How to get ArrayList and Scanner to play nice?

问题

import java.util.*;

public class CyclicShiftApp{

   public static void main(String[] args){
      Scanner scan = new Scanner(System.in);
      ArrayList<Integer> list = new ArrayList<Integer>();
      while(scan.hasNextInt()){
         list.add(scan.nextInt());
      }
      Integer[] nums = new  Integer[list.size()];
      nums = list.toArray(nums);
      for(int i = 0;i < nums.length; i++){
      System.out.println(nums[i]);
      }   
}

Thanks to poor-mans-debugging I've found that the while(scan.hasNextInt()) isnt actually adding anything. What might be going wrong? Is my google-fu weak or lack of know-how letting me down? I am rather new to programming, and so unfamiliar with Lists so thought this would be a nice first step but something isnt adding up. It also compiles fine so Its not syntax(anymore). Perhaps a casting issue?

回答1:

Does this work, master Samwise?

import java.util.*;

public class CyclicShiftApp{

public static void main(String[] args){
    Scanner scan = new Scanner(System.in);
    ArrayList<Integer> list = new ArrayList<Integer>();
    System.out.print("Enter integers please ");
    System.out.println("(EOF or non-integer to terminate): ");

    while(scan.hasNextInt()){
         list.add(scan.nextInt());
    }

    Integer [] nums = list.toArray(new Integer[0]);
    for(int i = 0; i < nums.length; i++){
       System.out.println(nums[i]);
    }
  }   
}

I'm assuming there's a reason why you need the list as an array, otherwise the conversion to an array is unnecessary. As Jon Skeet mentions in the comments, the loop will terminate only when the stream doesn't have a next int, ie. a non-integer value or a file's EOF if you're using 'java CyclicShiftApp < input_file.txt'.

回答2:

Your problem is here :

 while(scan.hasNextInt()){  <-- This will loop untill you enter any non integer value
     list.add(scan.nextInt());
  }

You just have to enter a character say e.g q once you finished entering all the integer values and then your program will print expected results.

Sample Input :14 17 18 33 54 1 4 6 q

回答3:

import java.util.*;
class SimpleArrayList{
public static void main(String args[])
{
Scanner sc = new Scanner(System.in);
ArrayList <Integer> al2 = new ArrayList<Integer>();
System.out.println("enter the item in list");
while(sc.hasNextInt())
{
al2.add(sc.nextInt());
}
Iterator it1 = al2.iterator();
/*loop will be terminated when it will not get integer value */
     while(it1.hasNext())
{
 System.out.println(it1.next());
}
}
}

回答4:

This is one of the simple and easiest way to use Scanner and ArrayList together.

import java.util.*;
public class Main
{
public static void main(String args[])
{
    Scanner sc=new Scanner(System.in);
    int num=sc.nextInt();
    ArrayList<Integer> list=new ArrayList<Integer>(num);
    for(int i=0;i<num;i++)
    {
    list.add(sc.nextInt());

   }
    Iterator itr=list.iterator();
        {
       while(itr.hasNext())
       {
           System.out.print(itr.next()+" ");
        }
        }
    }
}

来源：https://stackoverflow.com/questions/16098134/how-to-get-arraylistinteger-and-scanner-to-play-nice

标签

java