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deque vs. stack article
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‎core-java-modules/core-java-collections-4/pom.xml‎

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</dependencies>
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<properties>
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<assertj.version>3.18.0</assertj.version>
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<assertj.version>3.19.0</assertj.version>
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</properties>
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</project>

‎core-java-modules/core-java-collections-4/src/main/java/com/baeldung/collections/dequestack/ArrayLifoStack.java‎

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package com.baeldung.collections.dequestack;
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import java.util.ArrayDeque;
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import java.util.Collection;
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import java.util.Deque;
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import java.util.Iterator;
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public class ArrayLifoStack<E> implements LifoStack<E> {
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private final Deque<E> deque = new ArrayDeque<>();
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@Override
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public void push(E item) {
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deque.addFirst(item);
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}
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@Override
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public E pop() {
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return deque.removeFirst();
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}
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@Override
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public E peek() {
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return deque.peekFirst();
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}
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// implementing methods from the Collection interface
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@Override
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public int size() {
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return deque.size();
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}
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@Override
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public boolean isEmpty() {
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return deque.isEmpty();
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}
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@Override
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public boolean contains(Object o) {
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return deque.contains(o);
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}
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@Override
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public Iterator<E> iterator() {
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return deque.iterator();
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}
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@Override
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public Object[] toArray() {
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return deque.toArray();
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}
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@Override
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public <T> T[] toArray(T[] a) {
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return deque.toArray(a);
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}
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@Override
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public boolean add(E e) {
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return deque.add(e);
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}
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@Override
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public boolean remove(Object o) {
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return deque.remove(o);
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}
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@Override
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public boolean containsAll(Collection<?> c) {
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return deque.containsAll(c);
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}
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@Override
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public boolean addAll(Collection<? extends E> c) {
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return deque.addAll(c);
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}
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@Override
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public boolean removeAll(Collection<?> c) {
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return deque.removeAll(c);
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}
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@Override
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public boolean retainAll(Collection<?> c) {
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return deque.retainAll(c);
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}
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@Override
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public void clear() {
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deque.clear();
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}
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}

‎core-java-modules/core-java-collections-4/src/main/java/com/baeldung/collections/dequestack/LifoStack.java‎

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package com.baeldung.collections.dequestack;
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import java.util.Collection;
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public interface LifoStack<E> extends Collection<E> {
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E peek();
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E pop();
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void push(E item);
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}

‎core-java-modules/core-java-collections-4/src/test/java/com/baeldung/collections/dequestack/StackVsDequeUnitTest.java‎

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package com.baeldung.collections.dequestack;
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import org.junit.jupiter.api.Test;
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import java.util.ArrayDeque;
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import java.util.Deque;
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import java.util.Iterator;
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import java.util.Stack;
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import static org.assertj.core.api.Assertions.assertThat;
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class StackVsDequeUnitTest {
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@Test
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void givenAStack_whenAccessByIndex_thenElementCanBeRead() {
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Stack<String> myStack = new Stack<>();
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myStack.push("I am the 1st element."); //index 0
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myStack.push("I am the 2nd element."); //index 1
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myStack.push("I am the 3rd element."); //index 2
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//access by index
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assertThat(myStack.get(0)).isEqualTo("I am the 1st element.");
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}
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@Test
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void givenAStack_whenIterate_thenFromBottomToTop() {
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Stack<String> myStack = new Stack<>();
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myStack.push("I am at the bottom.");
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myStack.push("I am in the middle.");
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myStack.push("I am at the top.");
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Iterator<String> it = myStack.iterator();
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assertThat(it).toIterable().containsExactly(
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"I am at the bottom.",
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"I am in the middle.",
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"I am at the top.");
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}
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@Test
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void givenAStack_whenAddOrRemoveByIndex_thenElementCanBeAddedOrRemoved() {
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Stack<String> myStack = new Stack<>();
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myStack.push("I am the 1st element.");
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myStack.push("I am the 3rd element.");
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assertThat(myStack.size()).isEqualTo(2);
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//insert by index
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myStack.add(1, "I am the 2nd element.");
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assertThat(myStack.size()).isEqualTo(3);
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assertThat(myStack.get(1)).isEqualTo("I am the 2nd element.");
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//remove by index
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myStack.remove(1);
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assertThat(myStack.size()).isEqualTo(2);
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}
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@Test
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void givenADeque_whenAddOrRemoveLastElement_thenTheLastElementCanBeAddedOrRemoved() {
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Deque<String> myStack = new ArrayDeque<>();
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myStack.push("I am the 1st element.");
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myStack.push("I am the 2nd element.");
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myStack.push("I am the 3rd element.");
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assertThat(myStack.size()).isEqualTo(3);
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//insert element to the bottom of the stack
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myStack.addLast("I am the NEW element.");
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assertThat(myStack.size()).isEqualTo(4);
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assertThat(myStack.peek()).isEqualTo("I am the 3rd element.");
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//remove element from the bottom of the stack
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String removedStr = myStack.removeLast();
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assertThat(myStack.size()).isEqualTo(3);
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assertThat(removedStr).isEqualTo("I am the NEW element.");
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}
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@Test
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void givenADeque_whenIterate_thenFromTopToBottom() {
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Deque<String> myStack = new ArrayDeque<>();
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myStack.push("I am at the bottom.");
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myStack.push("I am in the middle.");
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myStack.push("I am at the top.");
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Iterator<String> it = myStack.iterator();
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assertThat(it).toIterable().containsExactly(
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"I am at the top.",
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"I am in the middle.",
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"I am at the bottom.");
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}
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}

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