2305. Java Core - Iterator
Iterator and Predicate

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Iterator and customize it for different purposes.

1. Iterator Interface

The Iterator interface is used for iterating (looping) various collection classes such as HashMap, ArrayList, LinkedList etc. It has three methods:

• hasNext() - Returns true if the iteration has more elements.
• next() - Returns the next element in the iteration.
• remove() - Removes from the underlying collection the last element returned by this iterator (optional operation).

1.1 Getting and Using Iterator

In general, to use an iterator to cycle through the contents of a collection, follow these steps:

• Obtain an iterator to the start of the collection by calling the collection’s iterator() method.
• Set up a loop that makes a call to hasNext(). Have the loop iterate as long as hasNext() returns true.
• Within the loop, obtain each element by calling next().

Example for how to use iterator.

import java.util.ArrayList;
import java.util.Iterator;
import java.util.List;

public class IteratorExample {
    public static void main(String args[]){
        List<String> names = new ArrayList();
        names.add("Peter");
        names.add("Paul");
        names.add("Johnny");

        Iterator<String> it = names.iterator();

        while (it.hasNext()) {
            String str = it.next();
            System.out.println(str);
        }
    }
}

Output.

Peter
Paul
Johnny

1.2 ConcurrentModificationException while using Iterator

We cannot add or remove elements to the collection while using iterator over it.

import java.util.ArrayList;
import java.util.List;

public class ModificationExceptionExample {
    public static void main(String args[]){
        List<String> books = new ArrayList<>();
        books.add("C");
        books.add("Java");
        books.add("Python");

        for (String book : books) {
            System.out.println(book);
            //We are adding element while iterating list
            books.add("C++");
        }
    }
}

ConcurrentModificationException occurs at line of books.add("C++").

Exception in thread "main" java.util.ConcurrentModificationException
    at java.util.ArrayList$Itr.checkForComodification(ArrayList.java:901)
    at java.util.ArrayList$Itr.next(ArrayList.java:851)
    at johnny.java.core.iterator.ModificationExceptionExample.main(ModificationExceptionExample.java:13)
C

2. Custom Iterator

2.1 Sorted Iterator

Create an iterator which accepts an integer array and returns its elements in increasing order. For example, the given array is [3,6,8,2,4,7,1], this iterator returns [1,2,3,4,6,7,8] if we keep calling the next() method.

Create a class with implementing Iterator interface. Override the hasNext() and next() method.

import java.util.Arrays;
import java.util.Iterator;

public class SortedIterator implements Iterator<Integer> {

    private Integer[] nums;
    private int index;
    public SortedIterator(int[] arr) {
        if (arr != null && arr.length > 0) {
            this.nums = new Integer[arr.length];
            Arrays.sort(arr);
            for (int i = 0; i < arr.length; i++) {
                nums[i] = arr[i];
            }
            index = 0;
        } else {
            nums = new Integer[]{};
            index = 0;
        }
    }

    @Override
    public boolean hasNext(){
        return index >= 0 && index < nums.length;
    }

    @Override
    public Integer next() {
        if (index < 0 || index >= nums.length) {
            return null;
        }

        return nums[index++];
    }
}

Testing.

public class SortedIteratorExample {
    public static void main(String args[]){
        SortedIterator iterator = new SortedIterator(new int[]{3,6,8,2,4,7,1});

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

Output.

1
2
3
4
6
7
8

2.2 Common Element Iterator

Give two sorted iterator, implement a common element iterator.
For example: Suppose we have iterator1, which contains int array [1,3,5,7,9]. And we also have iterator2, which contains int array [2,3,4,8,9]. The common element iterator should return the common elements in the above two iterators, [3,9].

Create a class with implementing Iterator interface. Override the hasNext() and next() method.

import java.util.Iterator;

public class CommonIterator implements Iterator {

    private SortedIterator itr1;
    private SortedIterator itr2;
    private Integer common;

    public CommonIterator(SortedIterator itr1, SortedIterator itr2) {
        this.itr1 = itr1;
        this.itr2 = itr2;
    }

