Java显式类型转换给出不同的结果

Question

I would like to know in which order does the explicit type conversion work and in what area does it work. In the next example I receive different results.

float S = 0;
for (int i = 1; i <= 10; i++)
    for (int j = 3; j <= 20; j++)
        S += (i * j - 5.)/(2. * i + j / 3);

the result is 621.8933

float S = 0;
for (float i = 1; i <= 10; i++)
    for (float j = 3; j <= 20; j++)
        S += (i * j - 5)/(2. * i + j / 3);

S = 607.3105

float S = 0;
for (int i = 1; i <= 10; i++)
    for (int j = 3; j <= 20; j++)
        S += (float)(i * j - 5)/(2 * i + j / 3);

S = 621.8933

float S = 0;
for (int i = 1; i <= 10; i++)
    for (int j = 3; j <= 20; j++)
        S += (i * j - 5)/(2 * i + j / 3.);

S = 607.3105

Obviously the correct result is 607.3105. Why doesn't it promote when I write (float) or multiply by 2., but it works when I divide by 3.? Thanks in advance.

Answer 1

Code 1 S += (i * j - 5.)/(2. * i + j / 3); :

• j is an int , thus complete divisor is int and an implicit cast to int is performed

Code 2 S += (i * j - 5)/(2. * i + j / 3);

• j , x are floats and all consecutive results are floats too

Code 3 S += (float)(i * j - 5)/(2 * i + j / 3);

• j is an int , thus complete divisor is int and an implicit cast to int is performed
• The explicit float - is performed after computing of the divisor

Code 4 S += (i * j - 5)/(2 * i + j / 3.);

• Due to '3.' the divisor becomes of type 'double'

Note that 3. is a double, 3.f is a float. This results in usage of different precision within the operation.