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A Generic Class with Two Type Parameters
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You can declare more than one type parameter in a generic type To specify two or more type parameters, simply use a comma-separated list For example, the following TwoGen class is a variation of the Gen class that has two type parameters:
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// A simple generic class with two type // parameters: T and V class TwoGen<T, V> { T ob1; V ob2; // Pass the constructor a reference to // an object of type T and an object of type V TwoGen(T o1, V o2) { ob1 = o1; ob2 = o2; }
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Generics
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// Show types of T and V void showTypes() { Systemoutprintln("Type of T is " + ob1getClass()getName()); Systemoutprintln("Type of V is " + ob2getClass()getName()); } T getob1() { return ob1; } V getob2() { return ob2; } } // Demonstrate TwoGen class SimpGen { public static void main(String args[]) { TwoGen<Integer, String> tgObj = new TwoGen<Integer, String>(88, "Generics"); // Show the types tgObjshowTypes(); // Obtain and show values int v = tgObjgetob1(); Systemoutprintln("value: " + v); String str = tgObjgetob2(); Systemoutprintln("value: " + str); } }
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The output from this program is shown here: Type of T is javalangInteger Type of V is javalangString value: 88 value: Generics Notice how TwoGen is declared:
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class TwoGen<T, V> {
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It specifies two type parameters: T and V, separated by a comma Because it has two type parameters, two type arguments must be passed to TwoGen when an object is created, as shown next:
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TwoGen<Integer, String> tgObj = new TwoGen<Integer, String>(88, "Generics");
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Part I:
The Java Language
In this case, Integer is substituted for T, and String is substituted for V Although the two type arguments differ in this example, it is possible for both types to be the same For example, the following line of code is valid:
TwoGen<String, String> x = new TwoGen<String, String>("A", "B");
In this case, both T and V would be of type String Of course, if the type arguments were always the same, then two type parameters would be unnecessary
The General Form of a Generic Class
The generics syntax shown in the preceding examples can be generalized Here is the syntax for declaring a generic class: class class-name<type-param-list> { // Here is the syntax for declaring a reference to a generic class: class-name<type-arg-list> var-name = new class-name<type-arg-list>(cons-arg-list);
Bounded Types
In the preceding examples, the type parameters could be replaced by any class type This is fine for many purposes, but sometimes it is useful to limit the types that can be passed to a type parameter For example, assume that you want to create a generic class that contains a method that returns the average of an array of numbers Furthermore, you want to use the class to obtain the average of an array of any type of number, including integers, floats, and doubles Thus, you want to specify the type of the numbers generically, using a type parameter To create such a class, you might try something like this:
// Stats attempts (unsuccessfully) to // create a generic class that can compute // the average of an array of numbers of // any given type // // The class contains an error! class Stats<T> { T[] nums; // nums is an array of type T // Pass the constructor a reference to // an array of type T Stats(T[] o) { nums = o; } // Return type double in all cases double average() { double sum = 00;
14:
Generics
for(int i=0; i < numslength; i++) sum += nums[i]doubleValue(); // Error!!! return sum / numslength; } }
In Stats, the average( ) method attempts to obtain the double version of each number in the nums array by calling doubleValue( ) Because all numeric classes, such as Integer and Double, are subclasses of Number, and Number defines the doubleValue( ) method, this method is available to all numeric wrapper classes The trouble is that the compiler has no way to know that you are intending to create Stats objects using only numeric types Thus, when you try to compile Stats, an error is reported that indicates that the doubleValue( ) method is unknown To solve this problem, you need some way to tell the compiler that you intend to pass only numeric types to T Furthermore, you need some way to ensure that only numeric types are actually passed To handle such situations, Java provides bounded types When specifying a type parameter, you can create an upper bound that declares the superclass from which all type arguments must be derived This is accomplished through the use of an extends clause when specifying the type parameter, as shown here: <T extends superclass> This specifies that T can only be replaced by superclass, or subclasses of superclass Thus, superclass defines an inclusive, upper limit You can use an upper bound to fix the Stats class shown earlier by specifying Number as an upper bound, as shown here:
// In this version of Stats, the type argument for // T must be either Number, or a class derived // from Number class Stats<T extends Number> { T[] nums; // array of Number or subclass // Pass the constructor a reference to // an array of type Number or subclass Stats(T[] o) { nums = o; } // Return type double in all cases double average() { double sum = 00; for(int i=0; i < numslength; i++) sum += nums[i]doubleValue(); return sum / numslength; } }
Part I:
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