why does the jdk's serialize object exist?

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• Question about Java generics and design of java.util.function.Function
  1. question about Wildcard

  Example：Student extends Person

      Person person = new Person();
      Student student = new Student();
  
      List<? super Student> list = new ArrayList<>();
      list.add(student); // success
      list.add(person); // compile error
  
      List<? extends Person> list2 = new ArrayList<>();
      list2.add(person); // compile error
      list2.add(student);// compile error
  

  I have read the answer below a question "capture#1-of ? extends Object is not applicable"

  You are using generic wildcard. You cannot perform add operation as class type is not determinate. You cannot add/put anything(except null) -- Aniket Thakur

  Official doc：The wildcard is never used as a type argument for a generic method invocation, a generic class instance creation, or a supertype

  But why could list.add(student) compile successfully ?

  2. Design of java.util.function.Function

  public interface Function<T, R>{
  
      //...
  
      default <V> Function<V, R> compose(Function<? super V, ? extends T> before) {
          Objects.requireNonNull(before);
          return (V v) -> apply(before.apply(v));
      }
  }
  

  Why before is designed to Function<? super V, ? extends T> rather than Function<V,T> when the type of return is Function<V,R> and type of the input is V ? (It still can pass compile and use flexibly)
• Is it possible to manually cache an entire web page?

  I need to cache a web page and then for future requests, check the cache (using url as key) and if found, return the web page from the cache instead of making a request.

  I'm using Smiley's ProxyServlet and the method where the servlet writes to the OutputStream seems perfect for caching. I've added just two lines of code:

  /**
   * Copy response body data (the entity) from the proxy to the servlet client.
   * TODO: CACHE entity here for retrieval in filter
   */
  protected void copyResponseEntity( HttpResponse proxyResponse, HttpServletResponse servletResponse,
          HttpRequest proxyRequest, HttpServletRequest servletRequest ) throws IOException
  {
      HttpEntity entity = proxyResponse.getEntity();
      if ( entity != null )
      {
          String key =  getCurrentUrlFromRequest( servletRequest );  // 1
          basicCache.getCache().put( key, proxyResponse.getEntity() ); // 2
          OutputStream servletOutputStream = servletResponse.getOutputStream();
          entity.writeTo( servletOutputStream );
      }
  }
  

  and it kinda works, it does store the HttpEntity in the cache. But when I go back to a browser and request the same url once again, when the code gets back in my Filter, I obtain the HttpEntity using the url as key, and I write it to the response, but I get a "Stream closed" error:

  java.io.IOException: Stream closed
      at java.base/java.util.zip.GZIPInputStream.ensureOpen(GZIPInputStream.java:63) ~[na:na]
      at java.base/java.util.zip.GZIPInputStream.read(GZIPInputStream.java:114) ~[na:na]
      at java.base/java.io.FilterInputStream.read(FilterInputStream.java:107) ~[na:na]
      at org.apache.http.client.entity.LazyDecompressingInputStream.read(LazyDecompressingInputStream.java:64) ~[httpclient-4.5.9.jar:4.5.9]
      at org.apache.http.client.entity.DecompressingEntity.writeTo(DecompressingEntity.java:93) ~[httpclient-4.5.9.jar:4.5.9]
      at com.myapp.test.foo.filters.TestFilter.doFilter(TestFilter.java:37) ~[classes/:na]
  

  Here's the filter:

  @Component
  @WebFilter( urlPatterns = "/proxytest", description = "a filter for test servlet", initParams = {
          @WebInitParam( name = "msg", value = "==> " ) }, filterName = "test filter" )
  public class TestFilter implements Filter
  {
  
      private FilterConfig filterConfig;
  
      @Autowired BasicCache basicCache;
  
  
      @Override
      public void doFilter( ServletRequest servletRequest, ServletResponse servletResponse, FilterChain filterChain )
              throws IOException, ServletException
      {
          String url = getCurrentUrlFromRequest( servletRequest ); // 1
          HttpEntity page = (HttpEntity) basicCache.getCache().get( url ); //2
          if ( null != page )  // 3
          {
              OutputStream servletOutputStream = servletResponse.getOutputStream();  // 4
              page.writeTo( servletOutputStream );  // 5 stream closed :(
          }
          else
          {
              filterChain.doFilter( servletRequest, servletResponse );
          }
  
