FilenameFilter

文件过滤器，获取指定的文件
dd

import java.io.File;
import java.io.FilenameFilter;
import java.util.regex.Pattern;
public class Filenamer {
    public static void main(String...args){
        File path=new File(".");
        String[] files;
        if(args.length==0){
            files=path.list();
        }else{
            files=path.list(new DirFilter(args[0]));
        }
        if(files==null){
            return;
        }
        for(String file:files){
            System.out.println("file:"+file);
        }
    }
}
class DirFilter implements FilenameFilter {
    private Pattern pattern;
    public DirFilter(String regex){
        pattern=Pattern.compile(regex);
    }
    @Override
    public boolean accept(File pathname,String name) {
        return pattern.matcher(name).matches();
    }
}

DirFilter重写了FilenameFilter的accept方法根据传入的正则判断出符合条件的文件名，符合返回true，不符合返回false

accept方法供list方法调用，list方法源码如下：

public String[] list(FilenameFilter filter) {
    String names[] = list();
    if ((names == null) || (filter == null)) {
        return names;
    }
    List<String> v = new ArrayList<>();
    for (int i = 0 ; i < names.length ; i++) {
        if (filter.accept(this, names[i])) {
            v.add(names[i]);
        }
    }
    return v.toArray(new String[v.size()]);
}

输入

读取文件到缓存

BufferedReader in=new BufferedReader(new FileReader(fileName));
while((str=in.readLine())!=null){
  strs.append(str);
}

读取字符串到内存

StringReader in=new StringReader(str);
//in。read返回字符的Ascall码，内容读取完则返回-1
while((c=in.read())!=-1){
  System.out.println((char)c);
}

读取字节到内存

//new ByteArrayInputStream()将字节数组转换成字节流
DataInputStream in=new DataInputStream(new ByteArrayInputStream(str.getBytes());
//in.readByte()返回下一个字节，当已经到达末尾，会抛出EOFException异常
while(in.available()!=0){
  System.out.println((char)in.readByte())
}

输出

写内容到文件

1 2	PrintWriter out=new PrintWriter(new BufferedWriter(new FileWriter(fileName))); out.println(str);

存储和恢复数据

try {
    DataOutputStream out=new DataOutputStream(new BufferedOutputStream(new FileOutputStream("test.txt")));
    out.writeChar('c');
    out.writeBoolean(true);
    out.writeDouble(12.3222222);
    out.writeUTF("str");
    out.close();
    DataInputStream in=new DataInputStream(new BufferedInputStream(new FileInputStream("test.txt")));
    //注意读取有顺序区分
    System.out.println(in.readChar()+" "+in.readBoolean()+" "+in.readDouble()+" "+in.readUTF());
    in.close();
} catch (IOException e) {
    e.printStackTrace();
}

c true 12.3222222 str

随机存储访问数据

String fileName="test.txt";
try {
    RandomAccessFile out=new RandomAccessFile(fileName,"rw");
    out.writeDouble(0.0);
    out.writeDouble(0.1);
    out.writeDouble(0.2);
    out.writeDouble(0.3);
    out.writeDouble(0.4);
    out.close();
    RandomAccessFile in=new RandomAccessFile(fileName,"rw");
    System.out.println(in.readDouble()+" "+in.readDouble()+" "+in.readDouble()+" "+in.readDouble()+" "+in.readDouble()+" ");
    //in.close();
    out=new RandomAccessFile(fileName,"rw");
    //一个double数值为8字节
    out.seek(8);
    out.writeDouble(1.0);
    out.close();
    //in=new RandomAccessFile(fileName,"r");
    //回到开始位置，重新读取数据
    in.seek(0);
    System.out.println(in.readDouble()+" "+in.readDouble()+" "+in.readDouble()+" "+in.readDouble()+" "+in.readDouble()+" ");
    in.close();
}catch (IOException e){
    e.printStackTrace();
}

0.0 0.1 0.2 0.3 0.4

0.0 1.0 0.2 0.3 0.4

读取二进制文件

//传入File对象获得二进制数据
public static byte[] readByte(File file){
  BufferedInputStream in=new BufferedInputStream(new FileInputStream(file));
  try{
    byte[] bytes=new byte[in.available()];
    in.read(bytes);
    return bytes;
  }catch(IOException e){
    e.printStackTrace();
  }
}
//通过文件名新建File对象，然后调用第一个静态方法
public static byte[] readByte(String fileName){
  return read(new File(fileName).getAbsoluteFile());
}

标准I/O

将System.out转换成PrintWriter

1 2	PrintWriter out=new PrintWriter(System.out,true); out.println("str");

1	public PrintWriter(OutputStream out, boolean autoFlush)

System.out为PrintWriter，而PrintWriter又为OutputStream所有可作为第一个参数，第二个参数为是否自动清空功能，需要设为true，否则可能无法看到输出

重定向

输出重定向：

try{
    PrintStream out=new PrintStream(new FileOutputStream("test.txt"));
    System.setOut(out);
    out.println("Hello world");
}catch (IOException e){
    e.printStackTrace();
}

将Hello world重定向输出到test.txt文件中，而标准输出是将其输出到控制台

输入重定向：

try{
    FileInputStream in=new FileInputStream("test.txt");
    System.setIn(in);
    Scanner scanner=new Scanner(System.in);
    while(scanner.hasNext()){
        System.out.println(scanner.nextLine());
    }
}catch (IOException e){
    e.printStackTrace();
}

Hello world

新I/O

try{
    FileChannel channel=new FileOutputStream("test.txt").getChannel();
    channel.write(ByteBuffer.wrap("FileOutputStream".getBytes()));
    channel.close();
    channel=new RandomAccessFile("test.txt","rw").getChannel();
    //移动到末尾处以便追加内容
    channel.position(channel.size());
    channel.write(ByteBuffer.wrap("RandomAccessFile".getBytes()));
    channel.close();
    channel=new FileInputStream("test.txt").getChannel();
    ByteBuffer buffer=ByteBuffer.allocate(1024);
    channel.read(buffer);
    //让channel做好读取字节的准备
    buffer.flip();
    while(buffer.hasRemaining()){
        System.out.println((char)buffer.get());
    }
}catch (IOException e){
    e.printStackTrace();
}

读写通过缓冲器由通道写出或读入，更快速高效

asCharBuffer

CharSet

1 2	String encoding=System.getProperty("file.encoding"); System.out.println(Chaset.forName(encoding).decode(buff));

为字节缓冲器编码

ByteBuffer

1	ByteBuffer.wrap("test".getBytes("UTF-16BE"));

将”test”字符串转换成以”UTF-16BE”编码的字节封装到字节缓冲器中

1	System.out.println(buff.asCharSet());

将字节缓冲区中的字节转换成字符集存入字符缓冲器中，输出该缓冲区会自动调用其toString方法

CharBuffer

补充

java获取当前路径的几种方法

//1
System.out.println(System.getProperty("user.dir"));
//2
File directory = new File("");
try{ 
    System.out.println(directory.getCanonicalPath());
    System.out.println(directory.getAbsolutePath());
}catch(Exceptin e){}

对于append，不能通过”\n”来实现换行，需要：

1	printStream.append(System.getProperty("line.separator"));