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;

}

}

AND here is the test!

public void testAddBlock() {

SimpleBlockchain testBc= new SimpleBlockchain("hello");

BlockchainNode newBc= new BlockchainNode ("number two", testBc.genesisNode.currHash, 1);

testBc.addBlock("number two");

assertEquals("number two", testBc.lastNode.contents);

assertEquals(1, testBc.lastNode.blockNumber);

assertEquals(newBc.currHash, testBc.lastNode.currHash);

assertSame(testBc.genesisNode.next, testBc.lastNode);

}

@Test

public void testValidate() {

SimpleBlockchain testBc= new SimpleBlockchain("hello");

testBc.addBlock("number two"); // assumes addBlock is correctly implemented

assertTrue(testBc.validate());

}

@Test

public void findTamperedNode() {

SimpleBlockchain testBc= new SimpleBlockchain("hello");

testBc.addBlock("number two"); // assumes addBlock is correctly implemented

testBc.lastNode.contents= "something else"; // A simple attack: Change the block chain contents

assertFalse(testBc.validate()); // No longer valid

assertSame(testBc.lastNode, testBc.findTamperedNode()); // Last node has been altered

}

public ArrayList createList(String[] items){

ArrayList list = new ArrayList<>();

if (items.length > 0){

list.add(new DNode(items[0],null,null));

for (int i = 1; i < items.length; i++){

DNode toAdd = new DNode(items[i],list.get(i-1),null);

list.add(toAdd);

list.get(i-1).next = toAdd;

}

}

return list;

}

}

Can anyone help plz !!!!