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# Automatic conversion of float to double in numeric promotion

Greenhorn
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Hi I am new to Java, and I am trying to figure out the automatic conversion of float to double.

for example: double x = 39.21; float y = 2.1; c= x+y; the book said this will not compile because floating point literals are assumed to be double. So, if we wanted this to work we would have to initialize float y= 2.1f . But what I don,t understand is if y is automatically assumed to be a double , making x and y the same data type. Why does this code not compile?.

2) Also short x=14; float y=13; double z =30;  c= x*y/z; here short is converted into an int and in the numeric operation x*y, 'x' gets converted to a float. My question is since float is not initialized with an f. shouldn't float value of y be assumed as a double and  and hence in x*y shouldn't 'x' be promoted to a double instead of float ?

Could really use some help related to this. Thanks.

Marshal
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Mau Dc wrote:for example: float y = 2.1; the book said this will not compile because floating point literals are assumed to be double. So, if we wanted this to work we would have to initialize float y= 2.1f .

Yes, literals like "2.1" are assumed to be double. And yes, if you want to assign that number to a floating-point variable you have to write "2.1f" to make it a float literal.

But what I don,t understand is if y is automatically assumed to be a double , making x and y the same data type. Why does this code not compile?.

But no! What makes you think that y is assumed to be a double? Yes, certain literals are assumed to be double, but that rule doesn't apply to variables. If you declare a variable to be of type float -- as in that example -- then it's a float variable.

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