    @Override
    public boolean hasNext(){
        if (common != null) {
            return true;
        }

        if (!itr1.hasNext() || !itr2.hasNext()) {
            return false;
        }

        Integer num1 = itr1.next();
        Integer num2 = itr2.next();

        if (num1 == num2) {
            common = num1;
            return true;
        }

        while (num1 != num2) {
            if (num1 < num2) {
                if (!itr1.hasNext()) {
                    return false;
                }
                num1 = itr1.next();
            } else {
                if (!itr2.hasNext()) {
                    return false;
                }
                num2 = itr2.next();
            }
        }

        common = num1;
        return true;
    }

    @Override
    public Integer next() {
        if (hasNext()) {
            int temp = common;
            common = null;
            return temp;
        } else {
            return null;
        }
    }
}

Testing.

import johnny.java.core.iterator.custom.CommonIterator;
import johnny.java.core.iterator.custom.SortedIterator;

public class CommonIteratorExample {
    public static void main(String args[]){
        SortedIterator itr1 = new SortedIterator(new int[]{1,3,5,7,9});
        SortedIterator itr2 = new SortedIterator(new int[]{2,3,4,8,9});
        CommonIterator iterator = new CommonIterator(itr1, itr2);

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

Output.

3
9

2.3 Removable Iterator

Given a list of integer, use it to implement iterator, override remove method. Notice that the remove method only remove the last visited element(visited by next() method).

import java.util.Iterator;
import java.util.List;

public class RemovableIterator implements Iterator<Integer> {

    private List<Integer> list;
    private int last = -1;
    private int curr = 0;
    public RemovableIterator(List<Integer> list) {
        this.list = list;
    }

    @Override
    public boolean hasNext(){
        if (curr < list.size()) {
            return true;
        }

        return false;
    }

    @Override
    public Integer next() {
        if (hasNext()) {
            last = curr;
            curr++;
            return list.get(last);
        }

        return null;
    }

    @Override
    public void remove() {
        if (last > 0) {
            //throw exception
        }

        list.remove(last);
        curr = last; // keep the current position
        last = -1;
    }
}

Test with with deleting elements 2 and 8.

import johnny.java.core.iterator.custom.RemovableIterator;

import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;

public class RemovableIteratorExample {
    public static void main(String args[]){
        List<Integer> list = new ArrayList<>(Arrays.asList(new Integer[]{3,5,6,8,2,4,7,1}));
        RemovableIterator iterator = new RemovableIterator(list);
        while (iterator.hasNext()){
            Integer number = iterator.next();
            System.out.println("Processing - " + number);

            if(number == 8 || number == 2) {
                iterator.remove();
            }
        }

        System.out.println("list after iteration = " + list);
    }
}

Output.

Processing - 3
Processing - 5
Processing - 6
Processing - 8
Processing - 2
Processing - 4
Processing - 7
Processing - 1
list after iteration = [3, 5, 6, 4, 7, 1]

2.4 Predicate Iterator

First, create a custom predicate, which returns true only if the given value is larger than 5.

public class BiggerThanFivePredicate implements Predicate<Integer> {
    @Override
    public boolean test(Integer val) {

        Integer five = 5;

        return val > five;
    }
}

Then, create the custom iterator with this predicate. Override the hasNext() and next() method. Use the custom predicate to filter out unmatched data.

public class PredicateIterator implements Iterator<Integer> {
    private Iterator<Integer> itr;
    private BiggerThanFivePredicate btf;
    private Integer next;
    public PredicateIterator(Iterator<Integer> itr, BiggerThanFivePredicate btf) {
        this.itr = itr;
        this.btf = btf;
    }

    @Override
    public boolean hasNext(){
        if (next != null) {
            return true;
        }

        while (itr.hasNext()) {
            Integer num = itr.next();
            if (btf.test(num)) {
                next = num;
                return true;
            }
        }

        return false;
    }

    @Override
    public Integer next() {
        if (hasNext()) {
            int temp = next;
            next = null;
            return temp;
        }

        return null;
    }
}

Testing.

import johnny.java.core.iterator.custom.BiggerThanFivePredicate;
import johnny.java.core.iterator.custom.PredicateIterator;

import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;

public class PredicateIteratorExample {
    public static void main(String args[]){
        List<Integer> list = new ArrayList<>(Arrays.asList(new Integer[]{3,5,6,8,2,4,7,1}));
        BiggerThanFivePredicate btf = new BiggerThanFivePredicate();
        PredicateIterator iterator = new PredicateIterator(list.iterator(), btf);

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

Output.

6
8
7

3. Source Files

• Source files for Java Iterator on GitHub

4. References

• Java Doc - Interface Iterator
• Java Predicate
• Java Iterator with examples

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