为什么可以将double转换为int，但是不能将double转换为String？

Question

I convert other type to String by String.valueOf() like following:

 String s = String.valueOf(2.5);

i also convert double to int by casting like following:

 int i = (int) 2.5 ;

i want casting double to String like following:

 String s = (String) 2.5; 

but i give following error:

 Cannot cast from double to String

why casting double to int is possible but casting double to String is not possible?

Answer 1

Casting from one primitive type to another is standard, well understood and supported in machine code. Ie Java just gives you the syntax to produce the appropriate machine code.

Even C language had support for this.

Converting to objects is different as it requires you to call a library and even though C++ has operator overloading to support such conversions, it is not a cast as such. C and Java do not support implicit conversion like this.

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BTW The simplest way to convert to a String is to use the following.

primitive p = ...
String s = "" + p;

This is not efficient as it uses a StringBuilder, however it's not efficient to create a String either so it's only not as bad, rather than being very good. btw I have libraries to turn primitives to/from text without creating objects.

However, I use this because it is more efficient for the developer and I waiting for a profiler to tell me this is not good enough in which case I wouldn't use String.valueOf() either.

Answer 2

double a = 44; 
String b = String.valueOf(a);

Answer 3

double and int are both numeric types, thus they can be converted by casting.

String is not a numeric type.

Answer 4

According to Java specifications, you need to convert the primitive type first:

Any type may be converted to type String by string conversion.

A value x of primitive type T is first converted to a reference value as if by giving it as an argument to an appropriate class instance creation expression (§15.9):

If T is boolean, then use new Boolean(x).

If T is char, then use new Character(x).

If T is byte, short, or int, then use new Integer(x).

If T is long, then use new Long(x).

If T is float, then use new Float(x).

If T is double, then use new Double(x).

This reference value is then converted to type String by string conversion.

Then

(...) the conversion is performed as if by an invocation of the toString method of the referenced object with no arguments; but if the result of invoking the toString method is null, then the string "null" is used instead

See 5.1.11. String Conversion in http://docs.oracle.com/javase/specs/jls/se7/html/jls-5.html

Answer 5

@Samiey Mehdi : I will give u what happens when U Type Cast From one format to another ..

Example

Double a = 2.5; // consider it

step 1:first machine will convert this number into binary format .. double a =(0000 0000 0010.0101);

step 2:when your type casting it into int .. first it will remove decimals.. lets make the binary into (0000 0000 0010)

step 3: it will truncate it into 32 bit after it is completely fallen into int range then it will print the value. int range( -2,147,483,648 to 2,147,483,647) it will print now Note: if the value of the non decimal is more than range of type cast it will lose the data or digit (shrinks it)and prints the remaining value

But In Case of String Format .. Step two fails .. it cant able to convert the value of binary digit into string range [bcos string don't have any range]. so it will print incompatible types [compile time error].................