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      "author": "luu78",
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      "body": "# Creating Your First Blockchain with Java. Part 1.\n*Note: I initially posted this tutorial on Medium, but thought It might interest some people on here :)*\n\nIn this tutorial we will :\n\n* Create your first (very) **basic ‘blockchain’**.\n* Implement a simple **proof of work** ( mining ) system.\n* **Marvel at the possibilities**.\n\n<br></br>\n( I will assume you have a basic understanding of [Object Oriented Programming](https://docs.oracle.com/javase/tutorial/java/concepts/) )\n<br></br>\n*It’s worth noting that this wont be a fully functioning, ready for production block chain. Instead this is a proof of concept implementation to help you understand what a blockchain is for future tutorials.*\n<br></br>\n*****\n\n# Setting Up.\n\nWe will be using Java but you should be able to follow along in any [OOP](https://en.wikipedia.org/wiki/Object-oriented_programming) language. I’ll be using Eclipse but you can use any new fancy text editor ( though you’ll miss out on a lot of good bloat ).\n\nYou will need:\n\n* Java and JDK installed. ( duh ).\n* Eclipse ( or another IDE/Text Editor ).\n\n<center>\n![](https://cdn-images-1.medium.com/max/800/1*3rE0ahnLzfQ7JHyxNJAH7Q.gif)\nDon’t worry if your eclipse looks different to mine. I’ll be using a dark theme in eclipse because ^\n</center>\n\nOptionally, you can grab[ GSON library by google](https://repo1.maven.org/maven2/com/google/code/gson/gson/2.6.2/gson-2.6.2.jar) (*who are they ???*). This will allow us to turn an object into Json \\o/. It’s a super useful library that we will also be using further down the line for peer2peer stuff, but feel free to use an alternate method.\n\n\nIn Eclipse create a (file > new > ) Java project. I’ll call my Project “**noobchain**” and create a new *Class* by the same name (**NoobChain**).\n<center>\n![](https://cdn-images-1.medium.com/max/800/1*VPKiJWgOiZszGvLgPNiqLA.png)\nDon’t be copying my project name now ( ͠° ͟ ͜ʖ ͡°)\n</center>\n\n\nNow you’re good to go :)\n\n*****\n\n# Making the Blockchain.\n\nA blockchain is just a chain/list of blocks. Each block in the blockchain will have its own digital signature, contain digital signature of the previous block, and have some data ( this data could be transactions for example ).\n\n<center>\n![](https://cdn-images-1.medium.com/max/800/1*627BG-7qMtaXNsX0n41C6Q.png)\nI sure hope Nakamoto never sees this. \n</center>\n\n> **Hash = Digital Signature.**\n\n**Each block doesn’t just contain the hash of the block before it, but its own hash is in part, calculated from the previous hash**. If the previous block’s data is changed then the previous block’s hash will change ( since it is calculated in part, by the data) in turn affecting all the hashes of the blocks there after. **Calculating and comparing the hashes allow us to see if a blockchain is invalid.**\n\nWhat does this mean ? …Changing any data in this list, will change the signature and **break the chain**.\n\n# So Firsts lets create class **Block** that make up the blockchain:\n<br><br>\n\n```\nimport java.util.Date;\n\npublic class Block {\n\n\tpublic String hash;\n\tpublic String previousHash;\n\tprivate String data; //our data will be a simple message.\n\tprivate long timeStamp; //as number of milliseconds since 1/1/1970.\n\n\t//Block Constructor.