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

    返回最大的double值（在正无穷方向上最接近的）小于或等于参数a的一个数学意义上的整数，特例：

    如果参数值等于数学上的整数，则返回结果与参数值相同

    如果参数是NaN（非数值）或是无穷或是+0或是-0，则返回值和参数值相同

    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（在负无穷方向最接近的）大于或等于参数a的一个数学意义上的整数，特例：

如果参数值已经等于一个数学意义上的整数，则返回结果与参数相同
如果参数是NaN（非数值）或者是无穷，或是+0，-0，则返回结果与参数相同
如果参数值小于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