Advanced Java Tutorial: Generic Methods

You can write a generic method that can receive different types of parameters when called. Depending on the parameter types passed to the generic method, the compiler handles each method call appropriately.

The following are the rules for defining generic methods:

All generic method declarations have a type parameter declaration part (separated by angle brackets) before the method return type. (in the example below).

Each type parameter declaration part contains one or more type parameters, separated by commas. A generic parameter, also called a type variable, is an identifier that specifies the name of a generic type.

Type parameters can be used to declare return value types and can serve as placeholders for the actual parameter types obtained by generic methods.

The declaration of the method body of a generic method is the same as that of other methods. Note that type parameters can only represent reference types, not primitive types (such as int, double, char, etc.).

Example

The following example demonstrates how to use generic methods to print elements of different strings:

public class GenericMethodTest
{
   // 泛型方法 printArray                         
   public static < E > void printArray( E[] inputArray )
   {
      // 输出数组元素            
         for ( E element : inputArray ){        
            System.out.printf( "%s ", element );
         }
         System.out.println();
    }
 
    public static void main( String args[] )
    {
        // 创建不同类型数组： Integer, Double 和 Character
        Integer[] intArray = { 1, 2, 3, 4, 5 };
        Double[] doubleArray = { 1.1, 2.2, 3.3, 4.4 };
        Character[] charArray = { &#39;H&#39;, &#39;E&#39;, &#39;L&#39;, &#39;L&#39;, &#39;O&#39; };
 
        System.out.println( "Array integerArray contains:" );
        printArray( intArray  ); // 传递一个整型数组
 
        System.out.println( "\nArray doubleArray contains:" );
        printArray( doubleArray ); // 传递一个双精度型数组
 
        System.out.println( "\nArray characterArray contains:" );
        printArray( charArray ); // 传递一个字符型型数组
    } 
}
编译以上代码，运行结果如下所示：
 
Array integerArray contains:
1 2 3 4 5 6
  
Array doubleArray contains:
1.1 2.2 3.3 4.4
  
Array characterArray contains:
H E L L O

Bounded type parameters:

Possible Sometimes, you want to limit the range of types that are allowed to be passed to a type parameter. For example, a method that operates on numbers may only wish to accept instances of Number or a subclass of Number. That's the purpose of bounded type parameters.

To declare a bounded type parameter, first list the name of the type parameter, followed by the extends keyword, and finally followed by its upper bound.

Example

The following example demonstrates how "extends" is used to mean "extends" (class) or "implements" (interface) in a general sense. The generic method in this example returns the maximum value of three comparable objects.

public class MaximumTest
{
   // 比较三个值并返回最大值
   public static > T maximum(T x, T y, T z)
   {                     
      T max = x; // 假设x是初始最大值
      if ( y.compareTo( max ) > 0 ){
         max = y; //y 更大
      }
      if ( z.compareTo( max ) > 0 ){
         max = z; // 现在 z 更大           
      }
      return max; // 返回最大对象
   }
   public static void main( String args[] )
   {
      System.out.printf( "Max of %d, %d and %d is %d\n\n",
                   3, 4, 5, maximum( 3, 4, 5 ) );
  
      System.out.printf( "Maxm of %.1f,%.1f and %.1f is %.1f\n\n",
                   6.6, 8.8, 7.7, maximum( 6.6, 8.8, 7.7 ) );
  
      System.out.printf( "Max of %s, %s and %s is %s\n","pear",
         "apple", "orange", maximum( "pear", "apple", "orange" ) );
   }
}
编译以上代码，运行结果如下所示：
 
Maximum of 3, 4 and 5 is 5
  
Maximum of 6.6, 8.8 and 7.7 is 8.8
  
Maximum of pear, apple and orange is pear

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