\n\tpublic Block(String data,String previousHash ) {\n\t\tthis.data = data;\n\t\tthis.previousHash = previousHash;\n\t\tthis.timeStamp = new Date().getTime();\n\t}\n}\n```\n<br><br>\n\nAs you can see our basic **Block** contains a `String hash` that will hold our digital signature. The variable `previousHash` to hold the previous block’s hash and`String data` to hold our block data.\n\n# **Next we will need a way to generate a digital signature**,\n\nthere are many cryptographic algorithms you can choose from, however SHA256 fits just fine for this example. We can `import java.security.MessageDigest;` to get access to the SHA256 algorithm.\n\nWe need to use SHA256 later down the line so lets create a handy helper method in a new **StringUtil** ‘utility’ *class* :\n<br><br>\n\n```\nimport java.security.MessageDigest;\n\npublic class StringUtil {\n\t//Applies Sha256 to a string and returns the result. \n\tpublic static String applySha256(String input){\t\t\n\t\ttry {\n\t\t\tMessageDigest digest = MessageDigest.getInstance(\"SHA-256\");\t        \n\t\t\t//Applies sha256 to our input, \n\t\t\tbyte[] hash = digest.digest(input.getBytes(\"UTF-8\"));\t        \n\t\t\tStringBuffer hexString = new StringBuffer(); // This will contain hash as hexidecimal\n\t\t\tfor (int i = 0; i < hash.length; i++) {\n\t\t\t\tString hex = Integer.toHexString(0xff & hash[i]);\n\t\t\t\tif(hex.length() == 1) hexString.append('0');\n\t\t\t\thexString.append(hex);\n\t\t\t}\n\t\t\treturn hexString.toString();\n\t\t}\n\t\tcatch(Exception e) {\n\t\t\tthrow new RuntimeException(e);\n\t\t}\n\t}\t\n}\n```\n<center>This is mostly a carbon copy of the\n[http://www.baeldung.com/sha-256-hashing-java](http://www.baeldung.com/sha-256-hashing-java)</center>\n<br><br>\n\n\n**Don’t worry too much if you don’t understand the contents of this helper method**, *all you need to know is that it takes a string and applies SHA256 algorithm to it, and returns the generated signature as a string.*\n\nNow lets use our **applySha256** helper, in a new method in the **Block** *class*, to calculate the hash. We must calculate the hash from all parts of the block we don’t want to be tampered with. So for our block we will include the `previousHash`, the `data` and `timeStamp`.\n<br><br>\n\n```\npublic String calculateHash() {\n\tString calculatedhash = StringUtil.applySha256( \n\t\t\tpreviousHash +\n\t\t\tLong.toString(timeStamp) +\n\t\t\tdata \n\t\t\t);\n\treturn calculatedhash;\n}\n```\n<br><br>\nand lets add this method to the **Block** *constructor*…\n<br><br>\n\n```\npublic Block(String data,String previousHash ) {\n\t\tthis.data = data;\n\t\tthis.previousHash = previousHash;\n\t\tthis.timeStamp = new Date().getTime();\n\t\tthis.hash = calculateHash(); //Making sure we do this after we set the other values.\n\t}\n  ```\n<br><br>\n# **Time for some testing…**\n\nIn our main **NoobChain** class lets create some blocks and print the hashes to the screen to see that everything is in working order.\n\n<center>\n![CalculatingHash.gif](https://steemitimages.com/DQmc8rsfW3KnnYq8Wq2kHnBrpSfihnuxc2tDSsSEU227Ajw/CalculatingHash.gif)\nLets test this…\n</center>\n\nThe first block is called the genesis block, and because there is no previous block we will just enter “0” as the previous hash.