Here is the code!

class BlockchainNode { // The basic BlockchainNode...

String currHash; // To store the hash

String contents; // To store the data

int blockNumber; // The number of the node in the block after the Genesis Node

BlockchainNode next; // The next BlockchainNode

BlockchainNode prev; // The previous BlockchainNode

BlockchainNode (String s) {

// Creates a Genesis node

currHash= StringUtil.applySha256(Integer.toString(0)+"AllZeros:This is the Genesis String!"+s);

contents= s;

blockNumber= 0;

next= null;

prev= null;

}

BlockchainNode (String s, String pH, int bN) {

// Creates a new blocknumbered bN with contents s

currHash= StringUtil.applySha256(Integer.toString(bN)+pH+s);

contents= s;

blockNumber= bN;

next= null;

prev= null;

}

}

class SimpleBlockchain {

// Class invariant: all nodes in the Blockchain satisfy "Blockchain Validity"

// i.e. Blockchain Validity holds of all nodes (except the Genesis node):

// StringUtil.applySha256(this.blockNumber+ prev.currHash+this.contents == this.currHash)

// AND the Genesis node has been correctly initialised

BlockchainNode genesisNode; // Created by calling BlockchainNode (String s)

BlockchainNode lastNode; // The last node in the Blockchain

SimpleBlockchain(String s){

genesisNode= new BlockchainNode(s);

lastNode= genesisNode;

}

void addBlock(String s) { //TODO

// Post: Creates a new valid block with contents s, and adds it

// to the current SimpleBlockchain so that the result satisfies the blockchain condition.

}

boolean validate () { //TODO

// Post: Returns true if the SimpleBlockchain is valid, i.e. satisfies the blockChain condition

// null SimplBlockchains are valid

return true;

}

BlockchainNode findTamperedNode() { //TODO

// Post: If validate returns false, returns the address of the first node which fails to validate

// If validate returns true, then returns null

return null;

}

}

/// This is what we will use for the hash function in making a blockchain

/// Do not change or remove this code

class StringUtil {

//Applies Sha256 to a string and returns the result.

public static String applySha256(String input){

try {

MessageDigest digest = MessageDigest.getInstance("SHA-256");

//Applies sha256 to our input,

byte[] hash = digest.digest(input.getBytes("UTF-8"));

StringBuffer hexString = new StringBuffer(); // This will contain hash as hexidecimal

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);

}

}

}

All methods to be implemented are labelled //TODO