Table of contents
Read and write file content - data stream
Character reading with Scanner
Write characters using OutputStreamWriter
Use PrintWriter to find our familiar methods
How to write data by character
Manipulating files in Java
An important function of the operating system is the management of files. Each operating system has its own set of system API calls. As a cross-platform language, Java has a layer of encapsulation for different operating systems. We only need to use The API for file operations provided by JDK can complete file operations on different systems.
In Java, a file (including a directory) is abstractly described through the java.io.File class. Note that having a File object does not mean that the file actually exists .
File overview
Attributes
| modifiers and types |
Attributes |
illustrate |
| static String |
pathSeparator |
System-dependent path separator, represented by String type |
| static char |
pathSeparator |
System-dependent path separator, char type representation |
Construction method
method
| modifiers and returns value type |
method signature |
illustrate |
| String |
getParent() |
Returns the file path of the parent directory of the File object |
| String |
getName() |
Returns the plain file name of the FIle object |
| String |
getPath() |
Returns the file path of the File object |
| String |
getAbsolutePath() |
Returns the absolute path of the File object |
| String |
getCanonicalPath() |
Returns the decorated absolute path of the File object |
| boolean |
exists() |
Determine whether the file described by the File object actually exists |
| boolean |
isDirectory() |
Determine whether the file represented by the File object is a directory |
| boolean |
isFile() |
Determine whether the file represented by the File object is an ordinary file |
| boolean |
createNewFile() |
According to the File object, an empty file is automatically created. return after successful creation times true |
| boolean |
delete() |
According to the File object, delete the file. Returns true after successful deletion |
| void |
deleteOnExit() |
According to the File object, the marked file will be deleted, and the delete action will go to Only when the JVM finishes running |
| String[] |
list() |
Returns all file names under the directory represented by the File object |
| File[] |
listFiles() |
Return all files under the directory represented by the File object, and the File object express |
| boolean |
mkdir() |
Create the directory represented by the File object |
| boolean |
mkdirs() |
Create the directory represented by the File object and, if necessary, intermediate record |
| boolean |
renameTo(File dest) |
Renaming files can also be regarded as our usual cut and paste operations. do |
| boolean |
canRead() |
Determine whether the user has read permission on the file |
| boolean |
canWrite() |
Determine whether the user has writable permissions on the file |
code example
Example 1
Observe the features and differences of the get series
import java.io.File;
import java.io.IOException;
public class Main {
public static void main(String[] args) throws IOException {
File file = new File("..\\hello-world.txt"); // 并不要求该文件真实存在
System.out.println(file.getParent());
System.out.println(file.getName());
System.out.println(file.getPath());
System.out.println(file.getAbsolutePath());
System.out.println(file.getCanonicalPath());
}
}operation result
..
hello-world.txt
..\hello-world.txt
D:\代码练习\文件示例1\..\hello-world.txt
D:\代码练习\hello-world.txtExample 2
Create and delete common files
import java.io.File;
import java.io.IOException;
public class Main {
public static void main(String[] args) throws IOException {
File file = new File("hello-world.txt"); // 要求该文件不存在，才能看到相同的现象
System.out.println(file.exists());
System.out.println(file.isDirectory());
System.out.println(file.isFile());
System.out.println(file.createNewFile());
System.out.println(file.exists());
System.out.println(file.isDirectory());
System.out.println(file.isFile());
System.out.println(file.createNewFile());
}
}operation result
false
false
false
true
true
false
true
falseExample 3
Observe the phenomenon of deleteOnExit
import java.io.IOException;
public class Main {
public static void main(String[] args) throws IOException {
File file = new File("some-file.txt"); // 要求该文件不存在，才能看到相同的现象
System.out.println(file.exists());
System.out.println(file.createNewFile());
System.out.println(file.exists());
file.deleteOnExit();
System.out.println(file.exists());
}
}operation result
false
true
true
trueAfter the program finishes running, the file is still deleted
Example 4
Watch directory creation
import java.io.File;
import java.io.IOException;
public class Main {
public static void main(String[] args) throws IOException {
File dir = new File("some-parent\\some-dir"); // some-parent 和 somedir 都不存在
System.out.println(dir.isDirectory());
System.out.println(dir.isFile());
System.out.println(dir.mkdir());
System.out.println(dir.isDirectory());
System.out.println(dir.isFile());
}
}operation result
false
false
false
false
falseWhen mkdir(), if the intermediate directory does not exist, it cannot be created successfully; mkdirs() can solve this problem.
