Java readLine()、read()、read(char cbuf[])、read(char cbuf[], int off, int len)详解

BufferReader继承Reader，主要有readLine()、read()、read(char cbuf[])、read(char cbuf[], int off, int len)方法，下面我来讲解这几个方法的用法和注意事项。

先在D盘下建立一个test.txt文件(Windows系统)，文件内容：

Hello
Android
2020-01-15

readLine()

该方法就是用来读取一行字符流数据，用法如下：

package s1;

import java.io.BufferedReader;
import java.io.File;
import java.io.FileNotFoundException;
import java.io.FileReader;
import java.io.IOException;

public class X {
	public static void main(String[] args) {
		File file = new File("D:\\test.txt");
		FileReader fileReader;
		BufferedReader bufferedReader = null;
		try {
			fileReader = new FileReader(file);
			bufferedReader = new BufferedReader(fileReader);
			StringBuilder stringBuilder = new StringBuilder();
			String s;
			while ((s = bufferedReader.readLine()) != null) {
				stringBuilder.append(s);
				stringBuilder.append("\n");
			}
			System.out.println(stringBuilder.toString());
		} catch (FileNotFoundException e) {
			e.printStackTrace();
		} catch (IOException e) {
			e.printStackTrace();
		} finally {
			if(bufferedReader != null) {
				try {
					bufferedReader.close();
				} catch (IOException e) {
					e.printStackTrace();
				}
			}
		}
	}
}

运行结果如下：

Hello
Android
2020-01-15

read()

该方法每次只读取一个字符，用法和readLine()差不多，就不多写了。

read(char cbuf[])

该方法是每次读取cbuf.length个字符到cbuf数组里面，源码里面该方法其实调用的是read(char cbuf[], int off, int len)。即read(cbuf, 0, cbuf.length)

read(char cbuf[], int off, int len)

该方法正确的含义是：读取len个字符，然后从cbuf[off]开始存放，因此必须注意len+off不能大于cbuf.length，否则会报数组越界异常。常见的错误理解是：BufferReader跳过off个字符，然后读取len个字符到cubf数组里。

package s1;

import java.io.BufferedReader;
import java.io.File;
import java.io.FileNotFoundException;
import java.io.FileReader;
import java.io.IOException;
import java.util.Arrays;

public class X {
	public static void main(String[] args) {
		File file = new File("D:\\test.txt");
		FileReader fileReader;
		BufferedReader bufferedReader = null;
		try {
			fileReader = new FileReader(file);
			bufferedReader = new BufferedReader(fileReader);
			char[] cbuf = new char[4];
			bufferedReader.read(cbuf, 2, cbuf.length - 2);
			System.out.println(Arrays.toString(cbuf));
		} catch (FileNotFoundException e) {
			e.printStackTrace();
		} catch (IOException e) {
			e.printStackTrace();
		} finally {
			if(bufferedReader != null) {
				try {
					bufferedReader.close();
				} catch (IOException e) {
					e.printStackTrace();
				}
			}
		}
	}
}

运行结果：

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