The Java Language in Java

Generation QR Code in Java The Java Language

X Y Z T Coordinates: 1 2 3 4 6 8 14 8 22 9 4 9 3 -2 -23 17 Notice these commented-out lines:

Because tdlocs is a Coords(TwoD) object, it cannot be used to call showXYZ( ) or showAll( ) because bounded wildcard arguments in their declarations prevent it To prove this to yourself, try removing the comment symbols, and then attempt to compile the program You will receive compilation errors because of the type mismatches In general, to establish an upper bound for a wildcard, use the following type of wildcard expression: < extends superclass> where superclass is the name of the class that serves as the upper bound Remember, this is an inclusive clause because the class forming the upper bound (that is, specified by superclass) is also within bounds You can also specify a lower bound for a wildcard by adding a super clause to a wildcard declaration Here is its general form: < super subclass> In this case, only classes that are superclasses of subclass are acceptable arguments This is an exclusive clause, because it will not match the class specified by subclass

As the preceding examples have shown, methods inside a generic class can make use of a class type parameter and are, therefore, automatically generic relative to the type parameter However, it is possible to declare a generic method that uses one or more type parameters of its own Furthermore, it is possible to create a generic method that is enclosed within a non-generic class Let s begin with an example The following program declares a non-generic class called GenMethDemo and a static generic method within that class called isIn( ) The isIn( ) method determines if an object is a member of an array It can be used with any type of object and array as long as the array contains objects that are compatible with the type of the object being sought

// Demonstrate a simple generic method class GenMethDemo { // Determine if an object is in an array static <T, V extends T> boolean isIn(T x, V[] y) {

14:

for(int i=0; i < ylength; i++) if(xequals(y[i])) return true; return false; } public static void main(String args[]) { // Use isIn() on Integers Integer nums[] = { 1, 2, 3, 4, 5 }; if(isIn(2, nums)) Systemoutprintln("2 is in nums"); if(!isIn(7, nums)) Systemoutprintln("7 is not in nums"); Systemoutprintln(); // Use isIn() on Strings String strs[] = { "one", "two", "three", "four", "five" }; if(isIn("two", strs)) Systemoutprintln("two is in strs"); if(!isIn("seven", strs)) Systemoutprintln("seven is not in strs"); // Oops! Won't compile! Types must be compatible if(isIn("two", nums)) Systemoutprintln("two is in strs");

// // } }

The output from the program is shown here: 2 is in nums 7 is not in nums two is in strs seven is not in strs Let s examine isIn( ) closely First, notice how it is declared by this line:

static <T, V extends T> boolean isIn(T x, V[] y) {

The type parameters are declared before the return type of the method Second, notice that the type V is upper-bounded by T Thus, V must either be the same as type T, or a subclass of T This relationship enforces that isIn( ) can be called only with arguments that are compatible with each other Also notice that isIn( ) is static, enabling it to be called independently of any object Understand, though, that generic methods can be either static or non-static There is no restriction in this regard

Part I:

Now, notice how isIn( ) is called within main( ) by use of the normal call syntax, without the need to specify type arguments This is because the types of the arguments are automatically discerned, and the types of T and V are adjusted accordingly For example, in the first call:

if(isIn(2, nums))

the type of the first argument is Integer (due to autoboxing), which causes Integer to be substituted for T The base type of the second argument is also Integer, which makes Integer a substitute for V, too In the second call, String types are used, and the types of T and V are replaced by String Now, notice the commented-out code, shown here:

// // if(isIn("two", nums)) Systemoutprintln("two is in strs");

If you remove the comments and then try to compile the program, you will receive an error The reason is that the type parameter V is bounded by T in the extends clause in V s declaration This means that V must be either type T, or a subclass of T In this case, the first argument is of type String, making T into String, but the second argument is of type Integer, which is not a subclass of String This causes a compile-time type-mismatch error This ability to enforce type safety is one of the most important advantages of generic methods The syntax used to create isIn( ) can be generalized Here is the syntax for a generic method: <type-param-list> ret-type meth-name(param-list) { // In all cases, type-param-list is a comma-separated list of type parameters Notice that for a generic method, the type parameter list precedes the return type