线程“主”中的异常要多少个数字？ java.lang.ArrayIndexOutOfBoundsException：4 at Calculator.main（Calculator.java:20）

Question

So i was making a calculator that uses exponents...

import java.util.Scanner;

public class Calculator {

    public static void main(String[] args) {
        // TODO Auto-generated method stub

        System.out.println("How many numbers do you want?");

        //starts the array 
        //[2] means numbers lower than 2
        int[] lnum = new int[2];

        for (int i=0; i<lnum.length; i++)
        {
            int[] num = new int[4];
            num[2] = 2;
            num[3] = 3;
            num[4] = 4; 

            Scanner x = new Scanner(System.in);
            int I = x.nextInt();

            if (I == num[2])
            {
                Scanner y = new Scanner(System.in);
                System.out.println("Enter a number...");
                int fnum = y.nextInt();
                System.out.println("Enter you're second number...");
                int snum = y.nextInt();
                int anum = (fnum + snum);
                System.out.println("The anser is " + anum);
            }

            if (I == num[3])
            {
                Scanner y = new Scanner(System.in);
                System.out.println("Enter a number...");
                int fnum = y.nextInt();
                System.out.println("Enter you're second number...");
                int snum = y.nextInt();
                System.out.println("Enter you're third number...");
                int tnum = y.nextInt();
                int anum = (fnum + snum + tnum);
                System.out.println("The anser is " + anum);
            }

            if (I == num[4])
            {
                Scanner y = new Scanner(System.in);
                System.out.println("Enter a number...");
                int fnum = y.nextInt();
                System.out.println("Enter you're second number...");
                int snum = y.nextInt();
                System.out.println("Enter you're third number...");
                int tnum = y.nextInt();
                System.out.println("Enter you're fourth number...");
                int ftnum = y.nextInt();
                int anum = (fnum + snum + tnum + ftnum);
                System.out.println("The anser is " + anum);
            }
        }
    }

}

and ended up with an error saying...

Exception in thread "main" How many numbers do you want?
java.lang.ArrayIndexOutOfBoundsException: 4
    at Calculator.main(Calculator.java:20)

Answer 1

Array indexes always start with 0, so if you have 4 elements, last element will be 4-1 ie 3. Here you have problem:

    num[2] = 2;
    num[3] = 3; //This is the last element
    num[4] = 4; //This is not last element

Change your logic accordingly.

Answer 2

您的num是一个int[4] ，这意味着它的索引范围为[0,3] ，但是您尝试访问无效的num[4] 。

Answer 3

Try this

    public static void main(String[] args) {
        // TODO Auto-generated method stub

        System.out.println("How many numbers do you want?");

        // starts the array
        // [2] means numbers lower than 2
        int[] lnum = new int[2];

        for (int i = 0; i < lnum.length; i++) {
            int[] num = new int[4];
            num[1] = 2;
            num[2] = 3;
            num[3] = 4;

            Scanner x = new Scanner(System.in);
            int I = x.nextInt();

            if (I == num[1]) {
                Scanner y = new Scanner(System.in);
                System.out.println("Enter a number...");
                int fnum = y.nextInt();
                System.out.println("Enter you're second number...");
                int snum = y.nextInt();
                int anum = (fnum + snum);
                System.out.println("The anser is " + anum);
            }

            if (I == num[2]) {
                Scanner y = new Scanner(System.in);
                System.out.println("Enter a number...");
                int fnum = y.nextInt();
                System.out.println("Enter you're second number...");
                int snum = y.nextInt();
                System.out.println("Enter you're third number...");
                int tnum = y.nextInt();
                int anum = (fnum + snum + tnum);
                System.out.println("The anser is " + anum);

            }

            if (I == num[3]) {
                Scanner y = new Scanner(System.in);
                System.out.println("Enter a number...");
                int fnum = y.nextInt();
                System.out.println("Enter you're second number...");
                int snum = y.nextInt();
                System.out.println("Enter you're third number...");
                int tnum = y.nextInt();
                System.out.println("Enter you're fourth number...");
                int ftnum = y.nextInt();
                int anum = (fnum + snum + tnum + ftnum);
                System.out.println("The anser is " + anum);
            }
        }
    }