Java程序跳过输入？

我试图使用基本的Java代码构建一个简单的计算器，并更好地理解OOP，所以，我这样写：

import java.util.Scanner;

public class Calc {

public static void main(String[] args) { Scanner input = new Scanner(System.in); System.out.println("Enter a number: "); int num1 = input.nextInt();

System.out.println("Enter an operation: ");
String opr;
opr = input.nextLine();

if(opr == "+") {
    System.out.println("Enter another Number: ");
    int num2 = input.nextInt();
    Operation op = new Operation(num1, num2);
    op.addition();
} else if (opr == "-") {
    System.out.println("Enter another Number: ");
    int num2 = input.nextInt();
    Operation op = new Operation(num1, num2);
    op.subtraction();
} else if (opr == "*") {
    System.out.println("Enter another Number: ");
    int num2 = input.nextInt();
    Operation op = new Operation(num1, num2);
    op.multiplication();
} else if (opr == "/") {
    System.out.println("Enter another Number: ");
    int num2 = input.nextInt();
    Operation op = new Operation(num1, num2);
    op.division();
} else {
    System.out.println("Please, Enter a valid operation!");
}

}

和另一个用于数学运算的类：

import java.util.Scanner;

public class Operation {

static int Num1; static int Num2;

Scanner input = new Scanner(System.in); public Operation(int x, int y) { x = Num1; y = Num2; }

public void addition() {

System.out.println(Num1 + Num2);

}

public void subtraction() {

System.out.println(Num1 - Num2);

}

public void multiplication() {

System.out.println(Num1 * Num2);

}

public void division() {

System.out.println(Num1 / Num2);

}

但它不接受操作输入，而是直接进入下一行代码，如下所示：

Enter a number: 4 Enter an operation: Please, Enter a valid operation!

任何人都可以指出我的错误吗？

注意：我一般是Java和编程方面的新手。所以，如果我的代码不是最好的，请不要介意我，我还在学习。

问题来源：stackoverflow