System.out

System.out的类型为PrintStream；
System.out.println(‘a’);
实际上调用是PrintStream的println(char c)方法；
而println(char c)方法的源代码为：

public void println(String x) {
    synchronized (this) {
        print(x);
        newLine();
    }
}

可见Println调用了print(char c)方法，print(char c)方法的源代码如下：

public void print(String s) {
    if (s == null) {
        s = "null";
    }
    write(s);
}

可见调用的是write(String s)方法，write(String s)的代码为：

private void write(String s) {
    try {
        synchronized (this) {
            ensureOpen();
            textOut.write(s);
            textOut.flushBuffer();
            charOut.flushBuffer();
            if (autoFlush && (s.indexOf('\n') >= 0))
                out.flush();
        }
    }
    catch (InterruptedIOException x) {
        Thread.currentThread().interrupt();
    }
    catch (IOException x) {
        trouble = true;
    }
}

当字符串中含有’\n’时会刷新out，此处的out是OutStream对象的实例。println(String s)最后调用newLine() 方法，newLine()的代码如下:

private void newLine() {
    try {
        synchronized (this) {
            ensureOpen();
            textOut.newLine();
            textOut.flushBuffer();
            charOut.flushBuffer();
            if (autoFlush)
                out.flush();
        }
    }
    catch (InterruptedIOException x) {
        Thread.currentThread().interrupt();
    }
    catch (IOException x) {
        trouble = true;
    }
}

newLine()会刷新out。