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voronyansky · 8 years ago

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Iterator

All we know iterator pattern it allow traverse collection without knowledge about internal collection work. This pattern have some benefits and pitfalls.

Benefits

1. Convinient traversal operation 2. Remove elements 3. Avoid NoSuchElementException by calling hasNext method

Pitfalls

1. When you call hasNext method pointer at the iterator does not move to the next value 2. When you call next method from iterator internal pointer move to the next value

Simple example of usage

public class HelloIterator { public static void main(String[] args) { List<String> strings = new ArrayList<>(); string.add("one"); string.add("two"); string.add("three");

Iterator<String> iterator = strings.iterator();

while (iterator.hasNext()) { System.out.println(iterator.next()); }

} } When you execute this code you should see this: one two three

Iterator implementation

private class More Itr implements Iterator<E> { int cursor; // index of next element to return int lastRet = -1; // index of last element returned; -1 if no such int expectedModCount = modCount;

public boolean More hasNext() { return cursor != size; }

@SuppressWarnings("unchecked") public E More next() { checkForComodification(); int i = cursor; if (i >= size) throw new NoSuchElementException(); Object[] elementData = ArrayList.this.elementData; if (i >= elementData.length) throw new ConcurrentModificationException(); cursor = i + 1; return (E) elementData[lastRet = i]; }

public void More remove() { if (lastRet < 0) throw new IllegalStateException(); checkForComodification();

try { ArrayList.this.remove(lastRet); cursor = lastRet; lastRet = -1; expectedModCount = modCount; } catch (IndexOutOfBoundsException ex) { throw new ConcurrentModificationException(); } }

@Override @SuppressWarnings("unchecked") public void More forEachRemaining(Consumer<? super E> consumer) { Objects.requireNonNull(consumer); final int size = ArrayList.this.size; int i = cursor; if (i >= size) { return; } final Object[] elementData = ArrayList.this.elementData; if (i >= elementData.length) { throw new ConcurrentModificationException(); } while (i != size && modCount == expectedModCount) { consumer.accept((E) elementData[i++]); } // update once at end of iteration to reduce heap write traffic cursor = i; lastRet = i - 1; checkForComodification(); }

final void checkForComodification() { if (modCount != expectedModCount) throw new ConcurrentModificationException(); } } First method hasNext() return true if cursor(pointer on the current element) not equals to the size of list. Next memthod next() execute checking that list not changed, if otherwise throw ConcurrentModificationException, further step is move cursor to the next position and update lastRet variable which contains last returned element and finally return element.

Your own iterator

If you would like to create your own iterator - it is possible, all your need is implement Iterator interface. Go to the create Iterator which return only odd numbers. public class OddIterator<Integer> implements Iterator<Integer> {

private final int[] values; private int cursor; private int size;

public OddIterator(final int[] values) { this.values = values; this.size = this.length; this.cursor = 0; }

@Override public boolean hasNext() { return this.cursor != this.size; }

@Override public Integer next() { return findOdd(); }

@Override public void remove() { throw UnsupportedOperationException(); }

@Override public void More forEachRemaining(Consumer<? super E> consumer) { throw UnsupportedOperationException(); }

private int findOdd() { int result 0; for (int index = cursor; index < this.size; index++) { if (index % / 2 == 0) { result = index; cursor = index; } } return result; }

} This iterator will be return only odd numbers from array. But it is a very simple implementation which not support remove and other more complex operations.

#java #iterator #pattern #patterns #code #tech #java collections #source code

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voronyansky · 8 years ago

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Java Servlet Tutorial

Preface

All java web application based on servlet, knowledge about servlet it is basic for start develop web apps on the Java. Inside all java web frameworks use servlets.

Let’s start

First you need to add to your pom.xml dependency on Servlet API. Scope value is provided - this means that this library which be provided by servlet container such as Tomcat or Jetty. Java web app should be packaged in war. Servlet Container provide for environment, sockets, manage resources and class loading for servlets. All java application which use maven should have three dirs at the src/main, it is java - hold source code, resources - hold config files and webapp - contains file which describe that this is web app. Third directory interesting for us. At this should be WEB-INF directory which must contain web.xml - deployment descriptor which needs for servlet container.

Configuration

When I try make my first servlet app, I get stuck - xml configuration does not works and I am start using annotations. XML configuration must be placed at the web.xml, otherwise annotations based configuration should be placed at the class file. XML Configuration: <web-app> <servlet> <servlet-name>HelloServlet</servlet-name> <servlet-class>me.vrnsky.HelloServlet</servlet-class> </servlet> <servlet-mapping> <servlet-name>HelloServlet</servlet-name> <url-pattern>/hello</url-pattern> </servlet-mapping> </web-app> Annotations configuration @WebServlet("/hello") public class HelloServlet {

}

Some code

Let’s create a first servlet - create a package at the java directory and create on it Index class. It must extends HttpServletClass which contains method for handle HTTP requests. And another three interesting method is init, service and destroy. What does this method ? First of it is create servlet object, second run at the endless loop and serve HTTP requests and last it call when servlet container shut down. Note that servlet container do not instant servlet object until receive request for servlet. It may change by set load-on-startup parameter. Another important thing that on all servlet container exist ONLY ONE object of servlet.

Override one of servlet method

By default servlet call doGet method, try to override it. We want to get some response from server in utf-8. For this we should write this code. Notice that servlet have a PrintWriter and OutputStreamWriter do not use it together, if you try to use this together you get an exception.

@WebServlet("/hello") public class HelloServlet {

@Override public void doGet(HttpServletRequest req, HttpServletResponse resp) throws Exception { resp.setContentType("text/utf-8"); PrintWriter writer = resp.getWriter(); writer.append("hello, I am a servlet!"); writer.flush(); } }

At this time most used method is doGet and doPost, other uses but less at the real web app.

doTrace - check trace of request

doPut - update data on the server

doDelete - delete data from server

doGet - get data from server

doPost - send data to server(not idempotent method, because produce different data)

doHead - ask about header without message

doOptions - return server options

doPatch - frequency update data on server

Another important thing

When servlet container receive an request it create two object HttpServletRequest and HttpServletRespone and give it to the thread of Servlet by this way servlet may serve a few user at one time, this thing allow improve speed of responding to the user.

A small task to the servlet and it’s solution

How to count how many users ask about some servlets? For solution I have two idea - thread local - which allow contains variable for current thread and atomic constant. First is not suitable for this and second is very suitable for it.

@WebServlet("/counter") public class Counter {

private static final AtomicInteger COUNTER = new AtomicInteger(0);