带你读《2022技术人的百宝黑皮书》——浅析设计模式3 —— 装饰者模式（3）

public class FilterInputStream extends InputStream {
 //维护一个 InputStream 对象
 protected volatile InputStream in; 
  //构造方法参数需要一个 inputStream
 protected FilterInputStream(InputStream in) {
 this.in = in;
 }
  //委托给 InputStream
 public int read() throws IOException {
 return in.read();
 }
  //委托给 InputStream
 public void close() throws IOException {
 in.close();
 }
 .......
 }

public class PushbackInputStream extends FilterInputStream {
 //缓冲区
 protected byte[] buf; 
 protected int pos;
 private void ensureOpen() throws IOException {
 if (in == null)
 throw new IOException("Stream closed");
 }
 //构造函数可以指定返回的字节个数
 public PushbackInputStream(InputStream in, int size) { 
  super(in);
 if (size <= 0) {
 throw new IllegalArgumentException("size <= 0");
 }
 //初始化缓冲区的大小
 this.buf = new byte[size];
 //设置读取的位置
 this.pos = size;
 }
 //默认回退一个
 public PushbackInputStream(InputStream in) {
 this(in, 1);
 }
  public int read() throws IOException {
 //确保流存在
 ensureOpen();
 //如果要读取的位置在缓冲区里面
 if (pos < buf.length) {
 //返回缓冲区中的内容
 return buf[pos++] & 0xff;
 }
 //否则调用超类的读函数
 return super.read();
 }
  //读取指定的长度
 public int read(byte[] b, int off, int len) throws IOException {
 ensureOpen();
 if (b == null) {
 throw new NullPointerException();
 } else if (off < 0 || len < 0 || len > b.length - off) {
 throw new IndexOutOfBoundsException();
 } else if (len == 0) {
 return 0;
 }
 //缓冲区长度减去读取位置
 int avail = buf.length - pos;
 //如果大于0，表明部分数据可以从缓冲区读取
 if (avail > 0) {
 //如果要读取的长度小于可从缓冲区读取的字符
if (len < avail) {
 //修改可读取值为实际要读的长度
 avail = len;
 }
 //将buf中的数据复制到b中
 System.arraycopy(buf, pos, b, off, avail);
 //修改pos的值
 pos += avail;
 //修改off偏移量的值
 off += avail;
 //修改len的值
 len -= avail;
 }
 //如果从缓冲区读取的数据不够
 if (len > 0) {
 //从流中读取
 len = super.read(b, off, len);
 if (len == -1) {
 return avail == 0 ? -1 : avail;
 }
 return avail + len;
 }
 return avail;
 }
 //不读字符b
 public void unread(int b) throws IOException {
 ensureOpen();
 if (pos == 0) {
 throw new IOException("Push back buffer is full");
 }
 //实际就是修改缓冲区中的值，同时pos后退
 buf[--pos] = (byte)b;
 }
  public void unread(byte[] b, int off, int len) throws IOException {
 ensureOpen();
 if (len > pos) {
 throw new IOException("Push back buffer is full");
 }
 //修改缓冲区中的值，pos后退多个
  pos -= len;
 System.arraycopy(b, off, buf, pos, len);
 }
 public void unread(byte[] b) throws IOException {
 unread(b, 0, b.length);
 }
}