Blockchain implementation in Java

Block.java

import java.util.Date;

public class Block {
    public String hash;
    public String previousHash;
    private String data;
    private long timestamp;

    public Block(String data, String previousHash) {
        this.data = data;
        this.previousHash = previousHash;
        this.timestamp = new Date().getTime();
        this.hash = calculateHash();
    }

    public String calculateHash() {
        String calculatedHash = StringUtil.applySha256(
                previousHash +
                Long.toString(timestamp) +
                data
                );
        return calculatedHash;
    }
}

NoobChain.java

public class NoobChain {
    public static void main(String[] args) {
        Block genesisBlock = new Block("Hi I'm the first block", "0");
        System.out.println("Hash for block 1: " + genesisBlock.hash);

        Block secondBlock = new Block("Yo I'm the second block", genesisBlock.hash);
        System.out.println("Hash for block 2: " + secondBlock.hash);

        Block thirdBlock = new Block("Hey I'm the third block", secondBlock.hash);
        System.out.println("Hash for block 3: " + thirdBlock.hash);
    }
}

StringUtil.Java

import java.security.MessageDigest;

public class StringUtil {
    public static String applySha256(String input) {
        try {
            MessageDigest digest = MessageDigest.getInstance("SHA-256");
            byte[] hash = digest.digest(input.getBytes("UTF-8"));
            StringBuffer hexString = new StringBuffer();

            for (int i = 0; i < hash.length; i++) {
                String hex = Integer.toHexString(0xff & hash[i]);
                if (hex.length() == 1) hexString.append('0');
                hexString.append(hex);
            }

            return hexString.toString();
        } catch(Exception e) {
            throw new RuntimeException(e);
        }
    }
}