Adding source code for the article tracked under BAEL-3232. (#7657)

java-blockchain/src/main/java/com/baeldung/blockchain/Block.java

@@ -0,0 +1,65 @@
+package com.baeldung.blockchain;
+
+import java.io.UnsupportedEncodingException;
+import java.security.MessageDigest;
+import java.security.NoSuchAlgorithmException;
+import java.util.Date;
+import java.util.logging.Level;
+import java.util.logging.Logger;
+
+public class Block {
+
+    private static Logger logger = Logger.getLogger(Block.class.getName());
+
+    private String hash;
+    private String previousHash;
+    private String data;
+    private long timeStamp;
+    private int nonce;
+
+    public Block(String data, String previousHash) {
+        this.data = data;
+        this.previousHash = previousHash;
+        this.timeStamp = new Date().getTime();
+        this.hash = calculateBlockHash();
+    }
+
+    public String mineBlock(int prefix) {
+        String prefixString = new String(new char[prefix]).replace('\0', '0');
+        while (!hash.substring(0, prefix)
+            .equals(prefixString)) {
+            nonce++;
+            hash = calculateBlockHash();
+        }
+        return hash;
+    }
+
+    public String calculateBlockHash() {
+        String dataToHash = previousHash + Long.toString(timeStamp) + Integer.toString(nonce) + data;
+        MessageDigest digest = null;
+        byte[] bytes = null;
+        try {
+            digest = MessageDigest.getInstance("SHA-256");
+            bytes = digest.digest(dataToHash.getBytes("UTF-8"));
+        } catch (NoSuchAlgorithmException | UnsupportedEncodingException ex) {
+            logger.log(Level.SEVERE, ex.getMessage());
+        }
+        StringBuffer buffer = new StringBuffer();
+        for (byte b : bytes) {
+            buffer.append(String.format("%02x", b));
+        }
+        return buffer.toString();
+    }
+
+    public String getHash() {
+        return this.hash;
+    }
+
+    public String getPreviousHash() {
+        return this.previousHash;
+    }
+
+    public void setData(String data) {
+        this.data = data;
+    }
+}

java-blockchain/src/main/resources/logback.xml

@@ -0,0 +1,13 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<configuration>
+    <appender name="STDOUT" class="ch.qos.logback.core.ConsoleAppender">
+        <encoder>
+            <pattern>%d{HH:mm:ss.SSS} [%thread] %-5level %logger{36} - %msg%n
+            </pattern>
+        </encoder>
+    </appender>
+
+    <root level="INFO">
+        <appender-ref ref="STDOUT" />
+    </root>
+</configuration>

java-blockchain/src/test/java/com/baeldung/blockchain/BlockchainUnitTest.java

@@ -0,0 +1,67 @@
+package com.baeldung.blockchain;
+
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+import java.util.ArrayList;
+import java.util.List;
+
+import org.junit.AfterClass;
+import org.junit.BeforeClass;
+import org.junit.Test;
+
+public class BlockchainUnitTest {
+
+    public static List<Block> blockchain = new ArrayList<Block>();
+    public static int prefix = 4;
+    public static String prefixString = new String(new char[prefix]).replace('\0', '0');
+
+    @BeforeClass
+    public static void setUp() {
+        Block genesisBlock = new Block("The is the Genesis Block.", "0");
+        genesisBlock.mineBlock(prefix);
+        blockchain.add(genesisBlock);
+        Block firstBlock = new Block("The is the First Block.", genesisBlock.getHash());
+        firstBlock.mineBlock(prefix);
+        blockchain.add(firstBlock);
+    }
+
+    @Test
+    public void givenBlockchain_whenNewBlockAdded_thenSuccess() {
+        Block newBlock = new Block("The is a New Block.", blockchain.get(blockchain.size() - 1)
+            .getHash());
+        newBlock.mineBlock(prefix);
+        assertTrue(newBlock.getHash()
+            .substring(0, prefix)
+            .equals(prefixString));
+        blockchain.add(newBlock);
+    }
+
+    @Test
+    public void givenBlockchain_whenValidated_thenSuccess() {
+        boolean flag = true;
+        for (int i = 0; i < blockchain.size(); i++) {
+            String previousHash = i == 0 ? "0"
+                : blockchain.get(i - 1)
+                    .getHash();
+            flag = blockchain.get(i)
+                .getHash()
+                .equals(blockchain.get(i)
+                    .calculateBlockHash())
+                && previousHash.equals(blockchain.get(i)
+                    .getPreviousHash())
+                && blockchain.get(i)
+                    .getHash()
+                    .substring(0, prefix)
+                    .equals(prefixString);
+            if (!flag)
+                break;
+        }
+        assertTrue(flag);
+    }
+
+    @AfterClass
+    public static void tearDown() {
+        blockchain.clear();
+    }
+
+}

java-blockchain/src/test/resources/.gitignore

@@ -0,0 +1,13 @@
+*.class
+
+#folders#
+/target
+/neoDb*
+/data
+/src/main/webapp/WEB-INF/classes
+*/META-INF/*
+
+# Packaged files #
+*.jar
+*.war
+*.ear