请各位大佬给我讲讲为什么我下面这串代码编译不过
public class StringBuffer
{
    public static void main(String[] args)
    {
        StringBuffer buffer = new StringBuffer();
        buffer.append("hello").append("world").append("welcome");
        
        String result = buffer.toString();
        System.out.println();
   
   
    }
}
谢谢