possible lossy conversion from int to short

问题

I have defined array gx, array arr to be short type. but why the operation at left may end up with int type and I must cast it into short? the compiler error is possible lossy conversion from int to short.

this is my code.

public PixImage sobelEdges() {

short gy=0;
for(int x=1;x<width-1;x++){
    for(int y=1;y<height-1;y++){
       // if(x=){
            for(int z=0;z<3;z++){
            gx[x][y][z]=arr[x-1][y-1][z]-arr[x+1][y-1][z]+2*arr[x-1][y][z]-2*arr[x+1][y][z]+arr[x-1][y+1][z]-arr[x+1][y+1][z];

            }
       // }    
    }     
}   
return this;
// Don't forget to use the method mag2gray() above to convert energies to
// pixel intensities.

}

Is that because the so called unbox? so that means every time I made a operation I need to cast?

回答1:

It's not unboxing; it's "binary numeric promotion". Section 5.6.2 of the JLS states:

When an operator applies binary numeric promotion to a pair of operands, each of which must denote a value that is convertible to a numeric type, the following rules apply, in order:

If any operand is of a reference type, it is subjected to unboxing conversion (§5.1.8).

Widening primitive conversion (§5.1.2) is applied to convert either or both operands as specified by the following rules:

If either operand is of type double, the other is converted to double.

Otherwise, if either operand is of type float, the other is converted to float.

Otherwise, if either operand is of type long, the other is converted to long.

Otherwise, both operands are converted to type int.

and

Binary numeric promotion is performed on the operands of certain operators:

The multiplicative operators *, /, and % (§15.17)

The addition and subtraction operators for numeric types + and - (§15.18.2)

The numerical comparison operators <, <=, >, and >= (§15.20.1)

The numerical equality operators == and != (§15.21.1)

The integer bitwise operators &, ^, and | (§15.22.1)

In certain cases, the conditional operator ? : (§15.25)

(emphasis mine)

When those values are added/multiplied, they are promoted to int before the math is done. At the end, you can cast back to short before assigning back to the array.

gx[x][y][z] = (short) (arr[x-1][y-1][z]-arr[x+1][y-1][z]+2*arr[x-1][y][z]
    -2*arr[x+1][y][z]+arr[x-1][y+1][z]-arr[x+1][y+1][z]);

You will need to cast it back every time you operate with primitive data types that are smaller than int, such as in your short example.

来源：https://stackoverflow.com/questions/24173965/possible-lossy-conversion-from-int-to-short

标签

java

arrays

int

short