java - 这段非常简单的代码有什么问题

Question

这个问题已经有答案了:

Why Wont My Java Scanner Take In input? (3 个回答)

已关闭 8 年前。

当我运行我的代码时，它会正常工作，直到它询问“您想使用哪个操作( sum ， subst ， multi ， div )”。无论用户选择什么，我的程序都没有任何响应!

为什么会发生这种情况？

import java.util.Scanner;
import java.io.*;

public class three3 {
    public static void main (String[] args) {
        int x;
        int y;
        int opera;
        String oper;

        Scanner in = new Scanner (System.in);
        System.out.println(" write the first number ");
        x = in.nextInt();

        System.out.println(" write the second number ");
        y = in.nextInt();

        System.out.println(" which operation do you want to use from ( sum , subst , multi , div )");
        oper = in.nextLine();

        if (oper == "sum") {
            opera=x+y;
            System.out.println(" the sum of two numbers is " + opera );
        }

        if (oper == "subst") {
            opera = x - y;
            System.out.println(" the subtraction of two numbers is " + opera );
        }

        if (oper == "multi") {
            opera = x * y;
            System.out.println(" the multi of two numbers is " + opera );
        }

        if (oper == "div") {
            opera = x / y;
            System.out.println(" the division of two numbers is " + opera );
        }
    }
}

Answer 1

因为这些 if 子句都没有被执行。您将 Strings 与 == 进行比较，这是错误的。请使用 oper.equals("sum") 来代替。请参阅this question以供引用。您的结论是始终对字符串使用equals。