External sort for large file using java

	import java.io.BufferedReader;
	import java.io.FileNotFoundException;
	import java.io.FileReader;
	import java.io.FileWriter;
	import java.io.IOException;
	import java.io.PrintWriter;
	import java.util.Arrays;
	import java.util.Random;
	public class ExternalSort {
	static int N = 2000000; // size of the file in disk
	static int M = 100000; // max items the memory buffer can hold
	public static void externalSort(String fileName) {
	String tfile = "temp-file-";
	int[] buffer = new int[M < N ? M : N];
	try {
	FileReader fr = new FileReader(fileName);
	BufferedReader br = new BufferedReader(fr);
	int slices = (int) Math.ceil((double) N / M);
	int i, j;
	i = j = 0;
	// Iterate through the elements in the file
	for (i = 0; i < slices; i++) {
	// Read M-element chunk at a time from the file
	for (j = 0; j < (M < N ? M : N); j++) {
	String t = br.readLine();
	if (t != null)
	buffer[j] = Integer.parseInt(t);
	else
	break;
	}
	// Sort M elements
	Arrays.sort(buffer);
	// Write the sorted numbers to temp file
	FileWriter fw = new FileWriter(tfile + Integer.toString(i) + ".txt");
	PrintWriter pw = new PrintWriter(fw);
	for (int k = 0; k < j; k++)
	pw.println(buffer[k]);
	pw.close();
	fw.close();
	}
	br.close();
	fr.close();
	// Now open each file and merge them, then write back to disk
	int[] topNums = new int[slices];
	BufferedReader[] brs = new BufferedReader[slices];
	for (i = 0; i < slices; i++) {
	brs[i] = new BufferedReader(new FileReader(tfile + Integer.toString(i) + ".txt"));
	String t = brs[i].readLine();
	if (t != null)
	topNums[i] = Integer.parseInt(t);
	else
	topNums[i] = Integer.MAX_VALUE;
	}
	FileWriter fw = new FileWriter("C:\\testd\\external-sorted.txt");
	PrintWriter pw = new PrintWriter(fw);
	for (i = 0; i < N; i++) {
	int min = topNums[0];
	int minFile = 0;
	for (j = 0; j < slices; j++) {
	if (min > topNums[j]) {
	min = topNums[j];
	minFile = j;
	}
	}
	pw.println(min);
	String t = brs[minFile].readLine();
	if (t != null)
	topNums[minFile] = Integer.parseInt(t);
	else
	topNums[minFile] = Integer.MAX_VALUE;
	}
	for (i = 0; i < slices; i++)
	brs[i].close();
	pw.close();
	fw.close();
	} catch (FileNotFoundException e) {
	e.printStackTrace();
	} catch (IOException e) {
	e.printStackTrace();
	}
	}
	static String generateInput(int n) {
	String fileName = "external-sort.txt";
	Random rand = new Random();
	try {
	FileWriter fw = new FileWriter(fileName);
	PrintWriter pw = new PrintWriter(fw);
	for (int i = 0; i < n; i++)
	pw.println(rand.nextInt(101));
	pw.close();
	} catch (IOException e) {
	e.printStackTrace();
	}
	return fileName;
	}
	public static void main(String[] args) {
	String fileName = generateInput(N);
	externalSort(fileName);
	}
	}

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