Java - 程序沒有突破循環？

Question

while (choice != 7) { 

    System.out.println("---  Mathematical Calculator  ---");

    System.out.println("");

    System.out.println("Pick an operation from the list - Use nos. 1 to 7");
    System.out.println("1) Multiplication");
    System.out.println("2) Division");
    System.out.println("3) Addition");
    System.out.println("4) Subtraction");
    System.out.println("5) Find the area of a regular object");
    System.out.println("6) Find the volume of a regular object");
    System.out.println("7) Exit");

    choice = userInput.nextInt(); 

    switch (choice) {

    case 1: {
        System.out.println("");
        System.out.println("You have chosen multiplication");
        System.out.println("Enter a number");
        double num1 = userInput.nextDouble();
        System.out.println("Enter another number");
        double num2 = userInput.nextDouble();

        double num3 = num1 * num2;

        System.out.println(num1 + " * " + num2 + " = " + num3);         
        num1 = num3;

        while (choice2 != 5 || choice2 != 6) {

            System.out.println(""); System.out.println("");
            System.out.println("If you would like to build on the answer obtained - pick an operation from the list - Use nos. 1 to 4");
            System.out.println("Else press 5 to return to the main menu");
            System.out.println("1) Multiplication");
            System.out.println("2) Division");
            System.out.println("3) Addition");
            System.out.println("4) Subtraction");
            System.out.println("5) Start new calculation");
            System.out.println("6) Exit");

            choice2 = userInput.nextInt();

            switch (choice2) {

            case 1: 
            {
                System.out.println("Enter number");
                num2 = userInput.nextDouble();
                num3 = num1 * num2;
                System.out.println(num1 + " * " + num2 + " = " + num3);
                num1 = num3;
                break;
            }

            case 2: 
            {
                System.out.println("Enter number");
                num2 = userInput.nextDouble();
                num3 = num1 / num2;
                System.out.println(num1 + " / " + num2 + " = " + num3);
                num1 = num3;
                reak;
            }

            case 3: 
            {
                System.out.println("Enter number");
                num2 = userInput.nextDouble();
                num3 = num1 + num2;
                System.out.println(num1 + " + " + num2 + " = " + num3);
                num1 = num3;
                break;
            }

            case 4: 
            {
                System.out.println("Enter number");
                num2 = userInput.nextDouble();
                num3 = num1 - num2;
                System.out.println(num1 + " - " + num2 + " = " + num3);
                num1 = num3;
                break;
            }

            case 5: choice = 0; break;

            case 6: choice = 7; break;

            default: System.out.println("Invalid choice");
        }
        choice2 = 0;
    }
    break;
}

我發布了一小段代碼。 我的問題是，當我在第二個查詢（ choice2 ）中輸入5或6時，我的程序繼續循環而不是退出循環並返回主菜單/終止程序。

希望得到一些關於我做錯的反饋。

Answer 1

你的while (choice2 != 5 || choice2 != 6)是錯誤的，因為它總會導致true。 如果choice2為5，則第二個子句為真。

更不用說你總是在循環結束時將choice2設置為0，所以即使你替換了|| ，循環也將永遠繼續 用&& 。

Answer 2

條件choice2 != 5 || choice2 != 6 choice2 != 5 || choice2 != 6始終為true，因為沒有數字同時等於5和6。

如果您想在輸入5或6時跳出循環，請使用&&代替：

while (choice2 != 5 && choice2 != 6)

確保在繼續下一次循環迭代之前設置choice2 。

請注意，您可以使用帶標簽的break構造從switch語句中跳出循環：

calc: // Label the loop for using labeled break
while (true) {
    ...
    switch(...) {
        case 1:
            break;          // This will exit the switch
        ...
        case 6: break calc; // This will exit the loop
    }
}