Java BigInt Implementation

[shuniu]

Here is my "Big Number" implementation in Java.
Adapted from the book "Programming Challenges".
Hope it helps!

[java]class BigInt implements Comparable {

int PLUS = 1;
int MINUS = -1;
int MAXDIGITS = 500;

char[] digits;
int signbit;
int lastdigit;

private BigInt() {
digits = new char[MAXDIGITS];
signbit = 1;
lastdigit = 0;
}

public BigInt(String s) {
if (s.charAt(0)=='-') {
signbit = -1;
s = s.substring(1,s.length());
} else {
signbit = 1;
}
digits = new char[MAXDIGITS];
for (int i=0; i<s.length(); i++) {
digits[i] = (char)((int)s.charAt(s.length()-i-1)-(int)'0');
}
lastdigit = s.length()-1;
}

public BigInt(int i) {
this(""+i);
}

public BigInt add(int i) {
return add(new BigInt(i));
}

public BigInt subtract(int i) {
return subtract(new BigInt(i));
}

public BigInt multiply(int i) {
return multiply(new BigInt(i));
}

public BigInt divide(int i) {
return divide(new BigInt(i));
}

public BigInt add(BigInt b) {
BigInt c = new BigInt();
if (signbit==b.signbit) {
c.signbit = signbit;
} else {
if (signbit==MINUS) {
signbit = PLUS;
c = b.subtract(this);
signbit = MINUS;
} else {
b.signbit = PLUS;
c = subtract(b);
b.signbit = MINUS;
}
return c;
}
c.lastdigit = Math.max(lastdigit, b.lastdigit)+1;
int carry = 0;
for (int i=0; i<=c.lastdigit; i++) {
c.digits[i] = (char) ((carry+digits[i]+b.digits[i])%10);
carry = (carry+digits[i]+b.digits[i])/10;
}
c.zeroJustify();
return c;
}

public BigInt subtract(BigInt b) {
BigInt c = new BigInt();
if (signbit==MINUS || b.signbit==MINUS) {
b.signbit = -1*b.signbit;
c = add(b);
b.signbit = -1*b.signbit;
return c;
}
if (compareTo(b)<0) {
c = b.subtract(this);
c.signbit = MINUS;
return c;
}
c.lastdigit = Math.max(lastdigit,b.lastdigit);
int borrow = 0;
for (int i=0; i<=c.lastdigit; i++) {
int v = digits[i] - borrow - b.digits[i];
if (digits[i] > 0) {
borrow = 0;
}
if (v<0) {
v = v+10;
borrow = 1;
}
c.digits[i] = (char)(v%10);
}
c.zeroJustify();
return c;
}

public BigInt multiply(BigInt b) {
BigInt c = new BigInt();
BigInt row = (BigInt)clone();
for (int i=0; i<=b.lastdigit; i++) {
for (int j=1; j<=b.digits[i]; j++) {
c = c.add(row);
}
row.digitShift(1);
}
c.signbit = signbit*b.signbit;
c.zeroJustify();
return c;
}

public BigInt divide(BigInt b) {
BigInt c = new BigInt();
c.signbit = signbit * b.signbit;
int asignbit = signbit;
int bsignbit = b.signbit;
signbit = PLUS;
b.signbit = PLUS;
c.lastdigit = lastdigit;
BigInt row = new BigInt();
for (int i=lastdigit; i>=0; i--) {
row.digitShift(1);
row.digits[0] = digits[i];
c.digits[i] = 0;
while (row.compareTo(b)>=0) {
c.digits[i]++;
row = row.subtract(b);
}
}
signbit = asignbit;
b.signbit = b.signbit;
c.zeroJustify();
return c;
}

public boolean isZero() {
return (lastdigit==0 && digits[0]==0);
}

private void digitShift(int d) {
if (!isZero()) {
for (int i=lastdigit; i>=0; i--) {
digits[i+d] = digits[i];
}
for (int i=0; i<d; i++) {
digits[i] = 0;
}
lastdigit = lastdigit+d;
}
}

public int compareTo(Object o) {
BigInt b = (BigInt)o;
if (signbit==MINUS && b.signbit==PLUS) {
return -1;
} else if (signbit==PLUS && b.signbit==MINUS) {
return 1;
}
if (lastdigit < b.lastdigit) {
return (-1*signbit);
} else if (lastdigit > b.lastdigit) {
return signbit;
}
for (int i=lastdigit; i>=0; i--) {
if (digits[i]>b.digits[i]) {
return signbit;
} else if (digits[i]<b.digits[i]) {
return (-1*signbit);
}
}
return 0;
}

public Object clone() {
BigInt c = new BigInt();
c.signbit = signbit;
c.lastdigit = lastdigit;
for (int i=0; i<=lastdigit; i++) {
c.digits[i] = digits[i];
}
return c;
}

private void zeroJustify() {
while(lastdigit>0 && digits[lastdigit]==0) {
lastdigit--;
}
if (lastdigit==0 && digits[0]==0) {
signbit = PLUS;
}
}

public String toString() {
String s = "";
if (signbit==MINUS) {
s += "-";
}
for (int i=lastdigit; i>=0; i--) {
s += (char)('0'+digits[i]);
}
return s;
}

public static void main (String[] args) {
while (true) {
BigInt a = new BigInt(readLine());
BigInt b = new BigInt(readLine());
System.out.println(a+"+"+b+"="+(a.add(b)));
System.out.println(a+"-"+b+"="+(a.subtract(b)));
System.out.println(a+"*"+b+"="+(a.multiply(b)));
System.out.println(a+"/"+b+"="+(a.divide(b)));
System.out.println();
}
}

static String readLine () {
int maxLg = 500;
byte lin[] = new byte [maxLg];
int lg = 0, car = -1;
String line = "";
try {
while (lg < maxLg) {
car = System.in.read();
if ((car < 0) || (car == '\n')) break;
lin [lg++] += car;
}
}
catch (Exception e) {
return (null);
}
if ((car < 0) && (lg == 0)) return (null); // eof
return (new String (lin, 0, lg)).trim();
}
}
[/java]