StringUtil.java

import java.security.Key;
import java.security.MessageDigest;
import java.security.PrivateKey;
import java.security.Signature;
import java.util.Base64;
import java.security.PublicKey;

public class StringUtil {
    public static String applySha256(String input) {
        try {
            MessageDigest digest = MessageDigest.getInstance("SHA-256");

            // Applies sha256 to our input,
            byte[] hash = digest.digest(input.getBytes("UTF-8"));
            StringBuilder hexString = new StringBuilder();                   // StringBuilder is slightly faster than StringBuffer

            for (byte b : hash) {
                String hex = Integer.toHexString(0xff & b);
                if (hex.length() == 1) hexString.append('0');
                hexString.append(hex);
            }
            return hexString.toString();
        }
        catch(Exception e) {
            throw new RuntimeException(e);
        }
    }

    // Applies ECDSA Signature and return the result as bytes
    public static byte[] applyECDSASig(PrivateKey privateKey, String input) {
        Signature dsa;

        byte[] output = new byte[0];
        try {
            dsa = Signature.getInstance("ECDSA", "BC");
            dsa.initSign(privateKey);
            byte[] strByte = input.getBytes();
            dsa.update(strByte);
            byte[] realSig = dsa.sign();
            output = realSig;
        } catch (Exception e) {
            throw new RuntimeException(e);
        }
        return output;
    }

    // Verify String signature
    public static boolean verifyECDSASig(PublicKey publicKey, String data, byte[] signature) {
        try {
            Signature ecdsaVerify = Signature.getInstance("ECDSA", "BC");
            ecdsaVerify.initVerify(publicKey);
            ecdsaVerify.update(data.getBytes());
            return ecdsaVerify.verify(signature);
        } catch(Exception e) {
            throw new RuntimeException(e);
        }
    }
    public static String getStringFromKey(Key key) {
        return Base64.getEncoder().encodeToString(key.getEncoded());
    }

    // Convert satoshis to display String
    // 100000000 satoshs = 1 noobCoins
    public static String toCoins(long satoshis){
        return String.format("%.8f", satoshis/100000000.0);
    }


}