问题
Widening and Boxing Java primitives.
I know it is not possible to widen a wrapper class from one to another as they are not from the same inheritence tree. Why though is it not possible to widen a primitive to another primitive type and autobox the widened primitive?
Given that a byte argument can be passed to a method that expects an int, why cant the byte in the following example be widened to an int and then boxed to an Integer?
class ScjpTest{
static void goInteger(Integer x){
System.out.println("Going with an Integer");
}
static void goInt(int x){
System.out.println("Going with an int");
}
public static void main(String args[]){
byte b = 5;
goInt(b);
goInteger(b);
}
}
In the above example, goInt(b) is accepted by the compiler but goInteger(b) is rejected.
回答1:
Short answer
The java language only supports some level of carelessness.
Longer answer
I believe that autoboxing was added to support developer carelessness. Specifically in situations like this: "I need an Integer as a parmeter to the method I want to call, but I have an int. Somehow, new Integer(int) never pops into my head. Instead, I'll just send an int and the java compiler will do the new Integer() call for me. Thanks java carelessness support group!"
The folks designing autoboxing were willing to support 1 level of carelessness (int => Integer and back), but were not willing to support auto casting of smaller primitive types to larger primitive types in conjunction with automatic creation and extration from primitive type wrapper classes. I suspect the descision matrix for this would be somewhat larger than the decision matrix for the current autoboxing scheme.
回答2:
Why? Because boxing / autoboxing is only some compiler sugar and not a new type system. It's badly designed and causes trouble at least as often as it simplifies things.
But here are some workarounds for your compile error:
goInteger((int) b);
// these are equivalent
goInteger(((Byte) b).intValue());
goInteger(Byte.valueOf(b).intValue());
回答3:
If we allow too many magic conversions, it'll get very confusing.
The existing conversion rules are already more than people care to understand. Even the language spec got it wrong! See this funny example Java casting: is the compiler wrong, or is the language spec wrong, or am I wrong?
回答4:
In Java, Boxing + Widening is allowed, but not Widening + Boxing.. For goInteger to be accepted, first widening of primitive datatype (byte -> int) is required, which ok and then Boxing is required (int -> Integer). Please find the 5 golden ruled of Widening, Boxing and Vararg:
- Primitive Widening > Boxing > Varargs.
- Widening and Boxing (WB) not allowed.
- Boxing and Widening (BW) allowed.
- While overloading, Widening + vararg and Boxing + vararg can only be used in a mutually exclusive manner i.e. not together.
- Widening between wrapper classes not allowed
I hope this will help you. With regards, Sudipta Deb.
回答5:
I think the order is pretty fascinating. I made out the following playground to see every possible combination. This are my functions:
static void doSomeThing(short i) {
System.out.println("short");
}
static void doSomeThing(short... i) {
System.out.println("short...");
}
static void doSomeThing(Short i) {
System.out.println("SHORT");
}
static void doSomeThing(Short... i) {
System.out.println("SHORT...");
}
static void doSomeThing(long i) {
System.out.println("long");
}
static void doSomeThing(long... i) {
System.out.println("long...");
}
static void doSomeThing(Long i) {
System.out.println("LONG");
}
static void doSomeThing(Long... i) {
System.out.println("LONG...");
}
static void doSomeThing(int i) {
System.out.println("int");
}
static void doSomeThing(int... i) {
System.out.println("int...");
}
static void doSomeThing(Integer i) {
System.out.println("INTEGER");
}
static void doSomeThing(Integer... i) {
System.out.println("INTEGER...");
}
static void doSomeThing(Object i) {
System.out.println("Object");
}
static void doSomeThing(Object... i) {
System.out.println("Object...");
}
Rules:
1.Searches for exactly the same type (int -> int)
2.Widening (int -> long)
3.Boxing (int-> Integer, it is NEVER possible to implicit box AND wide (int -> Long NOT possible without cast))
!!Multiple boxing go BEFORE var args!!
int -> Object will be chosen before int -> int...
4.Var args (int -> int...)
5.Widening + var args (int -> long...)
6.Boxing + var args (int -> Integer...)
7.Boxing + widening + var args (int -> Object...)
public class Main{
public static void main(String...args) {
//primitive int
int i = 0;
doSomeThing(i); //int
//commented out doSomeThing(int i){}
doSomeThing(i); //long. It is not possible to narrow, so short, short... Short and Short... will NEVER be called when the input is larger than a short.
//commented out doSomeThing(long i){}
doSomeThing(i); //INTEGER
//commented out doSomething(Integer i){}
doSomeThing(i); //Object. Notice that there can be multiple boxing before moving to var args
//Error occured: compiler if confused: can either execute int..., long..., Object... or Integer...
//Object... and Integer... are commented out, because in the real world int... will be called first
doSomeThing(i); //int...
//commented out int...
doSomeThing(i); //long...
//commented out long... and uncommented Integer...
doSomeThing(i); //Integer...
//commented out Integer... and uncommented Object...
doSomeThing(i); //Object...
//Integer
//Integer
Integer i = new Integer(0);
doSomeThing(i); //INTEGER
//commented out doSomeThing(Integer i)
doSomeThing(i); //Object
//commented out doSomeThing(Object i)
doSomeThing(i); //int
//commented out doSomeThing(int i)
doSomeThing(i); //long so NOT int... it goes widening again
//commented out doSomeThing(long i)
//Error occured: compliler refused: not both have int..., long..., Integer... and Object...
//int... and long... are commented out
doSomeThing(i); //INTEGER...
//commented out doSomeThing(Integer... i)
doSomeThing(i); //Object...
//commented out doSomeThing(Object... i)
//uncommented doSomeThing(int... and long...)
doSomeThing(i); //int...
//uncommented doSomeThing(int... i)
doSomeThing(i); //long...
}
来源：https://stackoverflow.com/questions/7014171/widening-and-boxing-java-primitives