BlockChain.java

public boolean isValid() throws NoSuchAlgorithmException { 
  var pre = ""; 
  for(;pre.length() < difficulty; pre += "0"); 
  for (var i=1; i < chain.size(); ++i) { 
    var previousBlock = chain.get(i-1); 
    var currentBlock = chain.get(i); 
    if (!currentBlock.getHash().startsWith(pre) || !currentBlock.previousBlockHash.equals(previousBlock.getHash())) { 
      return false; 
    } 
  } 
  return true;
}