教新手使用java如何对一个大的文本文件内容进行去重

	public static void distinct() {
		File ff = new File("G://password/all.txt");
		File distinctedFile = new File("G://password/all-distinced.txt");
		PrintWriter pw = null;
		Set<String> allHash = null;
		FileReader fr = null;
		BufferedReader br = null;
		try {
			pw = new PrintWriter(distinctedFile);
			allHash = new HashSet<String>();
			fr = new FileReader(ff);
			br = new BufferedReader(fr);
			String line = null;
			while((line=br.readLine())!=null){
				line = line.trim();
				if(line != ""){
					allHash.add(line);
				}
			}
		} catch (FileNotFoundException e) {
			e.printStackTrace();
		} catch (IOException e) {
			e.printStackTrace();
		} finally {
			try {
				if(null != fr){
					fr.close();
				}
			} catch (IOException e) {
				e.printStackTrace();
			}
			try {
				if(null != br){
					br.close();
				}
			} catch (IOException e) {
				e.printStackTrace();
			}
		}
		for(String s:allHash){
			pw.println(s);
		}
		pw.close();
	}

Exception in thread "main" java.lang.OutOfMemoryError: GC overhead limit exceeded
	at java.util.HashMap.newNode(HashMap.java:1734)
	at java.util.HashMap.putVal(HashMap.java:630)
	at java.util.HashMap.put(HashMap.java:611)
	at java.util.HashSet.add(HashSet.java:219)
	at encode.Main.distinct(Main.java:180)
	at encode.Main.main(Main.java:215)

import java.io.BufferedReader;
import java.io.File;
import java.io.FileNotFoundException;
import java.io.FileReader;
import java.io.IOException;
import java.io.PrintWriter;
import java.util.HashSet;
import java.util.Set;
public class DistinctFileUtil {
	
	public static File[] splitFile(String targetFile,int splitSize){
		File file = new File(targetFile);
		BufferedReader reader = null;
		PrintWriter[] pws = new PrintWriter[splitSize];
		File[] littleFiles = new File[splitSize];
		String parentPath = file.getParent();
		File tempFolder = new File(parentPath + File.separator + "test");
		if(!tempFolder.exists()){
			tempFolder.mkdir();
		}
		for(int i=0;i<splitSize;i++){
			littleFiles[i] = new File(tempFolder.getAbsolutePath() + File.separator + i + ".txt");
			if(littleFiles[i].exists()){
				littleFiles[i].delete();
			}
			try {
				pws[i] = new PrintWriter(littleFiles[i]);
			} catch (FileNotFoundException e) {
				e.printStackTrace();
			}
		}
		try {
			reader = new BufferedReader(new FileReader(file));
			String tempString = null;
			while ((tempString = reader.readLine()) != null) {
				tempString = tempString.trim();
				if(tempString != ""){
					//关键是将每行数据hash取模之后放到对应取模值的文件中，确保hash值相同的字符串都在同一个文件里面
					int index = Math.abs(tempString.hashCode() % splitSize);
					pws[index].println(tempString);
				}
			}
		} catch (Exception e) {
			e.printStackTrace();
		} finally {
			if (reader != null) {
				try {
					reader.close();
				} catch (IOException e1) {
					e1.printStackTrace();
				}
			}
			for(int i=0;i<splitSize;i++){
				if(pws[i] != null){
					pws[i].close();
				}
			}
		}
		return littleFiles;
	}
	
	public static void distinct(File[] littleFiles,String distinctFilePath,int splitSize){
		File distinctedFile = new File(distinctFilePath);
		FileReader[] frs = new FileReader[splitSize];
		BufferedReader[] brs = new BufferedReader[splitSize];
		PrintWriter pw = null;
		try {
			if(distinctedFile.exists()){
				distinctedFile.delete();
			}
			distinctedFile.createNewFile();
			pw = new PrintWriter(distinctedFile);
			Set<String> unicSet = new HashSet<String>();
			for(int i=0;i<splitSize;i++){
				if(littleFiles[i].exists()){
					System.out.println("开始对小文件：" + littleFiles[i].getName() + "去重");
					frs[i] = new FileReader(littleFiles[i]);
					brs[i] = new BufferedReader(frs[i]);
					String line = null;
					while((line = brs[i].readLine())!=null){
						if(line != ""){
							unicSet.add(line);
						}
					}
					for(String s:unicSet){
						pw.println(s);
					}
					unicSet.clear();
					System.gc();
				}
			}
		} catch (FileNotFoundException e) {
			e.printStackTrace();
		} catch (IOException e1){
			e1.printStackTrace();
		} finally {
			for(int i=0;i<splitSize;i++){
				try {
					if(null != brs[i]){
						brs[i].close();
					}
					if(null != frs[i]){
						frs[i].close();
					}
				} catch (IOException e) {
					e.printStackTrace();
				}
				//合并完成之后删除临时小文件
				if(littleFiles[i].exists()){
					littleFiles[i].delete();
				}
			}
			if(null != pw){
				pw.close();
			}
		}
	}
	public static void main(String[] args) throws IOException {
		int splitSize = 20;
		File[] files = splitFile("G://test/bigfile.txt",splitSize);
		distinct(files,"G://test/bigfile-distinct.txt",splitSize);
	}
}