A sample blockchain written in Java

SampleChain.java

import java.security.MessageDigest;
import java.util.*;
class Block {
    private String hash;
    private String previousHash;
    private String data;
    private long timeStamp;
    private int nonce;

    public Block(String data, String previousHash, long timeStamp){
        this.data = data;
        this.previousHash = previousHash;
        this.timeStamp = timeStamp;
        this.hash = calculateBlockHash();
    }

    public String getHash() {
        return this.hash;
    }

    public void setHash(String hash) {
        this.hash = hash;
    }

    public String getPreviousHash() {
        return this.previousHash;
    }

    public void setPreviousHash(String previousHash) {
        this.previousHash = previousHash;
    }

    public String getData() {
        return this.data;
    }

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

    public long getTimeStamp() {
        return this.timeStamp;
    }

    public void setTimeStamp(long timeStamp) {
        this.timeStamp = timeStamp;
    }

    public int getNonce() {
        return this.nonce;
    }

    public void setNonce(int nonce) {
        this.nonce = nonce;
    }

    public String calculateBlockHash() {
        String dataToHash = this.previousHash + this.data + Long.toString(timeStamp) + Integer.toString(nonce);
        MessageDigest digest = null;
        byte[] bytes = null;
        try{
            digest = MessageDigest.getInstance("SHA-256");
            bytes = digest.digest(dataToHash.getBytes("UTF-8"));
        } catch (Exception e) {
            e.printStackTrace();
        }
        StringBuffer buffer = new StringBuffer();
        for(byte b : bytes) {
            buffer.append(String.format("%02x", b));
        }
        return buffer.toString();
    }

    public String mineBlock(int prefix){
        String prefixString = new String(new char[prefix]).replace('\0', '0');
        while(!this.hash.substring(0, prefix).equals(prefixString)){
            this.nonce++;
            this.hash = calculateBlockHash();
        }
        return hash;
    }

}

public class SampleChain{
    public static List<Block> blockChain = new ArrayList<>();
    static int prefix = 4;
    static String prefixString = new String(new char[prefix]).replace('\0', '0');
    public static void main(String []args){
       Block genesisBlock = new Block("Genesis","0", new Date().getTime());
       blockChain.add(genesisBlock);
       Block newBlock = new Block("Second block", blockChain.get(blockChain.size() - 1).getHash(),new Date().getTime());
       newBlock.mineBlock(prefix);
       System.out.println(newBlock.getHash().substring(0, prefix).equals(prefixString));
        blockChain.add(newBlock);
        for(Block block: blockChain){
            System.out.println(block.getHash());
            System.out.println(block.getNonce());
        }
    }
}