import java.io.File;
import java.io.IOException;
public class Main {
public static void main(String[] args) throws IOException {
File dir = new File("some-parent\\some-dir"); // some-parent 和 somedir 都不存在
System.out.println(dir.isDirectory());
System.out.println(dir.isFile());
System.out.println(dir.mkdirs());
System.out.println(dir.isDirectory());
System.out.println(dir.isFile());
}
}operation result
false
false
true
true
falseExample 5
Watch for file renaming
import java.io.File;
import java.io.IOException;
public class Main {
public static void main(String[] args) throws IOException {
File file = new File("some-file.txt"); // 要求 some-file.txt 得存在，可以是普通文件，可以是目录
File dest = new File("dest.txt"); // 要求 dest.txt 不存在
System.out.println(file.exists());
System.out.println(dest.exists());
System.out.println(file.renameTo(dest));
System.out.println(file.exists());
System.out.println(dest.exists());
}
}operation result
true
false
true
false
trueReading and writing of file content - data flow
Data Flow Classification
InputStream overview
method
|
modifiers and
return value class
type
|
method signature
|
illustrate
|
|
int
|
read()
|
Read a byte of data, return
-1
means it has been completely read
|
|
int
|
read(byte[] b)
|
最多读取
b.length
字节的数据到
b
中，返回实际读到的数
量；
-1
代表以及读完了
|
|
int
|
read(byte[] b,
int off, int len)
|
最多读取
len - off
字节的数据到
b
中，放在从
off
开始，返
回实际读到的数量；
-1
代表以及读完了
|
| void |
close()
|
关闭字节流
|
说明
InputStream 只是一个抽象类，要使用还需要具体的实现类。关于 InputStream 的实现类有很多，基本可以认为不同的输入设备都可以对应一个 InputStream 类，我们现在只关心从文件中读取，所以使用 FileInputStream
FileInputStream 概述
|
签名
|
说明
|
|
FileInputStream(File file)
|
利用
File
构造文件输入流
|
|
FileInputStream(String name)
|
利用文件路径构造文件输入流
|
示例
这里我们把文件内容中填充中文看看，注意，写中文的时候使用 UTF-8 编码。hello.txt 中填写 "你好中国"
注意：这里我利用了这几个中文的 UTF-8 编码后长度刚好是 3 个字节和长度不超过 1024 字节的现状，但这种方式并不是通用的
import java.io.*;
// 需要先在项目目录下准备好一个 hello.txt 的文件，里面填充 "你好中国" 的内容
public class Main {
public static void main(String[] args) throws IOException {
try (InputStream is = new FileInputStream("hello.txt")) {
byte[] buf = new byte[1024];
int len;
while (true) {
len = is.read(buf);
if (len == -1) {
// 代表文件已经全部读完
break;
}
// 每次使用 3 字节进行 utf-8 解码，得到中文字符
// 利用 String 中的构造方法完成
// 这个方法了解下即可，不是通用的解决办法
for (int i = 0; i < len; i += 3) {
String s = new String(buf, i, 3, "UTF-8");
System.out.printf("%s", s);
}
}
}
}
}利用 Scanner 进行字符读取
上述例子中，我们看到了对字符类型直接使用 InputStream 进行读取是非常麻烦且困难的，所以，我们使用一种我们之前比较熟悉的类来完成该工作，就是 Scanner 类。
|
构造方法
|
说明
|
|
Scanner(InputStream is, String charset)
|
使用
charset
