如果int可用，您已经读取了它，基本上，您只需要重复这个步骤。您可以使用附加标志来指示程序是否能够成功读取两个整数：

int a1, a2;
boolean gotTwoInts = false;
if (input.hasNextInt()) {
    a1 = input.nextInt();
}
if (input.hasNextInt()) {
    a2 = input.nextInt();
    gotTwoInts = true;
}

if (!gotTwoInts) {
    System.out.println("Invalid input entered. Terminating... ");
    break;
}

更新

完整示例：

import java.util.Scanner;
public class Calculator {
    public static void main(String[] args) {
        Scanner input = new Scanner (System.in);

        System.out.println("List of operations: add subtract multiply divide alphabetize");
        System.out.println("Enter an operation:");
        String operationInput = input.next().toLowerCase();

        switch (operationInput){
            case "add": { // curly braces because local variables are block scoped.
                System.out.print("Enter two integers:");

                int a1 = -1, a2 = -1; // local variables need to be initialized
                boolean gotTwoInts = false;
                if (input.hasNextInt()) {
                    a1 = input.nextInt();
                }
                if (input.hasNextInt()) {
                    a2 = input.nextInt();
                    gotTwoInts = true;
                }

                if (!gotTwoInts) {
                    System.out.println("Invalid input entered. Terminating... ");
                    break;
                }

                System.out.println("Answer: " + (a1 + a2));
            }
        }
    }
}

你可以改变结构。您可以使用try块和catch块代替if ( ! input.hasNextInt())和else块。如果没有两个整数，input.nextint()将抛出异常。扫描仪API#nextInt

System.out.print("Enter two integers:");
try {
  int a1 = input.nextInt();
  int a2 = input.nextInt();
  System.out.println("Answer: " + (a1 + a2));
}
catch (InputMismatchException ex) {
   System.out.println("Invalid input entered. Terminating...");
   System.exit (100); }
catch (NoSuchElementException ex) {
   System.out.println("Missing input. Expected two integers."
                        + " Terminating...");
   System.exit (100); }
finally { 
   input.close();
}

我将使用方法nextLine–而不是方法nextInt，让用户在一行中输入两个整数，用空格分隔。
此外，我认为将代码划分为方法是一个好主意，而不是在方法main中编写所有代码。
似乎您还打算进行加法减法、乘法和除法运算，因此下面的部分代码可以被视为帮助您完成程序的模板。

import java.util.Scanner;

public class Calculator {
    private static final int NUMBER_OF_OPERANDS = 2;
    private static Scanner input = new Scanner(System.in);
    private static int[] operands = new int[NUMBER_OF_OPERANDS];

    private static int add() {
        getOperands();
        return operands[0] + operands[1];
    }

    private static void getOperands() {
        System.out.print("Enter two integers separated by space: ");
        String value = input.nextLine();
        String[] parts = value.split("\\s+");
        if (parts.length < NUMBER_OF_OPERANDS) {
            throw new RuntimeException("Less than two integers entered.");
        }
        operands[0] = Integer.parseInt(parts[0]);
        operands[1] = Integer.parseInt(parts[1]);
    }

    public static void main(String[] args) {
        System.out.println("List of operations: add subtract multiply divide alphabetize");
        System.out.print("Enter an operation: ");
        String operationInput = input.nextLine().toLowerCase();
        int result = 0;
        switch (operationInput) {
            case "add":// for addition
                result = add();
                break;
            case "subtract":
                break;
            case "multiply":
                break;
            case "divide":
                break;
            case "alphabetize":
                break;
            default:
                System.out.println("Invalid operation: " + operationInput);
                System.exit(1);
        }
        System.out.println("Result: " + result);
    }
}

请注意，RuntimeException是未经检查的exception，并且类java.lang.Integer的[static]方法parseInt可能会抛出NumberFormatException，这也是未经检查异常。