Java-IO之PushbackInputStream

来源：互联网发布：电火花电脉冲编程编辑：程序博客网时间：2024/05/21 17:48

1、当程序调用推回输入流的unread()方法时，系统会把指定数组的内容推回到该缓冲区中，而推回输入流每次调用read()方法时，总是先从推回缓冲区读取内容，只有完全读取了推回缓冲区里的内容后，但是还没有装满read()所需要的数组时才会从原输入流中读取

example

下面的程序demo5()试图找出程序中的”new PushbackReader”字符串，找到后然后打印出目标字符串之前的内容

import java.io.ByteArrayInputStream;import java.io.FileReader;import java.io.IOException;import java.io.InputStreamReader;import java.io.PushbackInputStream;import java.io.PushbackReader;public class PushbackIO {    public static void main(String[] args) throws Exception {        // demo1();        // demo2();        // demo3();        // demo4();        demo5();    }    private static void demo5() {        try (        // 创建一个PushbackReader对象，指定推回缓冲区的长度为64        PushbackReader pr = new PushbackReader(new FileReader("D:\\a.txt"), 64);        ) {            char[] buf = new char[32];            // 用以保存上次读取字符串的内容            String lastContent = "";            int hasRead = 0;            // 循环读取文件内容            while ((hasRead = pr.read(buf)) > 0) {                // 将读取的内容转化为字符串                String content = new String(buf, 0, hasRead);                int targetIndex = 0;                // 将上次读取的字符串和本次读取的字符串拼接起来                // 查看是否包含目标字符串，                // 如果包含目标字符串                if ((targetIndex = (lastContent + content)                        .indexOf("new PushbackReader")) > 0) {                    // 将本次的内容和上次的内容一起推回缓冲区//                  pr.unread((lastContent + content).toCharArray());                    // 重现定义一个长度为targetIndex的char类型的数组                    if (targetIndex > 32) {                        buf = new char[targetIndex];                    }                    // 再次读取指定长度的内容，即目标字符串之前的内容                    pr.read(buf, 0, targetIndex);                    // 答应读取指定长度的内容                    System.out.println(new String(buf, 0, targetIndex));//                  System.exit(0);                    System.out.println("------------------------------------");                    char[] c = new char[32];                    int len = 0;                    while((len = pr.read(c))>0){                        System.out.println(String.valueOf(c, 0, len));                    }                } else {                    // 打印上次读取的内容                    System.out.println(lastContent);                    // 将本次读取的内容设置为上次读取的内容                    lastContent = content;                }            }        } catch (IOException ioe) {            ioe.printStackTrace();        }    }    private static void demo4() throws Exception {        PushbackInputStream in = new PushbackInputStream(System.in);        in.unread('a');        int ch = in.read();        System.out.println((char) ch);        in.close();        PushbackReader reader = new PushbackReader(new InputStreamReader(                System.in));        reader.unread('b');        ch = reader.read();        System.out.println((char) ch);        reader.close();    }    private static void demo3() throws Exception {        String s = "abcdefg";        /*         * PushbackInputStream pbin = new PushbackInputStream(in,4)         * 这个构造函数创建的对象一次可以回推一个缓存         */        try (ByteArrayInputStream in = new ByteArrayInputStream(s.getBytes());                PushbackInputStream pbin = new PushbackInputStream(in, 4)) {            int n;            byte[] buffer = new byte[4];            while ((n = pbin.read(buffer)) != -1) {                System.out.println(new String(buffer));                // 取回推缓存中的一部分数据                if (new String(buffer).equals("abcd"))                    pbin.unread(buffer, 2, 2);                buffer = new byte[4];            }        }    }    private static void demo2() throws Exception {        String s = "abcdefg";        /*         * PushbackInputStream pbin = new PushbackInputStream(in,3)         * 这个构造函数创建的对象一次可以回推一个缓存         */        try (ByteArrayInputStream in = new ByteArrayInputStream(s.getBytes());                PushbackInputStream pbin = new PushbackInputStream(in, 3)) {            int n;            byte[] buffer = new byte[3];            while ((n = pbin.read(buffer)) != -1) {                System.out.println(new String(buffer));                if (new String(buffer).equals("abc"))                    pbin.unread(new byte[] { 'M', 'N', 'O' });                buffer = new byte[3];            }        }    }    private static void demo1() throws Exception {        String s = "abcdefg";        /*         * PushbackInputStream pbin = new PushbackInputStream(in)         * 这个构造函数创建的对象一次只能回推一个字节         */        try (ByteArrayInputStream in = new ByteArrayInputStream(s.getBytes());                PushbackInputStream pbin = new PushbackInputStream(in)) {            int n;            int i = 0;            while ((n = pbin.read()) != -1) {                System.out.println(++i + " : " + (char) n);                if ('b' == n)                    pbin.unread('U');            }        }    }}

0 0