code: private void ckeckKey(KeyType keyType) {
  		switch (keyType) {
  		case PublicKey:
  			if (null == this.publicKey) {
  				throw new NullPointerException("No public key provided");
  			}
  			break;
  		case PrivateKey:
  			if (null == this.privateKey) {
  				throw new NullPointerException("No private key provided");
  			}
  			break;
  		}
  	}
docs: 检查对应类型的Key是否存在
  
  @param keyType key类型