二维联系人数组（Java）

Question

I'm trying to make a basic program that allows the use to input and remove up to 10 contacts (a [10][4] array supposed to be filled with First Name, Last Name, Phone #, and age). However when I try to input the 5th contact eclipse gives me an error message. I'd like to know why I'm receiving an error message. (I assume it's something to do with the columns since it's whenever i'm inputting something >4, but i'm not sure what exactly.)

import java.util.Scanner;


    public class Lab2 {
        public static void main(String[] args){
            new Lab2 ();
        }

        // This will act as our program switchboard
        public Lab2 (){
            Scanner input = new Scanner(System.in);
            String[][] personalInfo = new String[10][4];

            System.out.println("Welcome to the contacts database.");
            System.out.println("Please select a number from the options below");
            System.out.println("");

            while(true){
                // Give the user a list of their options
                System.out.println("1: Add a new contact.");
                System.out.println("2: Remove an existing contact.");           
                System.out.println("0: Exit the database.");

                // Get the user input
                int userChoice = input.nextInt();

                switch(userChoice){
                    case 1: 
                        addContact(personalInfo);
                        break;
                    case 2: 
                        removeContact(personalInfo);
                        break;              
                    case 0: 
                        System.out.println("Thank you for using the contact database.");
                        System.exit(0);
                }
            }

        }

        private void addContact(String personalInfo[][])
     {

            Scanner input = new Scanner(System.in);
            System.out.println();
            System.out.println( " Hello user, please enter your contact info. (I.E. First and Last Names, Phone #, and Age. Max 10 Contacts)");
            String addedContact = input.nextLine(); 


     int j;
        for(int i = 0; i < personalInfo.length; i++)
        {
            for(j = 0; j < personalInfo.length; j++){
           if(personalInfo[i][j] == null)
           {
               personalInfo[i][j] = addedContact; 
           break;
           }


       }
        }
    }



        private void removeContact(String personalInfo[][]) {
    Scanner input = new Scanner(System.in);
    System.out.print( " Please enter an existing contact that you would like to remove. ");
    String deleteContact = input.nextLine();
    int i , j;

    for(i = 0; i < personalInfo.length; i++)
    {
        for(j = 0; j < personalInfo.length; j++){
    if(personalInfo[i][j].equals(deleteContact))
    {
        personalInfo[i][j] = null;
    break;
    }


        }
    }

        }   
    }

Answer 1

for(j = 0; j < personalInfo[i].length; j++){

Answer 2

You're working with jagged array (or array of arrays)

 String[][] personalInfo = new String[10][4];

so in order to itterate it you should use different lengths for dimensions (in general case the required length is personalInfo[i].length ):

for (int i = 0; i < personalInfo.length; ++i) // <- outer array
  for (int j = 0; j < personalInfo[i].length; ++j) // <- inner length depends on i
    if (personalInfo[i][j] == null) {
      personalInfo[i][j] = addedContact; 
      break;
    }

Answer 3

First things first:

String[][] personalInfo = new String[10][4];

This statement creates a 2dimensional array of Strings, thus creating an array that is capable of holding an array that is capable of holding strings. (in this case 10 arrays that keep 4 strings each)

Now the error:

String addedContact = input.nextLine(); 

This line puts the entire line you added in a single string.

Now the loops: 1. The outer loop loops correctly through the 10 arrays of strings. 2. The inner loop does not loop through the '2nd dimension', that is the actual array which will hold your contact. This loop should be as @Kannan Thangadurai described:

    for(j = 0; j < personalInfo[i].length; j++){
      (add relevant code here)
    }

The last error you made is more of a bug, and is in the way you read out the user input. You actually only ever put a string in the first array and so your four contacts that you saved will all be under personalinfo[1] then you try to add another contact in the fifth place of the array that does not exist, thus raising your error.