\n<br><br>\n\n```\npublic class NoobChain {\n\n\tpublic static void main(String[] args) {\n\t\t\n\t\tBlock genesisBlock = new Block(\"Hi im the first block\", \"0\");\n\t\tSystem.out.println(\"Hash for block 1 : \" + genesisBlock.hash);\n\t\t\n\t\tBlock secondBlock = new Block(\"Yo im the second block\",genesisBlock.hash);\n\t\tSystem.out.println(\"Hash for block 2 : \" + secondBlock.hash);\n\t\t\n\t\tBlock thirdBlock = new Block(\"Hey im the third block\",secondBlock.hash);\n\t\tSystem.out.println(\"Hash for block 3 : \" + thirdBlock.hash);\n\t\t\n\t}\n}\n```\n<br><br>\nThe output should look similar to this:\n\n<center>\n![outputHashTesting.png](https://steemitimages.com/DQmbgiQDj2UAE34gBMwdpjtdTpKK9aQmnRkj5D8z7kLvQS4/outputHashTesting.png)\nYour values will be different because your timestamp will be different.\n</center>\n\nEach block now has its own digital signature based on its information and the signature of the previous block.\n\nCurrently it’s not much of a block**chain**, so lets store our blocks in an *ArrayList* and also import gson to view it as Json. *(*[click here to find out how to import the gson library](https://medium.com/@cryptokass/importing-gson-into-eclipse-ec8cf678ad52)*)*\n<br><br>\n\n```\nimport java.util.ArrayList;\nimport com.google.gson.GsonBuilder;\n\npublic class NoobChain {\n\t\n\tpublic static ArrayList<Block> blockchain = new ArrayList<Block>(); \n\n\tpublic static void main(String[] args) {\t\n\t\t//add our blocks to the blockchain ArrayList:\n\t\tblockchain.add(new Block(\"Hi im the first block\", \"0\"));\t\t\n\t\tblockchain.add(new Block(\"Yo im the second block\",blockchain.get(blockchain.size()-1).hash)); \n\t\tblockchain.add(new Block(\"Hey im the third block\",blockchain.get(blockchain.size()-1).hash));\n\t\t\n\t\tString blockchainJson = new GsonBuilder().setPrettyPrinting().create().toJson(blockchain);\t\t\n\t\tSystem.out.println(blockchainJson);\n\t}\n\n}\n```\n<br><br>\nNow our output should look something closer to what we expect a blockchain to look like.\n\n#### Now we need a way to check the integrity of our blockchain.\n\nLets create an **isChainValid()** *Boolean* method in the **NoobChain** *class*, that will loop through all blocks in the chain and compare the hashes. This method will need to check the hash variable is actually equal to the calculated hash, and the previous block’s hash is equal to the **previousHash** variable.\n<br><br>\n\n```public static Boolean isChainValid() {\n\tBlock currentBlock; \n\tBlock previousBlock;\n\t\n\t//loop through blockchain to check hashes:\n\tfor(int i=1; i < blockchain.size(); i++) {\n\t\tcurrentBlock = blockchain.get(i);\n\t\tpreviousBlock = blockchain.get(i-1);\n\t\t//compare registered hash and calculated hash:\n\t\tif(!currentBlock.hash.equals(currentBlock.calculateHash()) ){\n\t\t\tSystem.out.println(\"Current Hashes not equal\");\t\t\t\n\t\t\treturn false;\n\t\t}\n\t\t//compare previous hash and registered previous hash\n\t\tif(!previousBlock.hash.equals(currentBlock.previousHash) ) {\n\t\t\tSystem.out.println(\"Previous Hashes not equal\");\n\t\t\treturn false;\n\t\t}\n\t}\n\treturn true;\n}\n```\n<br><br>\nAny change to the blockchain’s blocks will cause this method to return false.\n\nOn the bitcoin network nodes share their blockchains and the **longest valid chain is accepted** by the network. What’s to stop someone tampering with data in an old block then creating a whole new longer blockchain and presenting that to the network ? **Proof of work**. The *hashcash* proof of work system means it takes considerable time and \n computational power to create new blocks. Hence the attacker would need more computational power than the rest of the peers combined.