1、double java.lang.Math.floor(double a)

返回最大的double值（在正無窮方向上最接近的）小于或等于參數a的一個數學意義上的整數，特例：

如果參數值等于數學上的整數，則返回結果與參數值相同

如果參數是NaN（非數值）或是無窮或是+0或是-0，則返回值和參數值相同

Returns the largest (closest to positive infinity) double value that is less than or equal to the argument and is equal to a mathematical integer. Special cases:

If the argument value is already equal to a mathematical integer, then the result is the same as the argument.

If the argument is NaN or an infinity or positive zero or negative zero, then the result is the same as the argument.

Math.floor(2.0)=2.0
Math.floor(2.1)=2.0
Math.floor(2.5)=2.0
Math.floor(2.9)=2.0

Math.floor(-2.0)=-2.0

Math.floor(-2.1)=-3.0

Math.floor(-2.5)=-3.0

Math.floor(-2.9)=-3.0

2、double java.lang.Math.ceil(double
a)
返回最小的double值（在負無窮方向最接近的）大于或等于參數a的一個數學意義上的整數，特例：

如果參數值已經等于一個數學意義上的整數，則返回結果與參數相同
如果參數是NaN（非數值）或者是無窮，或是+0，-0，則返回結果與參數相同
如果參數值小于0或大于-1.0

Returns the smallest (closest to negative
infinity) double value that is greater than or equal
to the argument and is equal to a mathematical integer. Special
cases:

If the argument value is already equal to
a mathematical integer, then the result is the same as the
argument.
If the argument is NaN or an infinity or
positive zero or negative zero, then the result is the same as the
argument.
If the argument value is less than zero
but greater than -1.0, then the result is negative
zero.

Math.ceil(2.0)=2.0

Math.ceil(2.1)=3.0

Math.ceil(2.5)=3.0

Math.ceil(2.9)=3.0

Math.ceil(-2.0)=-2.0

Math.ceil(-2.1)=-2.0

Math.ceil(-2.5)=-2.0

Math.ceil(-2.9)=-2.0

double java.lang.Math.rint(double
a)

Returns the double value that
is closest in value to the argument and is equal to a mathematical
integer. If two double values that are mathematical
integers are equally close, the result is the integer value that is
even. Special cases:

If the argument value is already equal to
a mathematical integer, then the result is the same as the
argument.
If the argument is NaN or an infinity or
positive zero or negative zero, then the result is the same as the
argument.

Math.rint(2.0)=2.0

Math.rint(2.5)=2.0

Math.rint(2.6)=3.0

Math.rint(2.9)=3.0

Math.rint(-2.0)=-2.0

Math.rint(-2.5)=-2.0

Math.rint(-2.6)=-3.0

Math.rint(-2.9)=-3.0

long java.lang.Math.round(double
a)

Returns the closest long to
the argument. The result is rounded to an integer by adding 1/2,
taking the floor of the result, and casting the result to type
long. In other words, the result is equal to the value
of the expression:

(long)Math.floor(a + 0.5d)

Special cases:

If the argument is NaN, the result is 0.
If the argument is negative infinity or any value less than or equal to the value of Long.MIN_VALUE, the result is equal to the value of Long.MIN_VALUE.
If the argument is positive infinity or any value greater than or equal to the value of Long.MAX_VALUE, the result is equal to the value of Long.MAX_VALUE.

Math.round(2.0)=2
Math.round(2.5)=3
Math.round(2.6)=3
Math.round(2.9)=3

Math.round(-2.0)=-2

Math.round(-2.5)=-2

Math.round(-2.6)=-3

Math.round(-2.9)=-3

總結

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