protected byte[] encrypt(byte[] obj) ...{
    try ...{
        messagedigest md5 = messagedigest.getinstance("md5");
            md5.update(obj);
        return md5.digest();
    } catch (nosuchalgorithmexception e) ...{
        e.printstacktrace();
    }
}

protected byte[] encrypt(byte[] obj) ...{
    try ...{
        messagedigest sha = messagedigest.getinstance("sha");
            sha.update(obj);
        return sha.digest();
    } catch (nosuchalgorithmexception e) ...{
        e.printstacktrace();
    }
}

import java.security.keypair;
import java.security.keypairgenerator;
import java.security.interfaces.rsaprivatekey;
import java.security.interfaces.rsapublickey;

import javax.crypto.cipher;

/** *//**
 * <b>rsaencrypt</b>
 * <p>
 * @author maqujun
 * @see
 */
public class rsaencrypt ...{

    /** *//**
 * main method for rsaencrypt.
     * @param args
     */
    public static void main(string[] args) ...{
        try ...{
            rsaencrypt encrypt = new rsaencrypt();
            
            string encrypttext = "encrypttext";
            keypairgenerator keypairgen = keypairgenerator.getinstance("rsa");
            keypairgen.initialize(1024);
            keypair keypair = keypairgen.generatekeypair();
            // generate keys
            rsaprivatekey privatekey = (rsaprivatekey) keypair.getprivate();
            rsapublickey publickey = (rsapublickey) keypair.getpublic();
            
            byte[] e = encrypt.encrypt(publickey, encrypttext.getbytes());
            byte[] de = encrypt.decrypt(privatekey,e);
            system.out.println(encrypt.bytestostring(e));
            system.out.println(encrypt.bytestostring(de));
        } catch (exception e) ...{
            e.printstacktrace();
        }
    }
    
/** *//**
     * change byte array to string.
     * @return byte[]
     */
    protected string bytestostring(byte[] encrytpbyte) ...{
        string result = "";
        for (byte bytes : encrytpbyte) ...{
            result += (char) bytes.intvalue();
        }
        return result;
    }
    
/** *//**
     * encrypt string.
     * @return byte[]
     */
    protected byte[] encrypt(rsapublickey publickey, byte[] obj)  ...{
        if (publickey != null) ...{
            try ...{
                cipher cipher = cipher.getinstance("rsa");
                cipher.init(cipher.encrypt_mode, publickey);
                return cipher.dofinal(obj);
            } catch (exception e) ...{
                e.printstacktrace();
            }
        }
        return null;
    }

    /** *//**
     * basic decrypt method
     * @return byte[]
     */
    protected byte[] decrypt(rsaprivatekey privatekey, byte[] obj) ...{
        if (privatekey != null) ...{
                try ...{
                    cipher cipher = cipher.getinstance("rsa");
                    cipher.init(cipher.decrypt_mode, privatekey);
                    return cipher.dofinal(obj);
                } catch (exception e) ...{
                    e.printstacktrace();
                }
            }
    
        return null;
    }
}