问题
有没有办法转换对象以返回方法的值?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);
}
#1 热门回答(161 赞)
由于编译期间的泛型类型擦除,你必须使用aClass
实例。
public static <T> T convertInstanceOfObject(Object o, Class<T> clazz) {
try {
return clazz.cast(o);
} catch(ClassCastException e) {
return null;
}
}
声明that method:
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);
注意,当240242079传递给convertToInstanceOfObject
时,编译时间类型为Object
。
#2 热门回答(8 赞)
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,但我认为我确实提供了我在JVM字节代码级别的发现,以解释为什么OP遇到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
byte代码指令发生在main方法中,而不是convertInstanceOfObject
和convertInstanceOfObject
方法没有任何指令可以抛出ClassCastException
。因为当你执行main方法时,main方法没有捕获到ClassCastException
,因此你将获得aClassCastException
而不是printnull
的期望值。
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,在convertInstanceOfObject
方法中有一个invokevirtual
指令调用Class.cast()
方法,该方法将抛出ClassCastException
,这将被catch(ClassCastException e)
bock和returnnull
捕获;因此,"null"将打印到控制台,没有任何异常。
#3 热门回答(0 赞)
如果你不想依赖抛出异常(你可能不应该),你可以试试这个:
public static <T> T cast(Object o, Class<T> clazz) {
return clazz.isInstance(o) ? clazz.cast(o) : null;
}