Iterator Design Pattern in Java

By -

Type : Behavioral Design Pattern


Summary 



Iterator allows sequential traversal through a data structure without exposing its internal details.
Iterator design pattern provides a standard well versed methods to traverse collection. Client can use the Iterator to traverse the data structure without knowing its internal details.

Advantages


Iterator provides standard API that the client can use to navigate though the Data structure.
It helps in hiding the internal data of the Data Structure and provide a mechanism to access the information that is designed to be accessible through Iterator.

Details


Iterator design pattern is very useful in programming languages like java which provides iterator for the Collections. If we want to iterate though an aggregation of objects, we have to come up with some mechanism. Iterator design pattern helps with navigating though the collection.

Example


Say we have a Book object which contains list of Chapter objects and each Chapter contains list of pages. We want to navigate though the pages. May be we want to navigate pages chapter wise or book as a whole. We can write Iterators for encapsulating the logic behind the need.

BookPageIterator is the iterator which provides a mechanism to traverse the book from start to end.

Class Diagram



Sample Code

public class Book {

  List chapter;

  public Book(List chapter) {
    this.chapter = chapter;
  }

  public CustomBookIterator getIterator() {
    return new BookPageIterator(chapter, true);
  }
}


public class BookChapter {

  List pages;

}

public class BookPage {

  private String content;

}

// iterator interface
public interface CustomBookIterator {

  public BookPage nextPage();

  public boolean isLastBookPage();

}

// concrete iterator 
public class BookPageIterator implements CustomBookIterator {

  private List bookPageList;

  private int position;

  public BookPageIterator(List chapter, boolean s) {
    this.bookPageList = new ArrayList();
    for (BookChapter chap : chapter) {
      bookPageList.addAll(chap.getPages());
    }
  }

  public BookPageIterator(List pages) {
    this.bookPageList = pages;
  }

  @Override
  public BookPage nextPage() {
    BookPage nextPage = null;
    if (position < bookPageList.size() - 1) {
      nextPage = bookPageList.get(position);
      position++;
    }
    return nextPage;
  }

  @Override
  public boolean isLastBookPage() {

    boolean ret = false;

    ret = (position) == bookPageList.size() - 1;

    return ret;
  }

}

Comments

Post a Comment

Popular posts from this blog

Difference between volatile and synchronized

By -
volatile is a field modifier, while synchronized modifies code blocks and methods. So we can specify three variations of a simple accessor using those two keywords: int i1; int geti1() {return i1;} volatile int i2; int geti2() {return i2;} int i3; synchronized int geti3() {return i3;} geti1() accesses the value currently stored in i1 in the current thread . Threads can have local copies of variables, and the data does not have to be the same as the data held in other threads. In particular, another thread may have updated i1 in it's thread, but the value in the current thread could be different from that updated value. In fact Java has the idea of a "main" memory, and this is the memory that holds the current "correct" value for variables. Threads can have their own copy of data for variables, and the thread copy can be different from the "main" memory. So in fact, it is possible for the "main"
Read more

Converting Java Map to String

By -
Java Collections framework, String manipulation etc is something that we often encounter in Development process. For processing collections (like checking null/empty, Intersection, Disjunction) We do have some of the very use full libraries. Some of the Collection related libraries are Apche Commons Collections and Google  Collections(Guava). Problem Use Case This article explains how to convert a Java Map to String(and vice versa) using different libraries and technique. One way is to use StringBuilder(Or String) and loop though the Map and build the String by applying some sort of separator ( for key:value and entry). Here we have to take care of the null value etc. Without Any Library If we want to convert the map to a String with key value separator and also individual entry seperator in the resulting String, we have to write code for that. For a simple Map, we have to iterate though the map, take care of the null values etc. Following is a sample to get String built
Read more

Masking Credit Card number in Java

By -
Sometimes we need to mask crucial information like Credit Card Numbers, CVV numbers etc before storing  or logging the information. This example mask Credit Card Number (Except last 4 Digit) from a Text which contains information along with Credit Card Number. The following example demonstrates how we can easily mask the credit card with Matcher and Pattern Classes. This Sample Code uses Matcher and Pattern. Pattern Used in this sample is not optimized for Credit Card Numbers, this pattern will get any numerical numbers in the String Content.  Based on the Credit Card Type a more efficient and Strict RegEx can be used to mask the Credit Card. /**Mask the Credit card number but last four digit value **/   Pattern PATTERN = Pattern.compile( "[0-9]+" ); String message = content; Matcher matcher = PATTERN.matcher(message); String maskingChar = "*"; StringBuilder finalMask = new StringBuilder(maskingChar);
Read more