java实现任意整数相乘

public class Mutiply {

	public static void main(String[] args) {
		BigInteger multiplier = new BigInteger("-673589893565890368590578967285490682958485023495834902583490589345798583495723487563482756347856673589893565890368590578967285490682958485023495834902583490589345798583495723487563482756347856");
		BigInteger multiplicand = new BigInteger("-859085903425878805823409584039269057908349673285902345849035890878809586908504397562358476278584376328590385689457043257834927534095734956347853485");
		long st1 = System.currentTimeMillis();
		for(int index=0;index<1000;index++){
			multiply(multiplier, multiplicand);
		}
		System.out.println(System.currentTimeMillis()-st1);
		long st2 = System.currentTimeMillis();
		for(int index=0;index<1000;index++){
			multiplier.multiply(multiplicand);
		}
		System.out.println(System.currentTimeMillis()-st2);
	}
	
	public static String multiply(BigInteger multiplier, BigInteger multiplicand){
		boolean isResultNegative = false;
		if(multiplier.signum()==0 || multiplicand.signum()==0){
			return "0";
		}
		if(multiplier.signum()==-1 || multiplicand.signum()==-1){
			if(!(multiplier.signum()==-1 && multiplicand.signum()==-1)){
				isResultNegative = true;
			}
		}
		return (isResultNegative?"-" : "") + multiply(multiplier.toString().replace("-", ""), multiplicand.toString().replace("-", ""));
	}
	
	public static String multiply(String multiplier, String multiplicand){
		if(multiplier.length()>multiplicand.length()){
			String temp = multiplier;
			multiplier = multiplicand;
			multiplicand = temp;
		}
		int length = multiplicand.length()+multiplier.length();
		char[] result = new char[length];
		for(int indexOfResult=0;indexOfResult<result.length;indexOfResult++){
			result[indexOfResult] = '0';
		}
		
		String[] tempResults = new String[multiplier.length()];
		for(int indexOfMultiplier=0;indexOfMultiplier<multiplier.length();indexOfMultiplier++){
			char[] tempResult = new char[length];
			for(int indexOftempResult=0;indexOftempResult<tempResult.length;indexOftempResult++){
				tempResult[indexOftempResult] = '0';
			}
			int singleMultiplier = Integer.valueOf(String.valueOf(multiplier.charAt(multiplier.length()-1-indexOfMultiplier)));
			if(singleMultiplier!=0){
				int singleStep = 0;
				for(int indexOfmultiplicand=0;indexOfmultiplicand<multiplicand.length();indexOfmultiplicand++){
					int singleMultiplicand = Integer.valueOf(String.valueOf(multiplicand.charAt(multiplicand.length()-1-indexOfmultiplicand)));
					int singleResult = singleStep + singleMultiplier*singleMultiplicand;
					singleStep = singleResult/10;
					tempResult[tempResult.length-indexOfMultiplier-indexOfmultiplicand-1] = (""+singleResult%10).charAt(0);
				}
				tempResult[tempResult.length-indexOfMultiplier-multiplicand.length()-1] = ("" + singleStep).charAt(0);
			}
			
			tempResults[indexOfMultiplier] = String.valueOf(tempResult);
		}
		
		int step = 0;
		for(int indexOfResult=0; indexOfResult<length; indexOfResult++){
			int multiResult = step;
			for(int indexOfTempResults=0;indexOfTempResults<tempResults.length;indexOfTempResults++){
				multiResult += Integer.valueOf(String.valueOf(tempResults[indexOfTempResults].charAt(length-1-indexOfResult)));
			}
			step = multiResult/10;
			result[result.length-indexOfResult-1] = (""+multiResult%10).charAt(0);
		}
		return String.valueOf(result);
	}
}

结果对比：

578671582252594476786199322413773832540849019479781209035507587576779550181736165461400001640216752920490028498890769204733283108280630943510991915921171610703486725119777750465131309033130699124992461394973593046143133767470798477438073390294712136103115909945569596014299669909031490482950743553619069179063137722898136350387343581878160
578671582252594476786199322413773832540849019479781209035507587576779550181736165461400001640216752920490028498890769204733283108280630943510991915921171610703486725119777750465131309033130699124992461394973593046143133767470798477438073390294712136103115909945569596014299669909031490482950743553619069179063137722898136350387343581878160

虽然结果一致，但由于本人实现方式不算优，性能比JAVA默认的BigInteger实现整数相乘性能差很多，尤其是在计算遍数多的情况下。不是一个数量级的。