有人可以解释一下这段代码有什么问题吗？

Question

该程序执行以下操作:第一个输入从用户获得两个数字,第二个输入如果用户输入(+)符号,程序将两个数字相加,如果用户输入( - )符号,则从第二个数字中减去第一个数字.但没有结果显示该程序只是运行和终止.

import javax.swing.JOptionPane;

public class ObtainNumber {
public static void main(String []args) {

    String strNum1 = JOptionPane.showInputDialog("Enter the first number");
    String strNum2 = JOptionPane.showInputDialog("Enter the second number");

    int num1 = Integer.parseInt(strNum1);
    int num2 = Integer.parseInt(strNum2);

    String askForOperation = JOptionPane.showInputDialog("What operation needs to be done?");

    int sum;
    if (askForOperation == "+") {
            sum = num1 + num2;
            JOptionPane.showMessageDialog(null, "The result of the Addition is " + sum);
    }

    double subtract;
    if (askForOperation == "-") {
            subtract = num2 - num1;
            JOptionPane.showMessageDialog(null, "The result of the subtraction is " + subtract);

    }



    }

}

Answer 1

if (askForOperation == "+")  // Reference is not same

比较检查使用equals方法.

if ("+".equals(askForOperation))  // Compare content