\n\n<center>\n![BlockMining.gif](https://steemitimages.com/DQmUNBjqxHcjz2dD83N6AwzCy6Z2n8eq8MoJGZaLXs7mp2i/BlockMining.gif)\nhashcash, much wow.</center>\n\n### Lets start mining blocks !!!\n\nWe will require *miners* to do proof-of-work by **trying different variable values in the block until its hash starts with a certain number of 0’s.**\n\nLets add an *int* called **nonce** to be included in our **calculateHash()** method, and the much needed **mineBlock()** method :\n<br><br>\n```\nimport java.util.Date;\n\npublic class Block {\n\t\n\tpublic String hash;\n\tpublic String previousHash; \n\tprivate String data; //our data will be a simple message.\n\tprivate long timeStamp; //as number of milliseconds since 1/1/1970.\n\tprivate int nonce;\n\t\n\t//Block Constructor.  \n\tpublic Block(String data,String previousHash ) {\n\t\tthis.data = data;\n\t\tthis.previousHash = previousHash;\n\t\tthis.timeStamp = new Date().getTime();\n\t\t\n\t\tthis.hash = calculateHash(); //Making sure we do this after we set the other values.\n\t}\n\t\n\t//Calculate new hash based on blocks contents\n\tpublic String calculateHash() {\n\t\tString calculatedhash = StringUtil.applySha256( \n\t\t\t\tpreviousHash +\n\t\t\t\tLong.toString(timeStamp) +\n\t\t\t\tInteger.toString(nonce) + \n\t\t\t\tdata \n\t\t\t\t);\n\t\treturn calculatedhash;\n\t}\n\t\n\tpublic void mineBlock(int difficulty) {\n\t\tString target = new String(new char[difficulty]).replace('\\0', '0'); //Create a string with difficulty * \"0\" \n\t\twhile(!hash.substring( 0, difficulty).equals(target)) {\n\t\t\tnonce ++;\n\t\t\thash = calculateHash();\n\t\t}\n\t\tSystem.out.println(\"Block Mined!!! : \" + hash);\n\t}\n}\n```\n<center>In reality each miner will start iterating from a random point. Some miners may even try random numbers for nonce. Also it’s worth noting that at the harder difficulties solutions may require more than integer.MAX_VALUE, miners can then try changing the timestamp.</center>\n<br><br>\n\n\nThe **mineBlock()** method takes in an int called difficulty, this is the number of 0’s they must solve for. Low difficulty like 1 or 2 can be solved nearly instantly on most computers, i’d suggest something around 4–6 for testing. At the time of writing Litecoin’s difficulty is around 442,592.\n\nLets add the difficulty as a static variable to the NoobChain class :\n\n   ` public static int difficulty = 5;`\n\nWe should update the **NoobChain** *class* to trigger the **mineBlock()** *method* for each new block. The **isChainValid**() *Boolean* should also check if each block has a solved ( by mining ) hash.\n<br><br>\n\n```\nimport java.util.ArrayList;\nimport com.google.gson.GsonBuilder;\n\npublic class NoobChain {\n\t\n\tpublic static ArrayList<Block> blockchain = new ArrayList<Block>();\n\tpublic static int difficulty = 5;\n\n\tpublic static void main(String[] args) {\t\n\t\t//add our blocks to the blockchain ArrayList:\n\t\t\n\t\tblockchain.add(new Block(\"Hi im the first block\", \"0\"));\n\t\tSystem.out.println(\"Trying to Mine block 1... \");\n\t\tblockchain.get(0).mineBlock(difficulty);\n\t\t\n\t\tblockchain.add(new Block(\"Yo im the second block\",blockchain.get(blockchain.size()-1).hash));\n\t\tSystem.out.println(\"Trying to Mine block 2... \");\n\t\tblockchain.get(1).mineBlock(difficulty);\n\t\t\n\t\tblockchain.add(new Block(\"Hey im the third block\",blockchain.get(blockchain.size()-1).hash));\n\t\tSystem.out.println(\"Trying to Mine block 3... \");\n\t\tblockchain.get(2).mineBlock(difficulty);\t\n\t\t\n\t\tSystem.out.println(\"\\nBlockchain