java 流与文件

在java API当中，可以从其中读入一个字节序列的对象称作输入流，而可以向其中写入一个字节序列的对象称作输出流。形象的描述就是你从外部文件中的内容读取到java程序中的是输入流，从java程序到外部文件中的是输出流。

在输入输出流使用的时候，要在使用完毕的时候进行关闭，因为输入输出流占用系统的一定的资源。java的输入输出流有两种职责，一种是从文件和其他更外部的位置获取字节，另一种是将字节组装到更有用的数据类型中。例如：

FileInputStream fin=new FileInputStream("a.txt");

DataInputStream din=new DataInputStream(fin);

double s=din.readDouble();

为了更方便的控制输入输出流，可以在输入输出流中加入过滤功能。加入的位置如下所示：

FileInputStream fin=new FileInputStream("a.txt");

BufferedInputStream bis=new BufferedInputStream(fin);

DataInputStream din=new DataInputStream(bis);

double s=din.readDouble();

假如要读取ZIP文件，可以使用如下方法：

FileInputStream fin=new FileInputStream("a.txt");

ZipInputStream zis=new ZipInputStream(fin);

DataInputStream din=new DataInputStream(zis);

从上面的例子中可以分析出输入流的过滤序列。

下面是一段输入输出流的代码赏析：

第一段，数据单元的实体Bean，含有一些输入输出操作方法：

package stream;

import java.io.DataInput;

import java.io.DataOutput;

import java.io.IOException;

import java.io.PrintWriter;

import java.util.Calendar;

import java.util.Date;

import java.util.GregorianCalendar;

import java.util.Scanner;

public class Employee {

public static final int NAME_SIZE=40;

public static final int RECORD_SIZE=2*NAME_SIZE+8+4+4+4;

    private String name;

    private Double salary;

    private Date hireDay;

    

public Employee(){

}

public Employee(String n,double s,int year,int month,int day){

name=n;

salary=s;

GregorianCalendar calendar=new GregorianCalendar(year, month-1, day);

hireDay=calendar.getTime();

}

public String getName() {

return name;

}

public Double getSalary() {

return salary;

}

public Date getHireDay() {

return hireDay;

}

public void raiseSalary(double percent){

salary+=salary*percent/100;

}

public String toString(){

return getClass().getName()+"[name="+name+",salary="+salary+",hireDay="

+hireDay+"]";

}

public void writeData(PrintWriter out){

  GregorianCalendar calendar=new GregorianCalendar();

  calendar.setTime(hireDay);

  out.println(name+"|"+salary+"|"+calendar.get(Calendar.YEAR)+"|"

+(calendar.get(Calendar.MONTH)+1)+"|"+calendar.get(Calendar.DAY_OF_MONTH));

}

public void readData(Scanner in){

String line=in.nextLine();

String[] tokens=line.split("\\|");

name=tokens[0];

salary=Double.parseDouble(tokens[1]);

int y=Integer.parseInt(tokens[2]);

int m=Integer.parseInt(tokens[3]);

int d=Integer.parseInt(tokens[4]);

GregorianCalendar calendar=new GregorianCalendar(y, m-1,d);

hireDay=calendar.getTime();

}

public void writeData(DataOutput out) throws IOException{

DataIO.writeFixedString(name, NAME_SIZE, out);

out.writeDouble(salary);

GregorianCalendar calendar=new GregorianCalendar();

calendar.setTime(hireDay);

out.writeInt(calendar.get(Calendar.YEAR));

out.writeInt(calendar.get(Calendar.MONTH)+1);

out.writeInt(calendar.get(Calendar.DAY_OF_MONTH));

}

public void readData(DataInput in) throws IOException{

name=DataIO.readFixedString(NAME_SIZE, in);

salary=in.readDouble();

int y=in.readInt();

int m=in.readInt();

int d=in.readInt();

GregorianCalendar calendar=new GregorianCalendar(y, m-1, d);

hireDay=calendar.getTime();

}

}

DataIO这个类里面包含了读写固定长度的数据单元

package stream;

import java.io.DataInput;

import java.io.DataOutput;

import java.io.IOException;

public class DataIO {

    public static String readFixedString(int size,DataInput in)throws IOException{

     StringBuilder b=new StringBuilder(size);

     int i=0;

     boolean more=true;

     while(more&i<size){

     char ch=in.readChar();

     i++;

     if(ch==0)more=false;

     else b.append(ch);

     }

     in.skipBytes(2*(size-i));

     return b.toString();

    }

    

    public static void writeFixedString(String s,int size,DataOutput out) throws IOException{

     for(int i=0;i<size;i++){

     char ch=0;

     if(i<s.length())ch=s.charAt(i);

     out.writeChar(ch);

     }

    }

}

下面是两种输入数出流的运用：

1.文本输入输出流：

package stream;

import java.io.FileNotFoundException;

import java.io.FileReader;

import java.io.IOException;

import java.io.PrintWriter;

import java.util.Scanner;

public class TextFileTest {

   

private static Employee[] readData(Scanner in){

int n=in.nextInt();

in.nextLine();

Employee[] employees=new Employee[n];

for(int i=0;i<n;i++){

employees[i]=new Employee();

employees[i].readData(in);

}

return employees;

}

private static void writeData(Employee[] employees,PrintWriter out)throws IOException{

out.println(employees.length);

for(Employee e:employees){

e.writeData(out);

}

}

public static void main(String[]args){

Employee[] staff=new Employee[3];

staff[0]=new Employee("Carl Cracker",7500,1987,12,15);

staff[1]=new Employee("Harry Hacker",5000,1989,10,1);

staff[2]=new Employee("Tony Tester",4000,1990,3,15);

try {

PrintWriter out=new PrintWriter("employee.dat");

writeData(staff,out);

out.close();

Scanner in=new Scanner(new FileReader("employee.dat"));

Employee[] newstaff=readData(in);

in.close();

for(Employee e:newstaff){

System.out.println(e);

}

} catch (FileNotFoundException e) {

// TODO Auto-generated catch block

e.printStackTrace();

} catch (IOException e) {

// TODO Auto-generated catch block

e.printStackTrace();

}

}

}

2.RandomFile方式的

package stream;

import java.io.DataOutputStream;

import java.io.FileNotFoundException;

import java.io.FileOutputStream;

import java.io.IOException;

import java.io.RandomAccessFile;

public class RandomFileTest {

public static void main(String[]args){

Employee[] staff=new Employee[3];

staff[0]=new Employee("Carl Cracker",7500,1987,12,15);

staff[1]=new Employee("Harry Hacker",5000,1989,10,1);

staff[2]=new Employee("Tony Tester",4000,1990,3,15);

try {

DataOutputStream out=new DataOutputStream(new FileOutputStream("employee.dat"));

for(Employee e:staff)e.writeData(out);

out.close();

RandomAccessFile in=new RandomAccessFile("employee.dat", "r");

int n=(int) (in.length()/Employee.RECORD_SIZE);

Employee[] newstaff=new Employee[n];

for(int i=n-1;i>=0;i--){

newstaff[i]=new Employee();

in.seek(i*Employee.RECORD_SIZE);

newstaff[i].readData(in);

}

in.close();

for(Employee e:newstaff){

System.out.println(e);

}

} catch (FileNotFoundException e) {

// TODO Auto-generated catch block

e.printStackTrace();

} catch (IOException e) {

// TODO Auto-generated catch block

e.printStackTrace();

}

}

}

分别运行这两种输入数流，可以很轻松的发现它们产生文件内容的区别