java中遇到問題包含

Question

如何檢查變量是否等於或包含？我想這樣做，如果有人在哪裏投入一個無效的運營商，它會說再試一次。

String operator = "+, -, *,/, %"; 
double num1, num2, sum=0;  
System.out.println("First number: "); 
num1 = input.nextDouble(); 
System.out.println("Second number: "); 
num2 = input.nextDouble(); 
if (operator.contains("+")){ 
    sum = num1 + num2; 
} else if (operator != operator.contains("+,-,*,/,%"){ 

System.out.println（「Error」）; }

Answer 1

如果你想檢查應根據用戶輸入使用什麼操作，您可以使用switch聲明如下

String operator = ""; 
while (operator.isEmpty()) { 
    System.out.println("Enter Operator: "); 
    operator = input.next(); 
    switch (operator) { 
    case "+" : result = num1 + num2; break; 
    case "-" : result = num1 - num2; break; 
    case "*" : result = num1 * num2; break; 
    case "/" : result = num1/num2; break; 
    default : System.out.println("Invalid operator: " + operator); 
       operator = ""; 
    } 
    System.out.println("Result is " + result); 
} 

DEMO

如果你只是想檢查，如果操作者是有效的，你可以使用Listcontains()方法：

String[] operators = { "+", "-", "*", "/" , "%" }; 
List<String> ops = Arrays.asList(operators); 
: 
System.out.println("Enter Operator: "); 
String operator = input.next(); 
if (ops.contains(operator)) { 
    // do some calculation here 
}