I want to use an ArrayList in Java and implement it as a queue. Adding to the queue should be fairly easy using queue.add("element"). Although, removing popping items out it a bit trickier. The way I thought of doing it is the following:
public String pop(){ String s = queue.get(0); queue.remove(0); queue.trimToSize(); return s;
}is this the right way? will I be getting the next element of the queue next time I invoke the pop() method?
12 Answers
If you really need to use an ArrayList, remove() already does everything you want. You should be able to implement a queue just using add() and remove(0). Your pop() method could be implemented as:
public String pop() { return queue.remove(0);
}From the documentation:
remove
public E remove(int index)
Removes the element at the specified position in this list. Shifts any subsequent elements to the left (subtracts one from their indices).
Returns: the element that was removed from the list
Though, as others have suggested, an ArrayDeque would be a better fit for a queue.
Try below mention implementation using ArrayDeque
import java.util.ArrayDeque;
import java.util.Collection;
import java.util.Iterator;
public class QueueImpl<E>
{ private ArrayDeque<E> arrayDeque; /** * Constructs an empty array deque with an initial capacity sufficient to * hold 16 elements. **/ public QueueImpl() { arrayDeque = new ArrayDeque<E>(); } /** * Constructs a deque containing the elements of the specified collection, * in the order they are returned by the collection's iterator. **/ public QueueImpl(Collection<? extends E> c) { arrayDeque = new ArrayDeque<E>(c); } /** * Constructs an empty array deque with an initial capacity sufficient to * hold the specified number of elements. **/ public QueueImpl(int numElements) { arrayDeque = new ArrayDeque<E>(numElements); } /** Inserts the specified element at the tail of this queue. **/ public boolean add(E e) { return arrayDeque.add(e); } /** Atomically removes all of the elements from this queue. **/ public void clear() { arrayDeque.clear(); } /** Returns true if this queue contains the specified element. **/ public boolean contains(Object o) { return arrayDeque.contains(o); } /** Returns an iterator over the elements in this queue in proper sequence. **/ public Iterator<E> iterator() { return arrayDeque.iterator(); } /** * Inserts the specified element at the tail of this queue if it is possible * to do so immediately without exceeding the queue's capacity, returning * true upon success and false if this queue is full. **/ public boolean offer(E e) { return arrayDeque.offer(e); } /** * Retrieves, but does not remove, the head of this queue, or returns null * if this queue is empty. **/ public E peek() { return arrayDeque.peek(); } /** * Retrieves and removes the head of this queue, or returns null if this * queue is empty. **/ public E poll() { return arrayDeque.poll(); } /** * Removes a single instance of the specified element from this queue, if it * is present. **/ public boolean remove(Object o) { return arrayDeque.remove(o); } /** Returns the number of elements in this queue. **/ public int size() { return arrayDeque.size(); } /** * Returns an array containing all of the elements in this queue, in proper * sequence. **/ public Object[] toArray() { return arrayDeque.toArray(); } /** * Returns an array containing all of the elements in this queue, in proper * sequence; the runtime type of the returned array is that of the specified * array. **/ public <T> T[] toArray(T[] a) { return arrayDeque.toArray(a); } /** Returns a string representation of this collection. **/ public String toString() { return arrayDeque.toString(); } /** Inserts the specified element at the front of this deque. **/ public void addFirst(E e) { arrayDeque.addFirst(e); } /** Inserts the specified element at the end of this deque. **/ public void addLast(E e) { arrayDeque.addLast(e); } /** Retrieves, but does not remove, the first element of this deque. **/ public void getFirst() { arrayDeque.getFirst(); } /** Retrieves, but does not remove, the last element of this deque. **/ public void getLast() { arrayDeque.getLast(); } /** Inserts the specified element at the front of this deque. **/ public boolean offerFirst(E e) { return arrayDeque.offerFirst(e); } /** Inserts the specified element at the end of this deque. **/ public boolean offerLast(E e) { return arrayDeque.offerLast(e); } /** * Retrieves, but does not remove, the first element of this deque, or * returns null if this deque is empty. **/ public E peekFirst() { return arrayDeque.peekFirst(); } /** * Retrieves, but does not remove, the last element of this deque, or * returns null if this deque is empty. **/ public E peekLast() { return arrayDeque.peekLast(); } public static void main(String... arg) { QueueImpl<Integer> arrayDeque = new QueueImpl<Integer>(); arrayDeque.add(300); arrayDeque.add(200); arrayDeque.add(600); arrayDeque.add(-400); arrayDeque.add(240); System.out.println("the elements of the arrayDeque is "); Iterator<Integer> itr = arrayDeque.iterator(); while (itr.hasNext()) { System.out.print(itr.next() + "\t"); } System.out.println(); arrayDeque.offer(600); arrayDeque.offer(700); arrayDeque.offerFirst(3); arrayDeque.offerLast(10); System.out.println("the peak element of the arrayDeque is(by peeking) " + arrayDeque.peek()); System.out.println("the peak element of the arrayDeque is(by polling) " + arrayDeque.poll()); System.out.println("the last element of arrayDeque is (peeking)" + arrayDeque.peekLast()); System.out.println("element 300 removed " + arrayDeque.remove(300)); System.out.println("the arrayDeque contains 400 :" + arrayDeque.contains(400)); System.out.println("the arrayDeque contains 600 :" + arrayDeque.contains(600)); System.out.println("the size of the arrayDeque is " + arrayDeque.size()); System.out.println(arrayDeque); }
}Result:
the elements of the arrayDeque is
300 200 600 -400 240
the peak element of the arrayDeque is(by peeking) 3
the peak element of the arrayDeque is(by polling) 3
the last element of arrayDeque is (peeking)10
element 300 removed true
the arrayDeque contains 400 :false
the arrayDeque contains 600 :true
the size of the arrayDeque is 7
[200, 600, -400, 240, 600, 700, 10]
