大数加减乘除问题

下列代码给出的大数的加减乘代码的java代码，由于大数相除比较复杂，没有给出

/*大数问题(加、减、乘、除)*/
/*代码中给出了1、两个整数相加；2、两个整数相减；这个两种基本情况
 * 其他情况均可以转化成这两种运算；
 * 比如两个负数相加为2个整数相加加符号
 * 整数减负数为2个整数相加
 * 负数减整数为两个整数相加加负号
 * */
import org.junit.Test;

public class BigNumberPlus {
	@Test
	public void test(){
		//String rs = this.plus("0","000");
		//String rs = this.minus("1","100000");
		//String rs = this.singleMultipy("99999999999999999","9");
		//String rs = this.mutiMultiply("222221","222221");
		String rs = this.mutiMultiply("222221","1000000000000");
		System.out.println(rs);
	}
	/*两个整数相加*/
	public String plus(String a,String b){
		StringBuffer sb = new StringBuffer();
		int carry = 0;
		int i,j;
		for(i=a.length()-1,j=b.length()-1;i>=0||j>=0;i--,j--){
			int ca = i>=0?a.charAt(i)-'0':0;
			int cb = j>=0?b.charAt(j)-'0':0;
			int cv = (ca+cb+carry)%10;
			carry = (ca+cb+carry)/10;
			sb.insert(0,(char)(cv+'0'));
		}
		if(carry==1)
			sb.insert(0,1);
		return sb.toString();
	}
	/*两个整数相减*/
	public String minus(String a,String b){
		boolean minus_flag = false; 
		int carry = 0;
		StringBuffer sb = new StringBuffer();
		/*变成大数减小数*/
		if(a.length()<b.length()||(a.length()==b.length()&&(a.compareTo(b)<0))){//a < b
			String temp = a;
			a = b;
			b = temp;
			minus_flag = true;
		}
		for(int i=a.length()-1,j--){
			
			int ca = i>=0?a.charAt(i)-'0':0;
			int cb = j>=0?b.charAt(j)-'0':0;
			int cv;	
			if(ca-cb+carry>=0)
				cv = ca-cb+carry;
			else{
				cv = ca-cb+carry+10;
				carry = -1;
			}
			if(cv!=0)
				sb.insert(0,(char)(cv+'0'));
		}
		if(minus_flag)
			sb.insert(0,'-');
		return sb.toString();
	}
	/*多位数与多位数相乘（（正数乘以正数））*/
	public String mutiMultiply(String a,String b){
		
		String sum = "0";
		for(int i = b.length()-1;i>=0;i--){
			String tmp = singleMultipy(a,b.substring(i,i+1));
			for(int j=i;j<b.length()-1;j++){//tmp末尾要补0（乘以10），才能与上次结果相加
				tmp+="0";
			}
			sum = plus(sum,tmp);
		}
		
		return sum.toString();
	}
	/*一个多位数乘以一个个位数（正数乘以正数）*/
	public String singleMultipy(String a,String b){
		int carry = 0;
		int cb = b.charAt(0)-'0';
		StringBuffer sb = new StringBuffer();
		for(int i = a.length()-1;i>=0;i--){
			int ca = i>=0?a.charAt(i)-'0':0;
			int cv = (ca*cb+carry)%10;
			carry = (ca*cb+carry)/10;
			sb.insert(0,(char)(cv+'0'));
		}
		if(carry>0)
			sb.insert(0,(char)(carry+'0'));
		return sb.toString();
		
	}
}

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