IterableAndCollection

CollectionExamples.java

package com.iteratrlearning.collections.examples._1_what_are_collections;

import java.util.AbstractCollection;
import java.util.Collection;
import java.util.Iterator;

public class CollectionExamples {

  public static void main(String[] args) {

    System.out.println("Square Number Sequence (Iterable)");
    System.out.println("******************************************");

    Iterable<Integer> sequence = new SquareNumbersSequence();

    for (Integer num : sequence) {
      System.out.println(num);
    }

    System.out.println("\n\nSingleton Collection");
    System.out.println("******************************************");

    Collection<String> collection = new SingletonCollection<>();

    System.out.println(collection);
    collection.add("first");
    System.out.println(collection);
    collection.add("second");
    System.out.println(collection);
  }

  private static class SingletonCollection<T> extends AbstractCollection<T> {

    private T elem;

    @Override
    public boolean add(T t) {
      if (isEmpty()) {
        elem = t;
        return true;
      }
      return false;
    }

    @Override
    public Iterator<T> iterator() {
      return new SingleIterator();
    }

    @Override
    public int size() {
      if (elem == null) {
        return 0;
      } else {
        return 1;
      }
    }

    private class SingleIterator implements Iterator<T> {

      private int index;

      @Override
      public boolean hasNext() {
        return !isEmpty() && index == 0;
      }

      @Override
      public T next() {
        index++;
        return elem;
      }
    }
  }




  private static class SquareNumbersSequence implements Iterable<Integer> {


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

    private class TermIterator implements Iterator<Integer> {

      private int curr = 0;

      @Override
      public boolean hasNext() {
        return curr < 100;
      }

      @Override
      public Integer next() {
        curr++;
        return curr * curr;
      }
    }
  }

}