Question

根据Java docs，以下代码应该导致编译错误：

import java.util.*;

public class GenericTest1 {

    // Add T-array of objects to collection<T>
    static <T> void fromArrayToCollection(T[] a, Collection<T> c) {
        for (T o : a) {
            c.add(o);
        }
    }

   public static void main( String[] args ) {
       Number[] na = new Number[100];
       Collection<Number> cn = new ArrayList<Number>();


       // This should work and does
       fromArrayToCollection( na, cn );


       Collection<String> cs = new ArrayList<String>();

       // This should fail to copile and does
       fromArrayToCollection( na, cs );
   }
}

它确实：

GenericTest1.java:25: error: method fromArrayToCollection in class GenericTest1 cannot be applied to given types;
       fromArrayToCollection( na, cs );
       ^
  required: T[],Collection<T>
  found: Number[],Collection<String>
  reason: inference variable T has incompatible bounds
    equality constraints: String
    lower bounds: Number
  where T is a type-variable:
    T extends Object declared in method <T>fromArrayToCollection(T[],Collection<T>)

但是，这可以完美地编译和运行。

public class GenericTest2 {

    // Test for equality of two objects of type T
    static <T> boolean testEquality(T first, T second ) {
        return first.equals( second );
    }


   public static void main( String[] args ) {
       // Should work
       System.out.println( testEquality( "One", "One" ) );

       // Shouldn't this refuse to compile ?
       System.out.println( testEquality( "One", 1 ) );

       // Shouldn't this refuse to compile ?
       Number one = new Integer( 1 );
       System.out.println( testEquality( "One", one ) );

   }
}

输出是：

true
false
false

任何人都可以解释原因吗？

Answer 1

它有效，因为one（Number）和"One"（String）都是Object，1（{{ 1}}由于autoboxing）和Integer（"One"）。因此String被评估为T，等于被调用并返回Object。它不适用于false（及其他泛型）because a Collection<String> can not be cast to a Collection<Object>

Answer 2

testEquality( "One", 1 )

作为自动装箱的结果，1此处将转换为Integer(1)，这是一个对象。 String("One")和Integer(1)都从.equals继承了Object函数，因此可以无误地编译。

Answer 3

在第二次测试中，p=&a;只会被装入(int*)实例，该实例也是1的实例。

你能解释一下Java中Boxing和Generics的奇怪行为吗？

3 个答案: