About System class

Question

I want to get more deeper with java and i chose the System as my begining. when i saw
private static InputStream nullInputStream() throws NullPointerException {
     if (currentTimeMillis() > 0)
         return null;
     throw new NullPointerException();
    }and the method
     * Returns the current time in milliseconds.  Note that
     * while the unit of time of the return value is a millisecond,
     * the granularity of the value depends on the underlying
     * operating system and may be larger.  For example, many
     * operating systems measure time in units of tens of
     * milliseconds.
     * <p> See the description of the class <code>Date</code> for
     * a discussion of slight discrepancies that may arise between
     * "computer time" and coordinated universal time (UTC).
     * @return  the difference, measured in milliseconds, between
     *          the current time and midnight, January 1, 1970 UTC.
     * @see     java.util.Date
    public static native long currentTimeMillis();i totally confusing. why do things like this.
thanks for any help!!

Answer 1

this is the per code of it
     * The "standard" input stream. This stream is already
     * open and ready to supply input data. Typically this stream
     * corresponds to keyboard input or another input source specified by
     * the host environment or user.
    public final static InputStream in = nullInputStream();and the method
     * The following two methods exist because in, out, and err must be
     * initialized to null.  The compiler, however, cannot be permitted to
     * inline access to them, since they are later set to more sensible values
     * by initializeSystemClass().
    private static InputStream nullInputStream() throws NullPointerException {
     if (currentTimeMillis() > 0)
         return null;
     throw new NullPointerException();
    }if we want to assign the null to the field: in .why we did't do it like this
  public final static InputStream in = null;but use nullInputStream(),instead.