Read integers from a file, sort, and print.

BestJewSinceJC 0 5K Views

Since I keep seeing people asking how to read the Integers from a File, I figured this simple snippet might be useful. What it does is it creates a new Scanner Object based on the File Object "test.txt" then it reads all of the Integers from that file, skipping over anything else that was found in the file, and adding the Integers to an ArrayList. It then sorts the ArrayList and prints out the sorted numbers. The file must exist otherwise this example will crash.

import java.io.File;
import java.io.FileNotFoundException;
import java.util.ArrayList;
import java.util.Collections;
import java.util.Scanner;


public class IntegersFromFile {

	public static void main(String[] args) {
		Scanner file = null;
		ArrayList<Integer> list = new ArrayList<Integer>();
		
		try {
			file = new Scanner(new File("test.txt"));
		} catch (FileNotFoundException e) {
			e.printStackTrace();
		}
		
		while(file.hasNext()){
			if (file.hasNextInt()) list.add(file.nextInt());
			else file.next();
		}
		
		Collections.sort(list);
		
		for (Integer i: list) System.out.println(i);
		
	}

}
Member Avatar for kvass
kvass 70 Junior Poster

Out of curiosity, what sorting algorithm does the Collections class' sort method use?

Member Avatar for BestJewSinceJC
BestJewSinceJC 700 Posting Maven
Member Avatar for jade_91
jade_91 0 Light Poster

ok well i tried this but i want to modify it to sort text rather than numbers not sure how to do it I tried this

import java.io.File;
import java.io.FileNotFoundException;
import java.util.ArrayList;
import java.util.Collections;
import java.util.Scanner; 

 public class NameSort { 	

public static void main(String[] args) {	
	Scanner file = null;		
[ArrayList<String> list = new ArrayList<String>();]

 		try {	
		file = new Scanner(new File("names.txt"));
		} catch (FileNotFoundException e) {	
		e.printStackTrace();		} 	

	while(file.hasNext()){			
if (file.hasNextString()) list.add(file.nextString());
			else file.next();		}

 		Collections.sort(list); 		


for (String s: list) System.out.println(s); 	
	} 

}

but it doesnt want to work keeps coming back with errors any help would be much appreciated

Member Avatar for devninja
devninja 2 Light Poster

why do you have brackets around the arraylist decleration?

Member Avatar for tn464
tn464 0 Newbie Poster

if i were to want to get the total of the number in a files where would i insert the code

Member Avatar for Ezzaral
Ezzaral 2,714 Posting Sage

The read loop starts on line 20:
while(file.hasNext()){
That loop is where you'd perform any operations you want to do with each entry.

Edited by Ezzaral
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