遇到了JAVA中的.equals问题

Question

package thenewboston;
import java.util.Scanner;
public class apple {
    public static void main(String args[]) {
        Scanner myScan = new Scanner(System.in);
        System.out.println("Enter your first num");
        double fnum, snum, answer;
        String oper;
        fnum = myScan.nextDouble();
        System.out.println("Enter your second num");
        snum = myScan.nextDouble();
        System.out.println("Enter your operation");
        if (oper.equals("+")) {
        answer = fnum + snum;
        } else if (oper.equals("-")){
            answer = fnum - snum;
        } else if (oper.equals("*")) {
            answer = fnum * snum;
        } else if (oper.equals("/")){
            answer = fnum / snum;
        } else {
            System.out.println("Please choose a valid operation");
        }
        System.out.println("Your answer is: " + answer);
    }

}

您好。我试图使用.equals（）来创建一个基本的计算器，但是当我写oper.equals（“...”）时，oper上出现了一个错误。

Answer 1

您应该为操作指定值。

String oper = "*";

Answer 2

您可能会收到错误，因为在使用之前您从未真正将oper设置为任何内容。

...
snum = myScan.nextDouble();
System.out.println("Enter your operation");
oper = myScan.nextLine(); //<-- You were missing this.
if (oper.equals("+")) {
answer = fnum + snum;
...

Answer 3

您没有要求用户输入操作，因此，您的操作无法与您之后说的任何内容相匹配。