is Valid: \" + isChainValid());\n\t\t\n\t\tString blockchainJson = new GsonBuilder().setPrettyPrinting().create().toJson(blockchain);\n\t\tSystem.out.println(\"\\nThe block chain: \");\n\t\tSystem.out.println(blockchainJson);\n\t}\n\t\n\tpublic static Boolean isChainValid() {\n\t\tBlock currentBlock; \n\t\tBlock previousBlock;\n\t\tString hashTarget = new String(new char[difficulty]).replace('\\0', '0');\n\t\t\n\t\t//loop through blockchain to check hashes:\n\t\tfor(int i=1; i < blockchain.size(); i++) {\n\t\t\tcurrentBlock = blockchain.get(i);\n\t\t\tpreviousBlock = blockchain.get(i-1);\n\t\t\t//compare registered hash and calculated hash:\n\t\t\tif(!currentBlock.hash.equals(currentBlock.calculateHash()) ){\n\t\t\t\tSystem.out.println(\"Current Hashes not equal\");\t\t\t\n\t\t\t\treturn false;\n\t\t\t}\n\t\t\t//compare previous hash and registered previous hash\n\t\t\tif(!previousBlock.hash.equals(currentBlock.previousHash) ) {\n\t\t\t\tSystem.out.println(\"Previous Hashes not equal\");\n\t\t\t\treturn false;\n\t\t\t}\n\t\t\t//check if hash is solved\n\t\t\tif(!currentBlock.hash.substring( 0, difficulty).equals(hashTarget)) {\n\t\t\t\tSystem.out.println(\"This block hasn't been mined\");\n\t\t\t\treturn false;\n\t\t\t}\n\t\t}\n\t\treturn true;\n\t}\n}\n```\n<center>Notice we also check and print *isChainValid*.</center>\n<br><br>\n\n\nRunning this your results should look like :\n\n<center>\n![miningResults.png](https://steemitimages.com/DQmSfShzpYPYeRyMvMghHAJGfDfGjSjWyQAtTxoc9TeJdZp/miningResults.png)\n</center>\n\nMining each block took some time! ( around 3 seconds ) You should mess around with the difficulty value to see how that effects the time it takes to mine each block ;)\n\nIf someone were to **tamper**😒 with the data in your blockchain system:\n\n* Their blockchain would be invalid.\n* They would not be able to create a longer blockchain.\n* Honest blockchains in your network will have a time advantage on the longest\nchain.\n\n**A tampered blockchain will not be able to catch up with a longer & valid chain. ***\n\n*unless they have vastly more computation speed than all other nodes in your network combined. A future quantum computer or something.\n\n# You’re all done with your basic blockchain!\n\n<center>![giphy.gif](https://steemitimages.com/DQmWQBNZHWPjwNibkgr3h1kcVJGBbZ246Cr6JH2q9PgpVpj/giphy.gif)\nGo on pat yourself on the back.\n</center>\n\nYour blockchain:\n\n- **Is made up of blocks that store data.**\n- **Has a digital signature that chains your blocks together.**\n- **Requires proof of work mining to validate new blocks.**\n- **Can be check to see if data in it is valid and unchanged.**\n\n<center>\n**You can download these project files on**\n[Github](https://github.com/CryptoKass/NoobChain-Tutorial-Part-1).\n</center>\n\n<center>\n![more to go.gif](https://steemitimages.com/DQmSnifojy2EzCgg4w2sTXsvCNC2MfxN4f2dGKmhVVvFAnd/more%20to%20go.gif)\nYou can **follow to be notified** when next tutorials and other blockchain development articles are posted. Any feedback is also greatly appreciated.\nThanks.</center>\n\n# [Creating Your First Blockchain with Java. Part 2:](https://medium.com/programmers-blockchain/creating-your-first-blockchain-with-java-part-2-transactions-2cdac335e0ce) (I will update this with a steemit link once I post the second part here :D )\nWe cover **Transactions, Signatures **and **Wallets**.\n\n*contact: *[email protected]*",
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