为什么我没有得到任何输出？

Question

我没有从这个计算器中获得任何输出。有人能告诉我为什么吗？

import java.util.*; 

public class JavaMain { 

public static void main(String[] args) { 
    float num1, num2; 
    String op; 
    float d, m, a, s; 

    Scanner input = new Scanner(System.in); 

    System.out.println("Calculator"); 

    System.out.print("Please enter an operation: "); 
    op = input.next(); 


    System.out.print("First Number: "); 
    num1 = input.nextFloat(); 

    System.out.print("Second Number: "); 
    num2 = input.nextFloat(); 




     if (op == "+"){a = (num1 + num2); 
      System.out.println(num1 + "+" + num2 + "=" + a); 
     } 


     if (op == "-"){s = (num1 - num2); 
      System.out.println(num1 + "-" + num2 + "=" + s); 
     } 


     if (op == "/"){d = (num1/num2); 
      System.out.println(num1 + "/" +num2 + "=" + d); 
     } 


     if (op == "*"){m = (num1*num2); 
      System.out.println(num1 + "*" + num2 + "="+ m); 
     } 


    } 

}// why am i not getting any outputs 

Answer 1

您没有正确比较字符串。使用String.equals()方法进行字符串比较。 ==检查它们是否是相同的对象，如果它们在逻辑上相同则不相同。 请尝试以下代码：

if (op.equals("+")){ 
    a = (num1 + num2); 
    System.out.println(num1 + "+" + num2 + "=" + a); 
    } 

Answer 2

处理字符串时，我们需要使用字符串方法。那就是我们需要用equals（）方法来测试相等性。此外，你的if语句应该是if if else if like such ...

if (op.equals("+")) { 
     a = (num1 + num2); 
     System.out.println(num1 + "+" + num2 + "=" + a); 
    }else if(op.equals("-")){ 
     s = (num1 - num2); 
     System.out.println(num1 + "-" + num2 + "=" + s); 
    }else if(op.equals("/")){ 
     d = (num1/num2); 
     System.out.println(num1 + "/" +num2 + "=" + d); 
    }else{//if none of the above then the operation must be the remaining 
     m = (num1*num2); 
     System.out.println(num1 + "*" + num2 + "="+ m); 
    } 

你也可以使用switch case来代替。