FileReader使用
read()
项目根目录下有一个hello.txt
FileReader的read方法：
/**
* Reads a single character.
*
读取字符，当流结束的时候，返回-1
* @return The character read, or -1 if the end of the stream has been
* reached
*
* @exception IOException If an I/O error occurs
*/
public int read() throws IOException {
return sd.read();
}
由此我们可以写出：
@Test
public void test1() throws IOException {
//实例化File类的对象，指明要操作的文件
File file = new File("hello.txt");
//提供具体的流
FileReader fileReader = new FileReader(file);
int data;
while((data = fileReader.read()) != -1){
System.out.print((char)data);
}
//关闭流
fileReader.close();
}
优化上述代码：
@Test
public void test1(){
//实例化File类的对象，指明要操作的文件
File file = new File("hello.txt");
//提供具体的流
//读入的文件一定要存在，否则报异常
try(FileReader fileReader = new FileReader(file)){
int data;
while((data = fileReader.read()) != -1){
System.out.print((char)data);
}
}catch (IOException e){
e.printStackTrace();
}
}
read(char cbuf[])
将读取的字符放入到传入的数组中。返回值为int，返回每次读入到cbuf数组中的字符个数，到达文件末尾，返回-1。
当文件很大时，效率要比无参的read方法要高。
注意到read(char cbuf[])内部还调用了read(char cbuf[], int off, int len)方法，该方法实际工作用到可能性很小，这里不介绍。
/**
* Reads characters into an array. This method will block until some input
* is available, an I/O error occurs, or the end of the stream is reached.
*
* @param cbuf Destination buffer
*
* @return The number of characters read, or -1
* if the end of the stream
* has been reached
*
* @exception IOException If an I/O error occurs
*/
public int read(char cbuf[]) throws IOException {
return read(cbuf, 0, cbuf.length);
}
测试：
//实例化File类的对象，指明要操作的文件
File file = new File("hello.txt");
//提供具体的流,读入操作
try(FileReader fileReader = new FileReader(file)){
char[] cBuf = new char[5];
int len;
while ((len = fileReader.read(cBuf))!=-1){
for(int i =0 ;i<len;i++){
System.out.print(cBuf[i]);
}
}
}catch (IOException e){
e.printStackTrace();
}
改进写法：
//实例化File类的对象，指明要操作的文件
File file = new File("hello.txt");
//提供具体的流,读入操作
try(FileReader fileReader = new FileReader(file)){
char[] cBuf = new char[5];
int len;
while ((len = fileReader.read(cBuf))!=-1){
String str = new String(cBuf, 0, len);
System.out.print(str);
}
}catch (IOException e){
e.printStackTrace();
}
FileWriter使用
File file = new File("hello1.txt");
//提供FileWriter的对象，用于写出操作
try(FileWriter fw = new FileWriter(file)){
//写出操作
fw.write("hello FileWriter!
");
fw.write("hello world!");
}catch (IOException e){
e.printStackTrace();
}
如果使用FileWriter(File file)构造器，如果文件不存在的话，会自动创建文件；如果文件存在的话，会覆盖原来文件，写出新的内容。
此外还有一个构造器传入boolean参数的：如果传入true，则追加到文件末尾不覆盖
/**
* Constructs a {@code FileWriter} given the {@code File} to write and
* a boolean indicating whether to append the data written, using the platform's
* {@linkplain java.nio.charset.Charset#defaultCharset() default charset}.
*
* @param file the {@code File} to write
* @param append if {@code true}, then bytes will be written
* to the end of the file rather than the beginning
* @throws IOException if the file exists but is a directory rather than
* a regular file, does not exist but cannot be created,
* or cannot be opened for any other reason
* @since 1.4
*/
public FileWriter(File file, boolean append) throws IOException {
super(new FileOutputStream(file, append));
}
测试：
File file = new File("hello1.txt");
//提供FileWriter的对象，用于写出操作
try(FileWriter fw = new FileWriter(file, true)){
//写出操作
fw.write("hello FileWriter!
");
fw.write("hello world!");
}catch (IOException e){
e.printStackTrace();
}
实现文件复制
使用FileReader和FileWriter可以实现，代码也很简单
File src = new File("hello1.txt");
File dest = new File("hello2.txt");
try(FileReader fr = new FileReader(src);
FileWriter fw = new FileWriter(dest)){
char[] cBuf = new char[5];
int len;
while ((len = fr.read(cBuf))!=-1){
fw.write(cBuf, 0 ,len);
}
}catch (IOException e){
e.printStackTrace();
}
字符流不能处理图片
测试代码：
@Test
public void test5(){
File src = new File("a.jpg");
File dest = new File("b.jpg");
try(FileReader fr = new FileReader(src);
FileWriter fw = new FileWriter(dest)){
char[] cBuf = new char[5];
int len;
while ((len = fr.read(cBuf))!=-1){
fw.write(cBuf, 0 ,len);
}
}catch (IOException e){
e.printStackTrace();
}
}
发现生成的b.jpg打不开：
所以字符流不能处理图片等字节数据。