計算器沒有工作

Question

有人可以告訴我爲什麼這個計算器不工作？它只是不提供一個awnser。

import java.util.Scanner; 

public class Calcu { 

    public static void main(String[] args) 
    { 
     Scanner mati = new Scanner(System.in); 

    System.out.println("This program adds up or substracts two numbers"); 
    System.out.println("Enter an operator"); 
    String letter = mati.next();   //WAITS FOR THE PHRASE ADD OR SUBSTRACT 

    System.out.println("Enter your first number"); 
    int userNumberone = mati.nextInt();  // Get's first Number 

    System.out.println("Enter your second number"); 
    int userNumbertwo = mati.nextInt(); //Get's Following Number 

    if(letter == "add") { 
     int result = userNumberone + userNumbertwo; 
      System.out.println(result); 

    } else if(letter == "substract") { 
      int result1 = userNumberone - userNumbertwo; //If statement to add or substract. 
      System.out.println(result1); 

    } 
    } 
} 

Answer 1

您應該使用letter.equals("add")，而不是letters == "add"。這在這裏解釋：How do I compare strings in Java?