Simple Blockchain Example in Java

package com.helloacm;
 
class Block {
    static int BlockNumber = 0;
 
    public Block(Block prev, Block next, String data) {
        this.prev = prev;
        this.hash = data.hashCode();
        this.next = next;
        this.data = data;
        BlockNumber ++;
        this.blockNumber = BlockNumber;
    }
 
    private Block prev;
    private Block next;
    private long hash;
    private String data;
    private int blockNumber;
 
    public int getBlockNumber() {
        return this.blockNumber;
    }
 
    public String getData() {
        return this.data;
    }
 
    public Block getNext() {
        return this.next;
    }
 
    public Block getPrev() {
        return this.prev;
    }
 
    // check if current block is valid and connected to the first block
    public boolean isValid() {
        Block cur = this;
        while (cur.prev != null) {
            if (!validateHash()) {
                return false;
            }
            Block prevBlock = cur.prev;
            if (prevBlock != null) {
                if (prevBlock.next != cur) {
                    return false;
                }
            }
            cur = prevBlock;
        }
        return cur.validateHash() && cur.data == "0";
    }
 
    // connect the current block to next block
    public void generateBlock(Block next) {
        this.next = next;
        next.prev = this;
    }
 
    // current block should have correct hash
    public boolean validateHash() {
        return this.data.hashCode() == this.hash;
    }
}
 
public class Main {
    public static void main(String[] args) {
        var firstBlock = new Block(null, null, "0");
        var data = new String[] {"Hello", "World!", "How", "Are", "You?"};
        var prev = firstBlock;
        for (var s: data) {
            Block cur = new Block(prev, null, s);
            prev.generateBlock(cur);;
            prev = cur;
        }
        var root = firstBlock;
        while (root != null) {
            if (!root.isValid()) {
                throw new RuntimeException("Block Invalid.");
            }
            System.out.println(String.format("Block %d - Data %s", root.getBlockNumber(), root.getData()));
            root = root.getNext(); // get to next block
        }
    }
}