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The Apache Commons Codec package contains simple encoder and decoders for various formats such as Base64 and Hexadecimal. In addition to these widely used encoders and decoders, the codec package also maintains a collection of phonetic encoding utilities.
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/* ====================================================================
   Licensed to the Apache Software Foundation (ASF) under one or more
   contributor license agreements.  See the NOTICE file distributed with
   this work for additional information regarding copyright ownership.
   The ASF licenses this file to You under the Apache License, Version 2.0
   (the "License"); you may not use this file except in compliance with
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       http://www.apache.org/licenses/LICENSE-2.0

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   distributed under the License is distributed on an "AS IS" BASIS,
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   See the License for the specific language governing permissions and
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package org.apache.poi.ss.formula.functions;

import org.apache.poi.ss.formula.OperationEvaluationContext;
import org.apache.poi.ss.formula.eval.ErrorEval;
import org.apache.poi.ss.formula.eval.NumberEval;
import org.apache.poi.ss.formula.eval.OperandResolver;
import org.apache.poi.ss.formula.eval.RefEval;
import org.apache.poi.ss.formula.eval.ValueEval;

/**
 * Implementation for Excel Bin2Dec() function.
 * 

 * Syntax:
 Bin2Dec  (number)

 * 

 * Converts a binary number to decimal.
 * 

 * Number is the binary number you want to convert. Number cannot contain more than 10 characters (10 bits).
 * The most significant bit of number is the sign bit. The remaining 9 bits are magnitude bits.
 * Negative numbers are represented using two's-complement notation.
 * 
 * Remark
 * If number is not a valid binary number, or if number contains more than 10 characters (10 bits),
 * BIN2DEC returns the #NUM! error value.
 *
 * @author cedric dot walter @ gmail dot com
 */
public class Bin2Dec extends Fixed1ArgFunction implements FreeRefFunction {

    public static final FreeRefFunction instance = new Bin2Dec();

    public ValueEval evaluate(int srcRowIndex, int srcColumnIndex, ValueEval numberVE) {
        final String number;
        if (numberVE instanceof RefEval) {
            RefEval re = (RefEval) numberVE;
            number = OperandResolver.coerceValueToString(re.getInnerValueEval(re.getFirstSheetIndex()));
        } else {
            number = OperandResolver.coerceValueToString(numberVE);
        }
        if (number.length() > 10) {
            return ErrorEval.NUM_ERROR;
        }

        String unsigned;

        //If the leftmost bit is 0 -- number is positive.
        boolean isPositive;
        if (number.length() < 10) {
            unsigned = number;
            isPositive = true;
        } else {
            unsigned = number.substring(1);
            isPositive = number.startsWith("0");
        }

        String value;
        try {
            if (isPositive) {
                //bit9*2^8 + bit8*2^7 + bit7*2^6 + bit6*2^5 + bit5*2^4+ bit3*2^2+ bit2*2^1+ bit1*2^0
                int sum = getDecimalValue(unsigned);
                value = String.valueOf(sum);
            } else {
                //The leftmost bit is 1 -- this is negative number
                //Inverse bits [1-9]
                String inverted = toggleBits(unsigned);
                // Calculate decimal number
                int sum = getDecimalValue(inverted);
    
                //Add 1 to obtained number
                sum++;
    
                value = "-" + sum;
            }
        } catch (NumberFormatException e) {
            return ErrorEval.NUM_ERROR;
        }

        return new NumberEval(Long.parseLong(value));
    }

    private int getDecimalValue(String unsigned) {
        int sum = 0;
        int numBits = unsigned.length();
        int power = numBits - 1;

        for (int i = 0; i < numBits; i++) {
            int bit = Integer.parseInt(unsigned.substring(i, i + 1));
            int term = (int) (bit * Math.pow(2, power));
            sum += term;
            power--;
        }
        return sum;
    }

    private static String toggleBits(String s) {
        long i = Long.parseLong(s, 2);
        long i2 = i ^ ((1L << s.length()) - 1);
        String s2 = Long.toBinaryString(i2);
        while (s2.length() < s.length()) s2 = '0' + s2;
        return s2;
    }

    public ValueEval evaluate(ValueEval[] args, OperationEvaluationContext ec) {
        if (args.length != 1) {
            return ErrorEval.VALUE_INVALID;
        }
        return evaluate(ec.getRowIndex(), ec.getColumnIndex(), args[0]);
    }
}