Example Big Number implementation in Java

BigNumber.java

/**
 * Created by Nick on 1/28/2017.
 */
public class BigNumber {

    private Node head = null;

    public BigNumber(BigNumber other) {

        Node currentOther = other.getHead();
        Node current = head;
        Node prev = current;
        while (currentOther != null) {

            Node newNode = new Node();
            newNode.setNext(null);
            newNode.setData(currentOther.getData());

            if (head == null) {
                head = newNode;
                prev = newNode;
            } else {
                current = newNode;
                prev.setNext(current);
                prev = current;
            }
            currentOther = currentOther.getNext();
        }
    }

    public BigNumber(String number) {

        boolean isNumber = true;
        char[] chars = new StringBuffer(number).reverse().toString().toCharArray();
        for (char c : chars) {
            if (!Character.isDigit(c)) {
                isNumber = false;
                break;
            }
        }

        if (!isNumber) {
            chars = new char[1];
            chars[0] = '0';
        }

        Node current = head;
        Node prev = current;
        for (char c : chars) {

            Node newNode = new Node();
            newNode.setNext(null);
            newNode.setData(Character.digit(c, 10));

            if (head == null) {
                head = newNode;
                prev = newNode;
            } else {
                current = newNode;
                prev.setNext(current);
                prev = current;
            }
        }
    }

    public void append(BigNumber other) {
        if (other == null) {
            return;
        }
        Node current = other.getHead();
        while (current.getNext() != null) {
            current = current.getNext();
        }
        current.setNext(head);
        head = other.getHead();
    }

    public void add(BigNumber other) {

        if (other == null) {
            return;
        }

        Node currentOther = other.getHead();
        Node currentThis = this.getHead();

        int sum = 0;
        int carry = 0;
        int alen = other.toString().length();
        int blen = this.toString().length();
        int maxlen = (alen > blen ? alen : blen);

        if (blen < alen) {
            String padding = "";
            for (int i = 0; i < alen - blen; i++) {
                padding += "0";
            }
            BigNumber lpad = new BigNumber(padding);
            lpad.append(this);
            currentThis = lpad.getHead();
        }

        for (int i = 0; i < maxlen; i++) {

            int a = currentThis != null ? currentThis.getData() : 0;
            int b = currentOther != null ? currentOther.getData() : 0;
            int addition = a + b + carry;

            sum = addition % 10; // 0
            currentThis.setData(sum);
            carry = addition / 10; // 1

            if (currentOther.getNext() != null) {
                currentOther = currentOther.getNext();
            } else if (i != maxlen - 1) {
                currentOther.setData(0);
            }
            if (currentThis.getNext() != null) {
                currentThis = currentThis.getNext();
            } else if (i != maxlen - 1) {
                currentThis.setData(0);
            }
        }
        if (carry > 0) {
            if (currentThis.getNext() == null) {
                Node newNode = new Node();
                newNode.setNext(null);
                newNode.setData(carry);
                currentThis.setNext(newNode);
            }
        }
    }
    
    public void add(Long other) {
        if (other == null) {
            return;
        }
        this.add(new BigNumber(other.toString() + ""));
    }

    public void mult(BigNumber other) {

        if (other == null) {
            return;
        }

        Node currentThis, currentOther;
        if (other.toString().length() > this.toString().length()) {
            currentThis = other.getHead();
            currentOther = this.getHead();
        } else {
            currentThis = this.getHead();
            currentOther = other.getHead();
        }

        BigNumber solution = new BigNumber("0");
        Node temp = currentThis;
        int idx = 0;
        while (currentOther != null) {
            String pad = "";
            for (int j = 0; j < idx; j++) {
                pad += "0";
            }
            while (currentThis != null) {
                long res = (currentThis.getData() * currentOther.getData());
                solution.add(new BigNumber(res + pad));
                currentThis = currentThis.getNext();
                pad += "0";
            }
            currentOther = currentOther.getNext();
            currentThis = temp;
            ++idx;
        }
        this.head = solution.getHead();
    }

    public Node getHead() {
        return head;
    }

    @Override
    public String toString() {
        StringBuilder sb = new StringBuilder();
        Node current = head;
        while (current != null) {
            sb.insert(0, current.getData());
            current = current.getNext();
        }
        return sb.toString();
    }
}

Main.java

/**
 * Created by Nick on 1/28/2017.
 */
public class Main {

    public static void main(String[] args) {
        BigNumber bigNumber1 = new BigNumber("123");

        System.out.println("big number: " + bigNumber1);

        BigNumber bigNumber2 = new BigNumber("123234345456567678789890");

        System.out.println("big number: " + bigNumber2);

        BigNumber bigNumber3 = new BigNumber("12323badbeef4345456567678789890");

        System.out.println("big number: " + bigNumber3);

        BigNumber bigNumber4 = new BigNumber("-1");

        System.out.println("big number: " + bigNumber4);

        BigNumber bigNumber5 = new BigNumber(bigNumber1);

        System.out.println("big number: " + bigNumber5);

        bigNumber5.append(new BigNumber("75"));
        bigNumber5.append(new BigNumber("99"));

        System.out.println("big number: " + bigNumber5);

        BigNumber bigNumber10 = new BigNumber("999");
        bigNumber10.add(        new BigNumber("999"));
        bigNumber10.add(1234567890L);

        System.out.println("addition: " + bigNumber10);

        BigNumber bigNumber001 = new BigNumber("1234");
        BigNumber bigNumber002 = new BigNumber("4321");
        bigNumber001.mult(bigNumber002);

        System.out.println("multiplication: " + bigNumber001);

    }
}

Node.java

/**
 * Created by Nick on 1/28/2017.
 */
public class Node {

    int data;
    Node next;

    public int getData() {
        return data;
    }

    public void setData(int data) {
        this.data = data;
    }

    public Node getNext() {
        return next;
    }

    public void setNext(Node next) {
        this.next = next;
    }

    @Override
    public String toString() {
        return "Node{" +
                "data=" + data +
                '}';
    }
}

This is very simplistic implementation of a big number container represented as a linked list, similar in concept to Java's BigDecimal, the implementation isn't meant to be full or usable in a production code. The use of this code is as is, and I'm taking no responsibility for it.