java - 为什么 PrintStream 扩展 FilterOutputStream 而不是 OutputStream？

Question

System.out 和System.err 都是PrintStream；并且 PrintStream 扩展了 FilterOutputStream。

来自 FilterOutputStream 的 javadoc:

This class is the superclass of all classes that filter output streams. These streams sit on top of an already existing output stream (the underlying output stream) which it uses as its basic sink of data, but possibly transforming the data along the way or providing additional functionality.

The class FilterOutputStream itself simply overrides all methods of OutputStream with versions that pass all requests to the underlying output stream. Subclasses of FilterOutputStream may further override some of these methods as well as provide additional methods and fields.

(强调我的)

FilterOutputStream 本身扩展了 OutputStream。

我在这里不知所措。为什么 PrintStream 需要扩展 FilterOutputStream 而不是 OutputStream？

感谢示例代码...

Answer 1

FilterOutputStream 应用组合模式，它将所有调用委托(delegate)给它的实例变量 out:

/* The underlying output stream to be filtered. */
protected OutputStream out;

FilterOutputStream 也有抽象类 OutputStream 的默认实现:

public void write(int b) throws IOException {
    out.write(b);
}

public void write(byte b[]) throws IOException {
    write(b, 0, b.length);
}

public void write(byte b[], int off, int len) throws IOException {
    if ((off | len | (b.length - (len + off)) | (off + len)) < 0)
        throw new IndexOutOfBoundsException();

    for (int i = 0 ; i < len ; i++) {
        write(b[off + i]);
    }
}

public void flush() throws IOException {
    out.flush();
}

public void close() throws IOException {
    try {
      flush();
    } catch (IOException ignored) {
    }
    out.close();
}

现在，包括 PrintStream 在内的任何类都可以扩展 FilterOutputStream 并覆盖适当的方法。请注意，他们仍然需要将调用委托(delegate)给 out。例如 PrintStream#flush():

public void flush() {
    synchronized (this) {
        try {
            ensureOpen();
            out.flush();
        }
        catch (IOException x) {
            trouble = true;
        }
    }
}