Java: bitcoinj; how to download a full block?

Question

I would like to download a full block using Java and bitcoinj and then parse it to an array to be able to work with it. Here is some (very basic) pseudocode:

connect to network;  
download latestBlock;  
write/convert block to readable array;  
read blockArray;  
   sout: blockArray[23][55] // this should get me eg. a txid, address, ...  
// or similar. I just want to be able to read values from the block 

Is there a way to do this? I found the following solution here (Using bitcoinj to discover peers and download the block chain):

public class DumpLastBlock {
public static void main(String args[]) throws Exception {
    WalletAppKit kit = new WalletAppKit(MainNetParams.get(), new java.io.File("."), "test");
    kit.startAndWait();
    BlockChain chain = kit.chain();
    BlockStore bs = chain.getBlockStore();
    Peer peer = kit.peerGroup().getDownloadPeer();
    Block b = peer.getBlock(bs.getChainHead().getHeader().getHash()).get();
    System.out.println(b);
    }
}

Unfortunately, kit.startAndWait() isn't available anymore.

Can someone help me to get the latest block and be able to read its values?

Thank you very much :)

Answer 1

Example in bitcoinj source code is exactly what you are looking for:

BriefLogFormatter.init();
// Parse command line arguments
OptionParser parser = new OptionParser();
OptionSet opts = null;
List<String> nonOpts = null;
try {
    parser.accepts("localhost", "Connect to the localhost node");
    parser.accepts("help", "Displays program options");
    opts = parser.parse(args);
    if (opts.has("help")) {
        System.out.println("usage: org.bitcoinj.examples.FetchBlock [--localhost] <blockHash>");
        parser.printHelpOn(System.out);
        return;
    }
    nonOpts = opts.nonOptionArguments();
    if (nonOpts.size() != 1) {
        throw new IllegalArgumentException("Incorrect number of block hash, please provide only one block hash.");
    }
} catch (OptionException | IllegalArgumentException e) {
    System.err.println(e.getMessage());
    System.err.println("usage: org.bitcoinj.examples.FetchBlock [--localhost] <blockHash>");
    parser.printHelpOn(System.err);
    return;
}

// Connect to testnet and find a peer
System.out.println("Connecting to node");
final NetworkParameters params = TestNet3Params.get();
BlockStore blockStore = new MemoryBlockStore(params);
BlockChain chain = new BlockChain(params, blockStore);
PeerGroup peerGroup = new PeerGroup(params, chain);
if (!opts.has("localhost")) {
    peerGroup.addPeerDiscovery(new DnsDiscovery(params));
} else {
    PeerAddress addr = new PeerAddress(params, InetAddress.getLocalHost());
    peerGroup.addAddress(addr);
}
peerGroup.start();
peerGroup.waitForPeers(1).get();
Peer peer = peerGroup.getConnectedPeers().get(0);

// Retrieve a block through a peer
Sha256Hash blockHash = Sha256Hash.wrap(nonOpts.get(0));
Future<Block> future = peer.getBlock(blockHash);
System.out.println("Waiting for node to send us the requested block: " + blockHash);
Block block = future.get();
System.out.println(block);
peerGroup.stopAsync();