BlockChain.java

import java.security.NoSuchAlgorithmException; 
import java.util.ArrayList; 
import java.util.List; 

public class BlockChain { 
  public List<Block> chain; 
  
  public BlockChain() { 
    chain = new ArrayList<>(); 
  } 
  
  public void addBlock(Block block) throws NoSuchAlgorithmException { 
    block.previousBlockHash = chain.size() == 0 ? CryptoHelper.sha256("0") : chain.get(chain.size()-1).getHash(); 
    chain.add(block); 
  } 
  
  public boolean isValid() throws NoSuchAlgorithmException { 
    for (var i=1; i < chain.size(); ++i) { 
      var previousBlock = chain.get(i-1); 
      var currentBlock = chain.get(i); 
      if (!currentBlock.previousBlockHash.equals(previousBlock.getHash())) { 
        return false; 
      } 
    } 
    return true; 
  }
}