java.io DataInputStream与DataOutputStream理解（三）

DataInputStream与DataOutputStream类都是流的装饰者类（java中流的设计采用了装饰者模式），它是用来装饰其它基本的输入流的，比如FileInputStream、ByteArrayInputStream等。首先理解DataInputStream。
DataInputStream类，java的注释是“它允许应用程序以与机器无关方式从底层输入流中读取基本的java数据类型”。也就是说，通过DataInputStream类可以从被装饰的输入流类（FileInputStream、ByteArrayInputStream）中读取java基本数据类型。DataInputStream中的read、readByte、readDouble、readInt等方法比较简单，容易理解。重点理解readUTF方法，源码如下：

//将utf编码的输入流中字节转换为字符串，要想看懂这个方法的代码，需要了解utf-8编码//原理，如果不想深入理解，只要知道这个方法就是将输入流的字节数组按照utf-8编码方//式转换成字符串
public final static String readUTF(DataInput in) throws IOException {
        int utflen = in.readUnsignedShort();
        byte[] bytearr = null;
        char[] chararr = null;
        if (in instanceof DataInputStream) {
            DataInputStream dis = (DataInputStream)in;
            if (dis.bytearr.length < utflen){
                dis.bytearr = new byte[utflen*2];
                dis.chararr = new char[utflen*2];
            }
            chararr = dis.chararr;
            bytearr = dis.bytearr;
        } else {
            bytearr = new byte[utflen];
            chararr = new char[utflen];
        }

        int c, char2, char3;
        int count = 0;
        int chararr_count=0;

        in.readFully(bytearr, 0, utflen);

        while (count < utflen) {
            c = (int) bytearr[count] & 0xff;
            if (c > 127) break;
            count++;
            chararr[chararr_count++]=(char)c;
        }

        while (count < utflen) {
            c = (int) bytearr[count] & 0xff;
            switch (c >> 4) {
                case 0: case 1: case 2: case 3: case 4: case 5: case 6: case 7:
                    /* 0xxxxxxx*/
                    count++;
                    chararr[chararr_count++]=(char)c;
                    break;
                case 12: case 13:
                    /* 110x xxxx   10xx xxxx*/
                    count += 2;
                    if (count > utflen)
                        throw new UTFDataFormatException(
                            "malformed input: partial character at end");
                    char2 = (int) bytearr[count-1];
                    if ((char2 & 0xC0) != 0x80)
                        throw new UTFDataFormatException(
                            "malformed input around byte " + count);
                    chararr[chararr_count++]=(char)(((c & 0x1F) << 6) |
                                                    (char2 & 0x3F));//
                    break;
                case 14:
                    /* 1110 xxxx  10xx xxxx  10xx xxxx */
                    count += 3;
                    if (count > utflen)
                        throw new UTFDataFormatException(
                            "malformed input: partial character at end");
                    char2 = (int) bytearr[count-2];
                    char3 = (int) bytearr[count-1];
                    if (((char2 & 0xC0) != 0x80) || ((char3 & 0xC0) != 0x80))
                        throw new UTFDataFormatException(
                            "malformed input around byte " + (count-1));
                    chararr[chararr_count++]=(char)(((c     & 0x0F) << 12) |
                                                    ((char2 & 0x3F) << 6)  |
                                                    ((char3 & 0x3F) << 0));
                    break;
                default:
                    /* 10xx xxxx,  1111 xxxx */
                    throw new UTFDataFormatException(
                        "malformed input around byte " + count);
            }
        }
        // The number of chars produced may be less than utflen
        return new String(chararr, 0, chararr_count);
    }

DataOutputStream是应用程序将java基本类型数据写入到输出流中。例如将java基本类型写入到文件、字节数组等容器中。DataOutputStream继承FilterOutputStream，是装饰者类，用来装饰基本的输出流类，如FileOutputStream、ByteArrayInputStream。