getHeaderField为何总是return null

Question

各位大佬，我无意中查看源码的时候产生如下疑问：
java.net.URLConnection类中有一个getHeaderFieldDate方法，如下：

@SuppressWarnings("deprecation")
public long getHeaderFieldDate(String name, long Default) {
    String value = getHeaderField(name);
    try {
        return Date.parse(value);
    } catch (Exception e) { }
    return Default;
}

该方法中调用了一个getHeaderField方法，如下：

public String getHeaderField(String name) {
    return null;
}

getHeaderField方法总是返回null，这是为什么呢？谢谢！

Answer 1

    /**
     * Returns the value of the named header field.
     * <p>
     * If called on a connection that sets the same header multiple times
     * with possibly different values, only the last value is returned.
     *
     *
     * @param   name   the name of a header field.
     * @return  the value of the named header field, or {@code null}
     *          if there is no such field in the header.
     */
    public String getHeaderField(String name) {
        return null;
    }

事实上这个方法在常见实现类里都有被覆写：

比如常见的 HttpURLConnection 中:

public String getHeaderField(String var1) {
    try {
        this.getInputStream();
    } catch (IOException var3) {
        ;
    }

    return this.cachedHeaders != null 
        ? this.filterHeaderField(var1, this.cachedHeaders.findValue(var1)) 
        : this.filterHeaderField(var1, this.responses.findValue(var1));
}