compare elements in a listIterator

compare elements in a listIterator

I have a simple list of books like this

        mItems.add("Steve Jobs");          mItems.add("Inheritance (The Inheritance Cycle)");          mItems.add("The Hunger Games");          mItems.add("The LEGO Ideas Book");          mItems.add("Catching Fire (The Second Book of the Hunger Games)");          mItems.add("Death Comes to Pemberley");          mItems.add("Diary of a Wimpy Kid 6: Cabin Fever");          mItems.add("Explosive Eighteen: A Stephanie Plum Novel");          mItems.add("Elder Scrolls V: Skyrim: Prima Official Game Guide");  

I want to reorganize the list alphabetically by adding a header when the next book start with a different letter

So the final list should be like this

• C
• Catching Fire (The Second Book of the Hunger Games)
• D
• Death Comes to Pemberley
• Diary of a Wimpy Kid 6: Cabin Fever
• E
• Elder Scrolls V: Skyrim: Prima Official Game Guide
• Explosive Eighteen: A Stephanie Plum Novel
• I
• Inheritance (The Inheritance Cycle)

Etc...

Obviously I need to compare next and previous item so I used a ListIterator

mItems = new ArrayList();          mItems.add("Steve Jobs");          mItems.add("Inheritance (The Inheritance Cycle)");          mItems.add("The Hunger Games");          mItems.add("The LEGO Ideas Book");          mItems.add("Catching Fire (The Second Book of the Hunger Games)");          mItems.add("Death Comes to Pemberley");          mItems.add("Diary of a Wimpy Kid 6: Cabin Fever");          mItems.add("Explosive Eighteen: A Stephanie Plum Novel");          mItems.add("Elder Scrolls V: Skyrim: Prima Official Game Guide");          Collections.sort(mItems);            ArrayList books= new ArrayList();          int position = 0;          boolean isSeparator = false;          ListIterator it = mItems.listIterator();            while(it.hasNext()) {              isSeparator = false;                String name = it.next();                char[] nameArray;                // If it is the first item then need a separator              if (position == 0) {                  isSeparator = true;                  nameArray = name.toCharArray();              }              else {                  // Get the previous book's name                  String previousName =  it.previous();                    // Convert the previous and current book names                  // into char arrays                  char[] previousNameArray = previousName.toCharArray();                  nameArray = name.toCharArray();                    // Compare the first character of previous book and current book,                  if (nameArray[0] != previousNameArray[0]) {                      isSeparator = true;                  }                      // go next item                  it.next();              }               // item is a separator              if (isSeparator) {                  Book book= new Contact(String.valueOf(nameArray[0]), null, isSeparator);                  books.add( book);              }                // Create a Book object to store the name and if it's a separator or not              Book book= new Book (name, null, false);              books.add( book);                position++;          }  

The problem is that Iterator doesn't work in that way because next() and previous() method jump in the next/previous iteration. They don't get the next/previous value while staying in same iteration

Any advices ? Thank you very much

Answer by Jean Logeart for compare elements in a listIterator

Get all the initials, add them to the list, and sort:

Set initials = new HashSet<>();  for(String title: mItems) {      initials.add(title.substring(0, 1));  }  mItems.addAll(initials);  Collections.sort(mItems);  

Answer by andrey for compare elements in a listIterator

Simply store and compare previous and current element:

String prev = null;  String curr = null;  char c;  while(it.hasNext()) {      curr = it.next();      if (prev == null || prev.charAt(0) != curr.charAt(0)) {          c = curr.charAt(0);      }        // Do what you need to do with current name 'curr' and initial character 'c'        prev = curr;  }  

Answer by Nikitha Reddy for compare elements in a listIterator

    ListIterator it = mItems.listIterator();      int i = 0;      String [] listOfFirstChars = new String[26];      int indexForAddingCharacter = 0;      while(it.hasNext()) {                          String firstChar = it.next().subString(0,1);            if(firstChar != listOfFirstChars[indexForAddingCharacter]){            listOfFirstChars[indexForAddingCharacter] = firstChar;            indexForAddingCharacter++;            mItems.add(i, firstChar);            }      i++;                       }  

Let me know if this works :)

Answer by OldCurmudgeon for compare elements in a listIterator

If you wrap your sorted list in an Iterable you can add the extra sections yourself.

private static class InitialedList implements Iterable {        final List items;        public InitialedList(List mItems) {          this.items = mItems;      }        @Override      public Iterator iterator() {          return new Iterator() {              Iterator i = items.iterator();              // The next from the list.              String next = null;              // The last one we delivered.              String last = null;                @Override              public boolean hasNext() {                  return next != null || i.hasNext();              }                @Override              public String next() {                  // Peek at the next.                  if (next == null) {                      next = i.next();                  }                  // What to return.                  String it = null;                  // Is there a change in initial?                  if (next != null) {                      // Behaviour undefined if empty string in list.                      if (last == null || last.charAt(0) != next.charAt(0)) {                          it = next.substring(0, 1);                      } else {                          it = next;                          next = null;                      }                  }                  return last = it;              }            };      }  }    public void test() {      List mItems = new ArrayList<>();      mItems.add("Steve Jobs");      mItems.add("Inheritance (The Inheritance Cycle)");      mItems.add("The Hunger Games");      mItems.add("The LEGO Ideas Book");      mItems.add("Catching Fire (The Second Book of the Hunger Games)");      mItems.add("Death Comes to Pemberley");      mItems.add("Diary of a Wimpy Kid 6: Cabin Fever");      mItems.add("Explosive Eighteen: A Stephanie Plum Novel");      mItems.add("Elder Scrolls V: Skyrim: Prima Official Game Guide");      Collections.sort(mItems);      for (String s : new InitialedList(mItems)) {          System.out.println(s);      }  }  

prints:

C  Catching Fire (The Second Book of the Hunger Games)  D  Death Comes to Pemberley  Diary of a Wimpy Kid 6: Cabin Fever  E  Elder Scrolls V: Skyrim: Prima Official Game Guide  Explosive Eighteen: A Stephanie Plum Novel  I  Inheritance (The Inheritance Cycle)  S  Steve Jobs  T  The Hunger Games  The LEGO Ideas Book  

Answer by Alex Derkach for compare elements in a listIterator

If you're using java 8:

List newItems = mItems.stream()      .flatMap(s -> Stream.of(s.substring(0, 1), s))      .distinct()      .sorted()      .collect(toList());  

Now newItems contains the following:

C  Catching Fire (The Second Book of the Hunger Games)  D  Death Comes to Pemberley  Diary of a Wimpy Kid 6: Cabin Fever  E  Explosive Eighteen: A Stephanie Plum Novel  Elder Scrolls V: Skyrim: Prima Official Game Guide  I  Inheritance (The Inheritance Cycle)  S  Steve Jobs  T  The Hunger Games  The LEGO Ideas Book  

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