Simple codes generator java

The following function returns a string of codes is required

public String id(){
        
        Random ra =new Random();
        st="";
        String [] w= {"0","1","2","3","4","5","6","7","8","9","K","Q","p","m","n","b","w","z","x","g","t","y","f"};
        for(int i=0;i<4;i++)
        {
            st=st+w[ra.nextInt(23)];
        }
        return st;
        
    }

The idea is to use an array of strings is provided comprising a desired verification code character type, and a method using random numbers taking random characters required number of extraction, which in turn spliced ​​to the new string and returns, thus achieving a randomly generated code verification purposes.