First phase implementation

src/blockchain/Block.java

@@ -0,0 +1,33 @@
+package blockchain;
+
+public class Block {
+
+  private final Integer id;
+  private final String hashPreviousBlock;
+  private final String hash;
+  private final Long timestamp;
+
+
+  public Block(Integer id, String hashPreviousBlock, String hash, Long timestamp) {
+    this.id = id;
+    this.hashPreviousBlock = hashPreviousBlock;
+    this.hash = hash;
+    this.timestamp = timestamp;
+  }
+
+  public Integer getId() {
+    return id;
+  }
+
+  public String getHashPreviousBlock() {
+    return hashPreviousBlock;
+  }
+
+  public String getHash() {
+    return hash;
+  }
+
+  public Long getTimestamp() {
+    return timestamp;
+  }
+}

src/blockchain/Blockchain.java

@@ -0,0 +1,43 @@
+package blockchain;
+
+import java.util.Collections;
+import java.util.Date;
+import java.util.Iterator;
+import java.util.LinkedList;
+
+public class Blockchain implements Iterable<Block> {
+
+  private final LinkedList<Block> blocks;
+
+  public Blockchain() {
+    blocks = new LinkedList<>(Collections.singleton(
+        new Block(1,
+            "0",
+            StringUtil.applySha256("0"),
+            new Date().getTime()))
+    );
+  }
+
+  public void generate() {
+    Long timestamp = new Date().getTime();
+    Block tailBlock = blocks.getLast();
+    String hash = StringUtil.applySha256(tailBlock.getHash());
+    blocks.add(new Block(tailBlock.getId() + 1, tailBlock.getHash(), hash, timestamp));
+  }
+
+  public boolean validate() {
+    for (int i = 1; i < blocks.size(); i++) {
+      Block prev = blocks.get(i - 1);
+      Block cur = blocks.get(i);
+      if (!cur.getHashPreviousBlock().equals(prev.getHash())) {
+        return false;
+      }
+    }
+    return true;
+  }
+
+  @Override
+  public Iterator<Block> iterator() {
+    return blocks.iterator();
+  }
+}

src/blockchain/Main.java

@@ -1,7 +1,19 @@
 package blockchain;
 
 public class Main {
-    public static void main(String[] args) {
-        System.out.print("Hello world!");
+
+  public static void main(String[] args) {
+    Blockchain blockchain = new Blockchain();
+    for (int i = 0; i < 10; i++) {
+      blockchain.generate();
+    }
+
+    for (Block block : blockchain) {
+      System.out.printf("Block:\n");
+      System.out.printf("Id: %s\n", block.getId());
+      System.out.printf("Timestamp: %s\n", block.getTimestamp());
+      System.out.printf("Hash of the previous block:\n%s\n", block.getHashPreviousBlock());
+      System.out.printf("Hash of the block: \n%s\n\n", block.getHash());
     }
+  }
 }

src/blockchain/StringUtil.java

@@ -0,0 +1,28 @@
+package blockchain;
+
+import java.nio.charset.StandardCharsets;
+import java.security.MessageDigest;
+
+public class StringUtil {
+
+  /**
+   * Applies Sha256 to a string and returns a hash.
+   */
+  public static String applySha256(String input) {
+    try {
+      MessageDigest digest = MessageDigest.getInstance("SHA-256");
+      byte[] hash = digest.digest(input.getBytes(StandardCharsets.UTF_8));
+      StringBuilder hexString = new StringBuilder();
+      for (byte elem : hash) {
+        String hex = Integer.toHexString(0xff & elem);
+        if (hex.length() == 1) {
+          hexString.append('0');
+        }
+        hexString.append(hex);
+      }
+      return hexString.toString();
+    } catch (Exception e) {
+      throw new RuntimeException(e);
+    }
+  }
+}