Java面试题~随机文件读取

10W行以上的数据，存在txt文件中，每行数据包括id和描述。id是有序的。写一个Java方法，传入id，从文件中快速查询出对应的描述。时间控制在0.1s内

解决方案，使用二分查找+随机文件读取

import java.io.IOException;
import java.io.RandomAccessFile;
import java.util.Calendar;

/**
 * 问题描述
 * 在10W行数据的文本文件中，每行包括一个id和一个描述，给一个id快速查找对应的描述
 * id是有序的
 * @author song
 *
 */
public class Main {

	public static void main(String[] args) throws IOException {
		
		
		
		RandomAccessFile reader = new RandomAccessFile("1.txt", "r");
		
		long aver = 0;
		int testCount = 200;
		for(int i=0; i<testCount; i++){
			long startTime = Calendar.getInstance().getTimeInMillis();
			int searchId = (int) (Math.random() * 105000);
			System.out.print("No." + i +", ");
			binarySearch(reader, searchId);
			long endTime = Calendar.getInstance().getTimeInMillis();
			System.out.println(" |use time: " + (endTime - startTime) + " MilliSecond");
			aver += (endTime - startTime);
		}
		System.out.println("averange time: " + aver / testCount);
	}
	
	/**
	 * 从文件指针当前位置读取一个整数
	 * @param reader
	 * @return
	 * @throws IOException
	 */
	static int readInt(RandomAccessFile reader) throws IOException{
		long startpos = reader.getFilePointer();
		StringBuffer sb = new StringBuffer();
		int ch;
		while((ch=reader.read()) != ' '){
			sb.append((char)ch);
		}
		reader.seek(startpos);
		return Integer.parseInt(sb.toString());
	}
	
	
	/**
	 * 读文件的最后一行，最后文件指针定位在最后一行的第一个字符上
	 * @param reader
	 * @return 最后一行的id
	 * @throws IOException
	 */
	static int readEndline(RandomAccessFile reader) throws IOException{
		int endId = 0;
		
		long end = reader.getFilePointer();
		long cur = end - 1;
		int ch;
		boolean tag = false;
		while(cur >= 0){
			reader.seek(cur);
			ch = reader.read();
			if(ch == '\n'){
				endId = readInt(reader);
				tag = true;
				break;
			}
			cur--;
		}
		if(!tag){
			reader.seek(0);
			endId = readInt(reader);
		}
		return endId;
	}
	
	static boolean binarySearch(RandomAccessFile reader, int searchId) throws IOException{
		
		long start = 0;
		long end = reader.length() - 1;
		
		//读第一行
		reader.seek(start);
		int startId = readInt(reader);
		
		
		//读最后一行
		reader.seek(end);
		int endId = readEndline(reader);
		
		if(searchId < startId || searchId > endId){
			System.out.print("connot find: " + searchId);
			return false;
		}
		
		while(startId <= endId){
			//读居中一行
			reader.seek((start + end) / 2);
			int midId = readEndline(reader);
			long midpos = reader.getFilePointer();
			
			if(searchId == midId){
				System.out.print("find it: " + reader.readLine());
				return true;
			}
			else if(searchId > midId){
				//二分搜索(mid, end]区间
				reader.readLine();
				start = reader.getFilePointer();
				startId = readInt(reader);
			}
			else{
				//二分搜索[start, mid)区间
				end = midpos - 1;
				reader.seek(end);
				endId = readEndline(reader);
			}
		}
		System.out.print("connot find: " + searchId);
		return false;
	}
	
}