字符集进行
is
的扫描读取
|
import java.io.*;
import java.util.*;
// 需要先在项目目录下准备好一个 hello.txt 的文件，里面填充 "你好中国" 的内容
public class Main {
public static void main(String[] args) throws IOException {
try (InputStream is = new FileInputStream("hello.txt")) {
try (Scanner scanner = new Scanner(is, "UTF-8")) {
while (scanner.hasNext()) {
String s = scanner.next();
System.out.print(s);
}
}
}
}
}OutputStream 概述
|
修饰
符及
返回
值类
型
|
方法签名
|
说明
|
|
void
|
write(int b)
|
写入要给字节的数据
|
|
void
|
write(byte[]
b)
|
将
b
这个字符数组中的数据全部写入
os
中
|
| int |
write(byte[]
b, int off,
int len)
|
将
b
这个字符数组中从
off
开始的数据写入
os
中，一共写
len
个
|
|
void
|
close()
|
关闭字节流
|
|
void
|
flush()
|
重要：我们知道
I/O
的速度是很慢的，所以，大多的
OutputStream
为
了减少设备操作的次数，在写数据的时候都会将数据先暂时写入内存的
一个指定区域里，直到该区域满了或者其他指定条件时才真正将数据写
入设备中，这个区域一般称为缓冲区。但造成一个结果，就是我们写的
数据，很可能会遗留一部分在缓冲区中。需要在最后或者合适的位置，
调用
flush
（刷新）操作，将数据刷到设备中。
|
说明
OutputStream 同样只是一个抽象类，要使用还需要具体的实现类。我们现在还是只关心写入文件中，所以使用 FileOutputStream
利用 OutputStreamWriter 进行字符写入
import java.io.*;
public class Main {
public static void main(String[] args) throws IOException {
try (OutputStream os = new FileOutputStream("output.txt")) {
String s = "你好中国";
byte[] b = s.getBytes("utf-8");
os.write(b);
// 不要忘记 flush
os.flush();
}
}
}利用 PrintWriter 找到我们熟悉的方法
上述，我们其实已经完成输出工作，但总是有所不方便，我们接来下将 OutputStream 处理下，使用PrintWriter 类来完成输出，因为
import java.io.*;
public class Main {
public static void main(String[] args) throws IOException {
try (OutputStream os = new FileOutputStream("output.txt")) {
try (OutputStreamWriter osWriter = new OutputStreamWriter(os, "UTF-8")) {
try (PrintWriter writer = new PrintWriter(osWriter)) {
writer.println("我是第一行");
writer.print("我的第二行\r\n");
writer.printf("%d: 我的第三行\r\n", 1 + 1);
writer.flush();
}
}
}
}
}代码参考
如何按字节进行数据读
try (InputStream is = ...) {
byte[] buf = new byte[1024];
while (true) {
int n = is.read(buf);
if (n == -1) {
break;
}
// buf 的 [0, n) 表示读到的数据，按业务进行处理
}
}如何按字节进行数据写
try (OutputStream os = ...) {
byte[] buf = new byte[1024];
while (/* 还有未完成的业务数据 */) {
// 将业务数据填入 buf 中，长度为 n
int n = ...;
os.write(buf, 0, n);
}
os.flush(); // 进行数据刷新操作
}如何按字符进行数据读
try (InputStream is = ...) {
try (Scanner scanner = new Scanner(is, "UTF-8")) {
while (scanner.hasNextLine()) {
String line = scanner.nextLine();
// 根据 line 做业务处理
}
}
}如何按字符进行数据写
try (OutputStream os = ...) {
try (OutputStreamWriter osWriter = new OutputStreamWriter(os, "UTF-8")) {
try (PrintWriter writer = new PrintWriter(osWriter)) {
while (/* 还有未完成的业务数据 */) {
writer.println(...);
}
writer.flush(); // 进行数据刷新操作
}
}
}