<dependency>
  <groupId>org.web3j</groupId>
  <artifactId>crypto</artifactId>
  <version>3.3.1</version>
</dependency>

import org.bouncycastle.util.encoders.Hex;
import org.web3j.crypto.*;
import java.math.BigInteger;

// Generate random private key
// BigInteger privKey = Keys.createEcKeyPair().getPrivateKey(); 
​
BigInteger privKey = new BigInteger(
 "97ddae0f3a25b92268175400149d65d6887b9cefaf28ea2c078e05cdc15a3c0a", 16);
BigInteger pubKey = Sign.publicKeyFromPrivate(privKey);
ECKeyPair keyPair = new ECKeyPair(privKey, pubKey);
​
System.out.println("Private key: " + privKey.toString(16));
System.out.println("Public key: " + pubKey.toString(16));
System.out.println("Public key (compressed): " +
  compressPubKey(pubKey));

String msg = "Message for signing";
byte[] msgHash = Hash.sha3(msg.getBytes());
Sign.SignatureData signature =
  Sign.signMessage(msgHash, keyPair, false);
​
System.out.println("Msg: " + msg);
System.out.println("Msg hash: " + Hex.toHexString(msgHash));
System.out.printf(
  "Signature: [v = %d, r = %s, s = %s]\n",
  signature.getV() - 27,
  Hex.toHexString(signature.getR()),
  Hex.toHexString(signature.getS()));

BigInteger pubKeyRecovered =
  Sign.signedMessageToKey(msg.getBytes(), signature);
System.out.println("Recovered public key: " +
  pubKeyRecovered.toString(16));
​
boolean validSig = pubKey.equals(pubKeyRecovered);
System.out.println("Signature valid? " + validSig);