将对象转换为通用类型以进行返回

Question

有没有办法强制转换对象的方法值？我试过这种方式,但它在"instanceof"部分中给出了一个编译时异常:

public static <T> T convertInstanceOfObject(Object o) {
    if (o instanceof T) {
        return (T) o;
    } else {
        return null;
    }
}

我也试过这个,但它给出了一个运行时异常,ClassCastException:

public static <T> T convertInstanceOfObject(Object o) {
    try {
        T rv = (T)o;
        return rv;
    } catch(java.lang.ClassCastException e) {
        return null;
    }
}

是否有可能轻松地做到这一点:

String s = convertInstanceOfObject("string");
System.out.println(s); // should print "string"
Integer i = convertInstanceOfObject(4);
System.out.println(i); // should print "4"
String k = convertInstanceOfObject(345435.34);
System.out.println(k); // should print "null"

编辑:我写了正确答案的工作副本:

public static <T> T convertInstanceOfObject(Object o, Class<T> clazz) {
    try {
        return clazz.cast(o);
    } catch(ClassCastException e) {
        return null;
    }
}

public static void main(String args[]) {
    String s = convertInstanceOfObject("string", String.class);
    System.out.println(s);
    Integer i = convertInstanceOfObject(4, Integer.class);
    System.out.println(i);
    String k = convertInstanceOfObject(345435.34, String.class);
    System.out.println(k);
}

Answer 1

Class由于编译期间的泛型类型擦除,您必须使用实例.

public static <T> T convertInstanceOfObject(Object o, Class<T> clazz) {
    try {
        return clazz.cast(o);
    } catch(ClassCastException e) {
        return null;
    }
}

该方法的声明是:

public T cast(Object o)

这也可以用于数组类型.它看起来像这样:

final Class<int[]> intArrayType = int[].class;
final Object someObject = new int[]{1,2,3};
final int[] instance = convertInstanceOfObject(someObject, intArrayType);

请注意,someObject传递给convertToInstanceOfObject它时具有编译时类型Object.

Answer 2

我偶然发现了这个问题,这引起了我的兴趣.接受的答案是完全正确的,但我认为我确实在JVM字节代码级别上提供了我的发现,以解释OP遇到的原因ClassCastException.

我的代码与OP的代码几乎相同:

public static <T> T convertInstanceOfObject(Object o) {
    try {
       return (T) o;
    } catch (ClassCastException e) {
        return null;
    }
}

public static void main(String[] args) {
    String k = convertInstanceOfObject(345435.34);
    System.out.println(k);
}

和相应的字节代码是:

public static <T> T convertInstanceOfObject(java.lang.Object);
    Code:
       0: aload_0
       1: areturn
       2: astore_1
       3: aconst_null
       4: areturn
    Exception table:
       from    to  target type
           0     1     2   Class java/lang/ClassCastException

  public static void main(java.lang.String[]);
    Code:
       0: ldc2_w        #3                  // double 345435.34d
       3: invokestatic  #5                  // Method java/lang/Double.valueOf:(D)Ljava/lang/Double;
       6: invokestatic  #6                  // Method convertInstanceOfObject:(Ljava/lang/Object;)Ljava/lang/Object;
       9: checkcast     #7                  // class java/lang/String
      12: astore_1
      13: getstatic     #8                  // Field java/lang/System.out:Ljava/io/PrintStream;
      16: aload_1
      17: invokevirtual #9                  // Method java/io/PrintStream.println:(Ljava/lang/String;)V
      20: return

请注意,checkcast字节码指令发生在main方法不是convertInstanceOfObject和convertInstanceOfObject方法不具有可以抛出任何指令ClassCastException.因为ClassCastException当你执行main方法时,main方法没有捕获到它,所以你会得到一个ClassCastException而不是期望打印null.

现在我将代码修改为接受的答案:

public static <T> T convertInstanceOfObject(Object o, Class<T> clazz) {
        try {
            return clazz.cast(o);
        } catch (ClassCastException e) {
            return null;
        }
    }
    public static void main(String[] args) {
        String k = convertInstanceOfObject(345435.34, String.class);
        System.out.println(k);
    }

相应的字节代码是:

public static <T> T convertInstanceOfObject(java.lang.Object, java.lang.Class<T>);
    Code:
       0: aload_1
       1: aload_0
       2: invokevirtual #2                  // Method java/lang/Class.cast:(Ljava/lang/Object;)Ljava/lang/Object;
       5: areturn
       6: astore_2
       7: aconst_null
       8: areturn
    Exception table:
       from    to  target type
           0     5     6   Class java/lang/ClassCastException

  public static void main(java.lang.String[]);
    Code:
       0: ldc2_w        #4                  // double 345435.34d
       3: invokestatic  #6                  // Method java/lang/Double.valueOf:(D)Ljava/lang/Double;
       6: ldc           #7                  // class java/lang/String
       8: invokestatic  #8                  // Method convertInstanceOfObject:(Ljava/lang/Object;Ljava/lang/Class;)Ljava/lang/Object;
      11: checkcast     #7                  // class java/lang/String
      14: astore_1
      15: getstatic     #9                  // Field java/lang/System.out:Ljava/io/PrintStream;
      18: aload_1
      19: invokevirtual #10                 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
      22: return

请注意,方法中有一条invokevirtual指令convertInstanceOfObject调用Class.cast()方法,ClassCastException该方法将抛出哪些内容将被catch(ClassCastException e)bock 捕获并返回null; 因此,"null"将被打印到控制台,没有任何异常.

Answer 3

如果您不想依靠抛出异常（您可能不应该），可以尝试以下操作：

public static <T> T cast(Object o, Class<T> clazz) {
    return clazz.isInstance(o) ? clazz.cast(o) : null;
}