else if(msg.type.equals("pubKey")){
                    pubKey = msg.content;                     //get public key
                    String key = Arrays.toString(crypt.geteKey());
                    clients[findClient(ID)].send(new Message("symmKey", "SERVER", key, msg.sender));//!  //send symmetric key encrypted with public key
            }

public void keyEncryption() throws NoSuchAlgorithmException, NoSuchPaddingException, InvalidKeyException, IllegalBlockSizeException, BadPaddingException{
    Cipher cipher = Cipher.getInstance("RSA/ECB/PKCS1Padding");
    cipher.init(Cipher.ENCRYPT_MODE, pubKey);
    eKey = cipher.doFinal(key.getBlowfishKeyBytes());           //symmetric key encrypted with public key
    //System.out.println("2. cipherText= " + bytesToHex(symmKey));
}

else if(msg.type.equals("symmKey")){
                    symmKey = (String) msg.content;     //get encrypted symmetric key (must unlock with private key)
                }

package com.socket;

import java.io.Serializable;

public class Message implements Serializable{

    private static final long serialVersionUID = 1L;
    public String type, sender,content, recipient;

    public Message(String type, String sender, String content, String recipient){
        this.type = type; this.sender = sender; this.content = content; this.recipient = recipient;
    }

    @Override
    public String toString(){
        return "{type='"+type+"', sender='"+sender+"', content='"+content+"', recipient='"+recipient+"'}";
    }
}

private void jButton2ActionPerformed(java.awt.event.ActionEvent evt) {
    //KeyPair
    try {
        keyPair = new Keypair();
    } catch (NoSuchAlgorithmException ex) {
        jTextArea1.append("Security Error! You are not safe!");
    }
    Object pubKey = keyPair.getKeyPair().getPublic();
    username = jTextField3.getText();
    password = jPasswordField1.getText();

    if (!username.isEmpty() && !password.isEmpty()) {
        client.send(new Message("login", username, password, "SERVER"));
        client.send(new Message("pubKey",username, pubKey, "SERVER"));      //send Public key to Server
    }
}

Database exception : userExists()
53846 ERROR reading: cannot assign instance of sun.security.rsa.RSAPublicKeyImpl to field com.socket.Message.content of type java.lang.String in instance of com.socket.Message

pubKey = msg.content;