输入输入后，Java计算器停止

Question

我正在尝试为我的第一个java项目创建一个计算器。我向用户询问两个数字输入和一个符号来选择操作。它要求输入一旦到达if语句就停止计算答案。

import java.util.Scanner;
class Calculator{

    public static void main(String args[]){
        double a;
        double b;
        double answer;
        String symbol;
        Scanner in = new Scanner(System.in);
        System.out.print("Enter an operation");
        symbol = in.nextLine();
        System.out.print("Enter the first number");
        a = in.nextInt();
        System.out.print("Enter the second number");
        b = in.nextInt();

        if (symbol == "+"){
            answer = a+b;
            System.out.print(answer);

        }
        else if (symbol == "-"){
            answer = a-b;
            System.out.print(answer);

        }
        else if (symbol == "/"){
            answer = a/b;
            System.out.print(answer);

        }
        else if (symbol == "*"){
            answer = a*b;
            System.out.print(answer);

        }
    }
}

这是我的输出

Enter an operation +
Enter the first number 4
Enter the second number 5