String payload1=(char *)payload;为什么要强制转换char*

void receiveCallback(char* topic, byte* payload, unsigned int length) {//反调函数用于接收平台下发的消息
Serial.print("Message Received [");
Serial.print(topic);
Serial.print("]");
for (int i = 0; i < length; i++){ //按字符一个一个输出
Serial.print((char)payload[i]);
}
Serial.println("");
Serial.print("Message Length(Bytes)");
Serial.println(length);
payload[length] = '\0'; // 添加一个空值使其转化为字符数组
String payload1=(char *)payload;
int aNumber = atoi((char *)payload);//将字符数组转化为整型数组