Java番石榴 | LongMath 类的 log2(long x, RoundingMode mode) 带示例

Guava 的 LongMath 类的方法log2(long x, RoundingMode mode)接受两个参数，根据第二个参数指定的舍入模式计算第一个参数的以 2 为底的对数值。

句法：

public static int log2(long x, RoundingMode mode)

参数：该方法有 2 个参数，其中x是 long， mode是指定的舍入模式。

返回值：该方法返回x的Base-2对数，按照指定的取整方式取整。

例外：此方法抛出以下参数：

• IllegalArgumentException：如果 x <= 0。
• ArithmeticException：如果 mode 是 RoundingMode.UNNECESSARY 并且 x 不是 2 的幂。

枚举舍入模式

Enum Constant	Description
CEILING	Rounding mode to round towards positive infinity.
DOWN	Rounding mode to round towards zero.
FLOOR	Rounding mode to round towards negative infinity.
HALF_DOWN	Rounding mode to round towards “nearest neighbor” unless both neighbors are equidistant, in which case round down.
HALF_EVEN	Rounding mode to round towards the “nearest neighbor” unless both neighbors are equidistant, in which case, round towards the even neighbor.
HALF_UP	Rounding mode to round towards “nearest neighbor” unless both neighbors are equidistant, in which case round up.
UNNECESSARY	Rounding mode to assert that the requested operation has an exact result, hence no rounding is necessary.
UP	Rounding mode to round away from zero.

下面的例子说明了 Guava LongMath 的 log2() 方法的实现：

示例 1：

// Java code to show implementation of
// log2(long x, RoundingMode mode) method
// of Guava's LongMath class
  
import java.math.RoundingMode;
import com.google.common.math.LongMath;
  
class GFG {
  
    // Driver code
    public static void main(String args[])
    {
        long n1 = 1111;
  
        // Using log2(long x, RoundingMode mode)
        // method of Guava's LongMath class
        // The RoundingMode HALF_EVEN rounds towards
        // the "nearest neighbor" unless both neighbors
        // are equidistant, in which case, round towards
        // the even neighbor.
        System.out.println(LongMath.log2(
            n1,
            RoundingMode.HALF_EVEN));
  
        long n2 = 205;
  
        // Using log2(long x, RoundingMode mode)
        // method of Guava's LongMath class
        // The RoundingMode HALF_DOWN rounds towards
        // "nearest neighbor" unless both neighbors
        // are equidistant, in which case round down.
        System.out.println(LongMath.log2(
            n2,
            RoundingMode.HALF_DOWN));
    }
}

输出：

10
8

示例 2：

// Java code to show implementation of
// log2(long x, RoundingMode mode) method
// of Guava's LongMath class
  
import java.math.RoundingMode;
import com.google.common.math.LongMath;
  
class GFG {
  
    static long findlog2(long x, RoundingMode mode)
    {
        try {
            // Using log2(long x, RoundingMode mode)
            // method of Guava's LongMath class
            // The RoundingMode HALF_EVEN rounds towards
            // the "nearest neighbor" unless both neighbors
            // are equidistant, in which case, round towards
            // the even neighbor.
            // This should throw "IllegalArgumentException"
            // as x <= 0
            long ans = LongMath.log2(x, mode);
  
            // Return the answer
            return ans;
        }
        catch (Exception e) {
            System.out.println(e);
            return -1;
        }
    }
  
    // Driver code
    public static void main(String args[])
    {
        long x = -122;
  
        try {
  
            // Function calling
            findlog2(x, RoundingMode.HALF_EVEN);
        }
        catch (Exception e) {
            System.out.println(e);
        }
    }
}

输出：

java.lang.IllegalArgumentException: x (-122) must be > 0

参考： https://google.github.io/guava/releases/20.0/api/docs/com/google/common/math/LongMath.html#log2-long-java.math.RoundingMode-