Block.java

public class Block {
  public long nonce; 
  private long timestamp; 
  public String transactionData; 
  public String previousBlockHash; 
  
  public Block(String transactionData) { 
    nonce = 0; 
    timestamp = System.currentTimeMillis(); 
    this.transactionData = transactionData; 
  } 
  
  public String getHash() throws NoSuchAlgorithmException { 
    return CryptoHelper.sha256(transactionData + String.valueOf(timestamp) 
      + String.valueOf(previousBlockHash) 
      + String.valueOf(nonce));
  } 
  
  @Override 
  public String toString() { 
    return String.format("%s\t%d\t%d\t%s", previousBlockHash, timestamp, nonce, transactionData); 
  } 
}