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// Display price of Winesap Systemoutprintln("Winesap costs " + AppleWinesapgetPrice() + " cents\n"); // Display all apples and prices Systemoutprintln("All apple prices:"); for(Apple a : Applevalues()) Systemoutprintln(a + " costs " + agetPrice() + " cents"); } }
The output is shown here: Winesap costs 15 cents All apple prices: Jonathan costs 10 cents GoldenDel costs 9 cents RedDel costs 12 cents Winesap costs 15 cents Cortland costs 8 cents This version of Apple adds three things The first is the instance variable price, which is used to hold the price of each variety of apple The second is the Apple constructor, which is passed the price of an apple The third is the method getPrice( ), which returns the value of price When the variable ap is declared in main( ), the constructor for Apple is called once for each constant that is specified Notice how the arguments to the constructor are specified, by putting them inside parentheses after each constant, as shown here:
Jonathan(10), GoldenDel(9), RedDel(12), Winesap(15), Cortland(8);
These values are passed to the p parameter of Apple( ), which then assigns this value to price Again, the constructor is called once for each constant Because each enumeration constant has its own copy of price, you can obtain the price of a specified type of apple by calling getPrice( ) For example, in main( ) the price of a Winesap is obtained by the following call:
AppleWinesapgetPrice()
The prices of all varieties are obtained by cycling through the enumeration using a for loop Because there is a copy of price for each enumeration constant, the value associated with one constant is separate and distinct from the value associated with another constant This is a powerful concept, which is only available when enumerations are implemented as classes, as Java does Although the preceding example contains only one constructor, an enum can offer two or more overloaded forms, just as can any other class For example, this version of Apple provides a default constructor that initializes the price to 1, to indicate that no price data is available:
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Enumerations, Autoboxing, and Annotations (Metadata)
// Use an enum constructor enum Apple { Jonathan(10), GoldenDel(9), RedDel, Winesap(15), Cortland(8); private int price; // price of each apple // Constructor Apple(int p) { price = p; } // Overloaded constructor Apple() { price = -1; } int getPrice() { return price; } }
Notice that in this version, RedDel is not given an argument This means that the default constructor is called, and RedDel s price variable is given the value 1 Here are two restrictions that apply to enumerations First, an enumeration can t inherit another class Second, an enum cannot be a superclass This means that an enum can t be extended Otherwise, enum acts much like any other class type The key is to remember that each of the enumeration constants is an object of the class in which it is defined
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