Generic Classes and Methods - Java: How to Program Early Objects - page 842

When the compiler encounters a method call, it attempts to locate a method declara-
tion with the same name and with parameters that match the argument types in the call.
In this example, each printArray call matches one of the printArray method declara-
tions. For example, line 14 calls printArray with integerArray as its argument. The
compiler determines the argument's type (i.e., Integer[] ) and attempts to locate a print-
Array method that specifies an Integer[] parameter (lines 22-29), then sets up a call to
that method. Similarly, when the compiler encounters the call at line 16, it determines the
argument's type (i.e., Double[] ), then attempts to locate a printArray method that spec-
ifies a Double[] parameter (lines 32-39), then sets up a call to that method. Finally, when
the compiler encounters the call at line 18, it determines the argument's type (i.e., Char-
acter[] ), then attempts to locate a printArray method that specifies a Character[]
parameter (lines 42-49), then sets up a call to that method.

Common Features in the Overloaded printArray Methods
Study each printArray method. The array element type appears in each method's header
(lines 22, 32 and 42) and for -statement header (lines 25, 35 and 45). If we were to replace
the element types in each method with a generic name— T by convention—then all three
methods would look like the one in Fig. 20.2. It appears that if we can replace the array
element type in each of the three methods with a single generic type , then we should be able
to declare one printArray method that can display the String representations of the ele-
ments of any array that contains objects. The method in Fig. 20.2 is similar to the generic
printArray method declaration you'll see in Section 20.3. The one shown here will not
compile —we use this simply to show that the three printArray methods of Fig. 20.1 are
identical except for the types they process.

1 public static void printArray( [] inputArray) T

2 {

3 // display array elements

4 for ( element : inputArray) T

5 System.out.printf( "%s " , element);

6
7 System.out.println();

8 }

Fig. 20.2 | printArray method in which actual type names are replaced with a generic type
name (in this case T ).

20.3 Generic Methods: Implementation and Compile-
Time Translation
If the operations performed by several overloaded methods are identical for each argument
type, the overloaded methods can be more conveniently coded using a generic method.
You can write a single generic method declaration that can be called with arguments of
different types. Based on the types of the arguments passed to the generic method, the
compiler handles each method call appropriately. At compilation time , the compiler en-
sures the type safety of your code, preventing many runtime errors.
Figure 20.3 reimplements Fig. 20.1 using a generic printArray method (lines 22-29
of Fig. 20.3). The printArray calls in lines 14, 16 and 18 are identical to those of