# BigIntegerMath log2() function | Guava | Java

• Last Updated : 15 Nov, 2018

The method log2(BigInteger x, RoundingMode mode) of Guava’s BigIntegerMath class returns the base-2 logarithm of x, rounded according to the specified rounding mode.

Syntax:

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```public static int log2(BigInteger x, RoundingMode mode)
```

Parameters: This method takes the following parameters:

• x: The BigInteger number whose log is to be found.
• mode: The rounding mode for calculating the log of 2.

Return Value: This method returns the base-2 logarithm of x, rounded according to the specified rounding mode.

Exceptions: This method throws the following exceptions:

• IllegalArgumentException: if x <= 0.
• ArithmeticException: if mode is RoundingMode.UNNECESSARY and x is not a power of 2.

#### Enum RoundingMode

Enum ConstantDescription
CEILINGRounding mode to round towards positive infinity.
DOWNRounding mode to round towards zero.
FLOORRounding mode to round towards negative infinity.
HALF_DOWNRounding mode to round towards “nearest neighbor” unless both neighbors are equidistant, in which case round down.
HALF_EVENRounding mode to round towards the “nearest neighbor” unless both neighbors are equidistant, in which case, round towards the even neighbor.
HALF_UPRounding mode to round towards “nearest neighbor” unless both neighbors are equidistant, in which case round up.
UNNECESSARYRounding mode to assert that the requested operation has an exact result, hence no rounding is necessary.
UPRounding mode to round away from zero.

Below examples illustrates the BigIntegerMath.log2() method:

Example 1:

 `// Java code to show implementation of``// log2(BigInteger x, RoundingMode mode) method``// of Guava's BigIntegerMath class`` ` `import` `java.math.*;``import` `com.google.common.math.BigIntegerMath;`` ` `class` `GFG {`` ` `    ``// Driver code``    ``public` `static` `void` `main(String args[])``    ``{``        ``BigInteger a1 = BigInteger.valueOf(``1024``);`` ` `        ``// Using log2(BigInteger x, RoundingMode mode)``        ``// method of Guava's BigIntegerMath 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(``"Log base 2 of "` `+ a1``                           ``+ ``" with rounding mode HALF_EVEN = "``                           ``+ BigIntegerMath``                                 ``.log2(a1,``                                        ``RoundingMode.HALF_EVEN));`` ` `        ``BigInteger a2 = BigInteger.valueOf(``15``);`` ` `        ``// Using log2(BigInteger x, RoundingMode mode)``        ``// method of Guava's BigIntegerMath class``        ``// The RoundingMode HALF_DOWN rounds towards``        ``// "nearest neighbor" unless both neighbors``        ``// are equidistant, in which case round down.``        ``System.out.println(``"Log base 2 of "` `+ a2``                           ``+ ``" with rounding mode HALF_DOWN = "``                           ``+ BigIntegerMath``                                 ``.log10(a2,``                                        ``RoundingMode.HALF_DOWN));``    ``}``}`

Output:

```Log base 2 of 1024 with rounding mode HALF_EVEN = 10
Log base 2 of 15 with rounding mode HALF_DOWN = 1
```

Example 2: To show IllegalArgumentException

 `// Java code to show implementation of``// log2(BigInteger x, RoundingMode mode) method``// of Guava's BigIntegerMath class`` ` `import` `java.math.*;``import` `com.google.common.math.BigIntegerMath;`` ` `class` `GFG {`` ` `    ``// Driver code``    ``public` `static` `void` `main(String args[])``    ``{`` ` `        ``try` `{``            ``BigInteger a1 = BigInteger.valueOf(-``5``);`` ` `            ``// Using log2(BigInteger x, RoundingMode mode)``            ``// method of Guava's BigIntegerMath 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 a1 <= 0``            ``System.out.println(``"Log base 2 of "` `+ a1``                               ``+ ``" with rounding mode HALF_EVEN = "``                               ``+ BigIntegerMath``                                     ``.log2(a1,``                                            ``RoundingMode.HALF_EVEN));``        ``}`` ` `        ``catch` `(Exception e) {``            ``System.out.println(``"Exception: "` `+ e);``        ``}``    ``}``}`

Output:

```Exception: java.lang.IllegalArgumentException: x (-5) must be > 0
```

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