RandomAccessFile类
    只能访问文件，不能操作其他io设备
    支持随机访问
    在读写等长记录文件有优势

实例：

import java.io.*;

class Employee {
	private String name;
	private int age;
	public static final int LEN = 8;
	
	String getName() {
		return name;
	}
	
	int getAge() {
		return age;
	}
	
	Employee(String name, int age) {
		if (name.length() > LEN) { // 为了构造等长记录
			this.name = name.substring(0, LEN-1);
		} else {
			this.name = name;
			while (this.name.length() < LEN) {
				this.name += '\u0000';
			}
		}
		this.age = age;
	}
}

public class RandomAccessFileTest {
	public static void main(String [] args) {
		Employee e1 = new Employee("Ronnie", 37);
		Employee e2 = new Employee("John", 37);
		Employee e3 = new Employee("Mark", 37);
		
		try {
			RandomAccessFile randFile = new RandomAccessFile("employee.txt", "rw");
			
			//randFile.write(e1.getName().getBytes()); // 如果name有中文，会出现问题，因为一个英文字符转换为一个字节，一个中文字符转换为两个字节，可以用writeChars函数改写
			randFile.writeChars(e1.getName()); //-
			randFile.writeInt(e1.getAge());
			//randFile.write(e2.getName().getBytes());
			randFile.writeChars(e1.getName()); //-
			randFile.writeInt(e2.getAge());
			//randFile.write(e3.getName().getBytes());
			randFile.writeChars(e1.getName()); //-
			randFile.writeInt(e3.getAge());
			
			randFile.close();
		} catch (Exception e) {
			e.printStackTrace();
		}
		
		try {
			//byte[] nameBuf = new byte[Employee.LEN];
			
			RandomAccessFile randFile = new RandomAccessFile("employee.txt", "r");
			
			//randFile.skipBytes(12);
			randFile.skipBytes(20); //-
			//int len = randFile.read(nameBuf);
			//String name = new String(nameBuf, 0, len);
			String name = "";//-
			for (int i = 0; i < Employee.LEN; ++i) { //-
				name += randFile.readChar(); //-
			}//-
			System.out.println(name.trim() + ":" + randFile.readInt());
			name = ""; //-
			
			randFile.seek(0); // 绝对定位
			//len = randFile.read(nameBuf);
			//name = new String(nameBuf, 0, len);
			for (int i = 0; i < Employee.LEN; ++i) { //-
				name += randFile.readChar(); //-
			} //-
			System.out.println(name.trim() + ":" + randFile.readInt());
			name = ""; //-
			
			//randFile.skipBytes(12);
			randFile.skipBytes(20); //-
			//len = randFile.read(nameBuf);
			//name = new String(nameBuf, 0, len);
			for (int i = 0; i < Employee.LEN; ++i) { //-
				name += randFile.readChar(); //-
			} //-
			System.out.println(name.trim() + ":" + randFile.readInt());
			
			randFile.close();
		} catch (Exception e) {
			e.printStackTrace();
		}
	}
}

注释部分为按字节写入时的程序，带//-为原来的代码

其他函数请参照jdk文档