String.format or printf

都可以格式化输出
System.out.printf("Total: R$ %.2f",price[x-1]*y);
System.out.println("Total: R$ "+String.format("%.2f", price[x-1] * y));

System.out.println(String.format("%.4f",Math.sqrt(a+b)));
System.out.printf("%.4f",Math.sqrt(a+b))