Xử lý số lớn bằng Java

@ltd em mới học java nên đang gà lắm. chỉ mới biết thế này thôi
http://ideone.com/otClJP

package oop.asg02;

import java.io.IOException;

public class BigInteger
{
	private String s;
    public BigInteger(long init) {
        s = String.valueOf(init);
    }
   
    public BigInteger(String init) throws IOException {
    	if (init == "") 
			s = "0";
		else s = init;
    	while (s.charAt (0) == '0' && s.length() > 1) 
			s = s.substring (1);
		for(int i=0; i<init.length(); i++){
			if(init.charAt(i)< '0' | init.charAt(i)> '9'){
				throw new IOException ();
    		}
    	}
    }

    public String toString() {
		return s;
    }

    public boolean equals(Object other) {
		return toString().equals(other.toString());
    }

    public long toLong() {
        return Long.parseLong(s);  
    }

	public BigInteger add(BigInteger other)throws IOException {	
		while(this.s.length()>other.s.length()){
			other.s = "0" + other.s;
		}
		while(this.s.length()<other.s.length()){
			s = "0" + s;
		}
		
		String result = "";
		int remember = 0, temp = 0;
		
		for(int i = s.length()- 1;i >= 0;i--){		
			temp = (int)(s.charAt(i)-'0') + (int)(other.s.charAt(i)-'0')  + remember;
			if(temp >=10){
				temp = temp % 10;
				remember = 1;
			}
			else{
				remember = 0;
			}
			result = Integer.toString(temp) + result;
		}
		if(remember == 1){
			result = '1' + result ;
		}
		return new BigInteger(result);
	}
	public int compareTo(BigInteger other){
		if(this.s.length() < other.s.length())
			return -1;
		if(this.s.length() > other.s.length())
			return 1;
		if(this.s.length() == other.s.length()){
			for(int i=0; i<this.s.length(); i++){
				if(this.s.charAt(i) < other.s.charAt(i))
					return -1;
				if(this.s.charAt(i) > other.s.charAt(i))
					return 1;
			}
		}
		return 0;
	}

	public BigInteger subtract(BigInteger other)throws IOException {
		int remember = 0;
		int temp = 0;
		String result = new String ();
		while(this.s.length()>other.s.length()){
			other.s = "0" + other.s;
		}
		while(this.s.length()<other.s.length()){
			s = "0" + s;
		}
		BigInteger s1 = new BigInteger(s);
		if(s1.compareTo(other)==1 || s1.compareTo(other)==0)
		{
			for(int i = s.length()- 1;i >= 0;i--){		
					temp = (int)(this.s.charAt(i))-'0' - ((int)(other.s.charAt(i))-'0' + remember);
					if(temp <0)
					{
						temp = (int)(this.s.charAt(i))-'0' + 10 - ((int)(other.s.charAt(i))-'0' + remember);
						remember = 1;
					}
					else{
						remember=0;
					}
					result = Integer.toString(temp) + result;
				}
				return new BigInteger(result);
		}
		else{
			for(int i = s.length()- 1;i >= 0;i--){		
				temp = (int)(other.s.charAt(i))-'0' - ((int)(this.s.charAt(i))-'0' + remember);
				if(temp <0)
				{
					temp = (int)(other.s.charAt(i))-'0' + 10 - ((int)(this.s.charAt(i))-'0' + remember);
					remember = 1;
				}
				else{
					remember=0;
				}
				result = Integer.toString(temp) + result;
			}
			return new BigInteger("-" + result);

		}
	}
	public BigInteger clone(){
		BigInteger init = null;
		try{
			init = new BigInteger(s);
		} catch (IOException e){
			System.out.println("day co ki tu char!");
		}
		return init;
	}
}