      }
  
  
      public String getCurrentUrlFromRequest( ServletRequest request )
      {
          if ( !( request instanceof HttpServletRequest ) ) return null;
  
          return getCurrentUrlFromRequest( (HttpServletRequest) request );
      }
  
      public String getCurrentUrlFromRequest( HttpServletRequest request )
      {
          StringBuffer requestURL = request.getRequestURL();
          String queryString = request.getQueryString();
  
          if ( queryString == null ) return requestURL.toString();
  
          return requestURL.append( '?' ).append( queryString ).toString();
      }
  
      @Override
      public void destroy()
      {
      }
  
      @Override
      public void init( FilterConfig filterConfig ) throws ServletException
      {
          this.filterConfig = filterConfig;
      }
  
  }
  

  oh, and the BasicCache class just in case:

  @Component
  public class BasicCache
  {
  
      private UserManagedCache<String, HttpEntity> userManagedCache;
  
      public BasicCache()
      { 
          userManagedCache = UserManagedCacheBuilder.newUserManagedCacheBuilder( String.class, HttpEntity.class )
                  .build( true );
      }
  
      public UserManagedCache getCache()
      {
          return userManagedCache;
      }
  
      public void destroy()
      {
          if ( null != userManagedCache )
          {
              userManagedCache.close();
          }
      }
  }
  

  I am stuck with this very localized / manual / whatever you want to call it kind of caching -- I can't use the obvious "just hook up ehcache / redis / whatever and let it do it's thing". So while I know those fine caches can cache entire webpages, I don't know if they allow me to work in this admittedly unusual way.

  So I'm hoping SO can show me how to get this done. I first tried just wiring in a ConcurrentHashMap for my basic cache but that didn't work either, so I thought I'd see if I could tap into whatever magic the big caching guns have, but so far I can't.

  Thanks for any help!
• How to reset CountDownTimer without having to pause first

  Novice looking for some help~

  I have a CountDownTimer with start, pause, and reset buttons. When the timer is running, I'd like to be able to hit the reset button to restart it from the original value (edittext.) Currently, the only way I am able to accomplish this is to hit "pause" then "reset."

  To give an example of what I'm hoping to accomplish:

  If timer is set to 10s, 10-9-8-reset-10-9-8-7-6-5-reset-10-9....

  Thanks in advance!

  I've looked through the forums and android dev site

      mButtonStartPause.setOnClickListener(new View.OnClickListener() {
                  @Override
                  public void onClick(android.view.View view) {
                      if (mTimerRunning) {
                          pauseTimer();
                      } else {
                          startTimer();
                      }
                  }
              });
              mButtonReset.setOnClickListener(new View.OnClickListener() {
                  @Override
                  public void onClick(android.view.View view) {
                      resetTimer();
                  }
              });
              return View;
          }
  
          private void setTime(long milliseconds) {
              mStartTimeInMillis = milliseconds;
              resetTimer();
          }
  
          private void startTimer() {
              mCountdowntimer = new CountDownTimer(mTimeLeftInMillis + 1000, 100) {
                  @Override
                  public void onTick(long millisUntilFinished) {
                      mTimeLeftInMillis = millisUntilFinished;
                      updateCountDownText();
                  }
  
                  @Override
                  public void onFinish() {
                      mCountdowntimer.start();
                      updateWatchInterface();
                  }
              }.start();
  
              mTimerRunning = true;
              updateWatchInterface();
          }
  
          private void pauseTimer() {
              mCountdowntimer.cancel();
              mTimerRunning = false;
              updateWatchInterface();
          }
  
          private void resetTimer() {
              mTimeLeftInMillis = mStartTimeInMillis;
              updateCountDownText();
              updateWatchInterface();
          }
  
         `enter code here` private void updateCountDownText() {
              int minutes = (int) (mTimeLeftInMillis / 1000)/60;
              int seconds = (int) (mTimeLeftInMillis / 1000)% 60;
  
              String timeLeftFormatted = String.format(Locale.getDefault(),
  
              timeLeftFormatted = String.format(Locale.getDefault(), "%02d:%02d", minutes, seconds));
              mTextViewCountdown.setText(timeLeftFormatted);
          }