Can we write our own iterator in Java

Question

If I have a list containing  alice  bob  abigail  charlie  and I want to write an iterator such that it iterates over elements that begin with  a   can I write my own   How can I do that

User · Answer

Good example for Iterable to compute factorial

FactorialIterable fi = new FactorialIterable(10);
Iterator<Integer> iterator = fi.iterator();
while (iterator.hasNext()){
     System.out.println(iterator.next());
}

Short code for Java 1.8

new FactorialIterable(5).forEach(System.out::println);

Custom Iterable class

public class FactorialIterable implements Iterable<Integer> {

    private final FactorialIterator factorialIterator;

    public FactorialIterable(Integer value) {
        factorialIterator = new FactorialIterator(value);
    }

    @Override
    public Iterator<Integer> iterator() {
        return factorialIterator;
    }

    @Override
    public void forEach(Consumer<? super Integer> action) {
        Objects.requireNonNull(action);
        Integer last = 0;
        for (Integer t : this) {
            last = t;
        }
        action.accept(last);
    }

}

Custom Iterator class

public class FactorialIterator implements Iterator<Integer> {

    private final Integer mNumber;
    private Integer mPosition;
    private Integer mFactorial;


    public FactorialIterator(Integer number) {
        this.mNumber = number;
        this.mPosition = 1;
        this.mFactorial = 1;
    }

    @Override
    public boolean hasNext() {
        return mPosition <= mNumber;
    }

    @Override
    public Integer next() {
        if (!hasNext())
            return 0;

        mFactorial = mFactorial * mPosition;

        mPosition++;

        return  mFactorial;
    }
}

User · Answer

Here is the complete answer to the question     import java util Arrays  import java util Iterator  import java util List  import java util NoSuchElementException   class ListIterator implements Iterator lt String gt       List lt String gt  list      int pos   0       public ListIterator List lt String gt  list            this list   list              Override     public boolean hasNext             while pos  lt  list size                 if  list get pos  startsWith  a                    return true              pos                      return false               Override     public String next             if  hasNext                return list get pos             throw new NoSuchElementException             public class IteratorTest        public static void main String   args            List lt String gt  list   Arrays asList  alice    bob    abigail    charlie            ListIterator itr   new ListIterator list            while itr hasNext                System out println itr next        prints alice  abigail            ListIterator is the iterator for the array which returns the elements that start with  a     There is no need for implementing an Iterable interface  But that is a possibility     There is no need to implement this generically     It fully satisfies the contract for hasNext   and next    ie if hasNext   says there are still elements  next   will return those elements  And if hasNext   says no more elements  it returns a valid NoSuchElementException exception

User · Answer

The best reusable option is to implement the interface Iterable and override the method iterator     Here s an example of a an ArrayList like class implementing the interface  in which you override the method Iterator     import java util Iterator   public class SOList lt Type gt  implements Iterable lt Type gt         private Type   arrayList      private int currentSize       public SOList Type   newArray            this arrayList   newArray          this currentSize   arrayList length              Override     public Iterator lt Type gt  iterator             Iterator lt Type gt  it   new Iterator lt Type gt                   private int currentIndex   0                Override             public boolean hasNext                     return currentIndex  lt  currentSize  amp  amp  arrayList currentIndex     null                              Override             public Type next                     return arrayList currentIndex                                 Override             public void remove                     throw new UnsupportedOperationException                                     return it            This class implements the Iterable interface using Generics  Considering you have elements to the array  you will be able to get an instance of an Iterator  which is the needed instance used by the  foreach  loop  for instance   You can just create an anonymous instance of the iterator without creating extending Iterator and take advantage of the value of currentSize to verify up to where you can navigate over the array  let s say you created an array with capacity of 10  but you have only 2 elements at 0 and 1   The instance will have its owner counter of where it is and all you need to do is to play with hasNext    which verifies if the current value is not null  and the next    which will return the instance of your currentIndex  Below is an example of using this API     public static void main String   args           create an array of type Integer     Integer   numbers   new Integer   1  2  3  4  5           create your list and hold the values      SOList lt Integer gt  stackOverflowList   new SOList lt Integer gt  numbers           Since our class SOList is an instance of Iterable  then we can use it on a foreach loop     for Integer num   stackOverflowList            System out print num                 creating an array of Strings     String   languages   new String    C    C      Java    Python    Scala            create your list and hold the values using the same list implementation      SOList lt String gt  languagesList   new SOList lt String gt  languages        System out println             Since our class SOList is an instance of Iterable  then we can use it on a foreach loop     for String lang   languagesList            System out println lang              will print  12345   C   C     Java   Python   Scala   If you want  you can iterate over it as well using the Iterator instance      navigating the iterator while  allNumbers hasNext          Integer value   allNumbers next        if  allNumbers hasNext              System out print value                else           System out print value               will print 1  2  3  4  5   The foreach documentation is located at http   download oracle com javase 1 5 0 docs guide language foreach html  You can take a look at a more complete implementation at my personal practice google code   Now  to get the effects of what you need I think you need to plug a concept of a filter in the Iterator    Since the iterator depends on the next values  it would be hard to return true on hasNext    and then filter the next   implementation with a value that does not start with a char  a  for instance  I think you need to play around with a secondary Interator based on a filtered list with the values with the given filter

User · Answer

You can implement your own Iterator   Your iterator could be constructed to wrap the Iterator returned by the List  or you could keep a cursor and use the List s get int index  method   You just have to add logic to your Iterator s next method AND the hasNext method to take into account your filtering criteria   You will also have to decide if your iterator will  support the remove operation

User · Answer

This is the complete code to write an iterator such that it iterates over elements that begin with  a     import java util Iterator   public class AppDemo        public static void main String args               Bag lt String gt  bag1   new Bag lt  gt              bag1 add  alice            bag1 add  bob             bag1 add  abigail            bag1 add  charlie              for  Iterator lt String gt  it1   bag1 iterator    it1 hasNext                    String s   it1 next                if  s    null                  System out println s                        Custom Iterator class  import java util ArrayList  import java util Iterator   public class Bag lt T gt         private ArrayList lt T gt  data       public Bag              data   new ArrayList lt  gt                public void add T e             data add e               public Iterator lt T gt  iterator              return new BagIterator               public class BagIterator lt T gt  implements Iterator lt T gt             private int index           private String str           public BagIterator                  index   0                      Override         public boolean hasNext                   return index  lt  data size                          Override         public T next                  str    String  data get index                if  str startsWith  a                    return  T  data get index                  index                 return null

User · Answer

Sure  An iterator is just an implementation of the java util Iterator interface  If you re using an existing iterable object  say  a LinkedList  from java util  you ll need to either subclass it and override its iterator function so that you return your own  or provide a means of wrapping a standard iterator in your special Iterator instance  which has the advantage of being more broadly used   etc

[java] Can we write our own iterator in Java?

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