1、DataOutputStreams 核心底层

DataOutputStreams是数据输出流,实现八种基本类型数据的输出。虽然DataOutputStreams的用法看似复杂,实则简单,它只要一个核心功能,如下所示:

public final void writeByte(int v) throws IOException {
    out.write(v);
}

所有数据类型的输出都转换为字节数组,然后调用上面的核心函数进行数据输出。例如下面的int和long的输出:

public final void writeShort(int v) throws IOException {
    out.write((v >>> 8) & 0xFF);
    out.write((v >>> 0) & 0xFF);
}
public final void writeInt(int v) throws IOException {
    out.write((v >>> 24) & 0xFF);
    out.write((v >>> 16) & 0xFF);
    out.write((v >>>  8) & 0xFF);
    out.write((v >>>  0) & 0xFF);
}
private byte writeBuffer[] = new byte[8];

public final void writeLong(long v) throws IOException {
    writeBuffer[0] = (byte)(v >>> 56);
    writeBuffer[1] = (byte)(v >>> 48);
    writeBuffer[2] = (byte)(v >>> 40);
    writeBuffer[3] = (byte)(v >>> 32);
    writeBuffer[4] = (byte)(v >>> 24);
    writeBuffer[5] = (byte)(v >>> 16);
    writeBuffer[6] = (byte)(v >>>  8);
    writeBuffer[7] = (byte)(v >>>  0);
    out.write(writeBuffer, 0, 8);
}

2、逻辑类型数据的输出

逻辑数值的输出是通过1和0来表示的:

public final void writeBoolean(boolean v) throws IOException {
    out.write(v ? 1 : 0);
}

3、字符类型数据的输出

字符类型数据分为两种情况:单字节编码的字符串和多字节编码的字符串(UTF字符串)。

public final void writeBytes(String s) throws IOException {
    int len = s.length();
    for (int i = 0 ; i < len ; i++) {
        out.write((byte)s.charAt(i));
    }
}
public final void writeUTF(String str) throws IOException {
    ...
}

4、应用举例

public class Test
{
    public static void main(String[] args)
    {
        try
        {
            File file = new File("E:\\out.txt");
            FileOutputStream out = new FileOutputStream(file);
            DataOutputStream dos = new DataOutputStream(out);
            dos.writeInt(100);
            dos.writeBytes("Hello world!");
            dos.writeUTF("世界,你好!");

        }
        catch (FileNotFoundException e)
        {
            System.out.println("文件不存在");
        }
        catch (IOException e)
        {
            System.out.println("写入过程存在异常");
        }
    }
}

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