maven-resolver-util/src/main/java/org/eclipse/aether/util/graph/visitor/Stack.java

/*
 * Licensed to the Apache Software Foundation (ASF) under one
 * or more contributor license agreements.  See the NOTICE file
 * distributed with this work for additional information
 * regarding copyright ownership.  The ASF licenses this file
 * to you under the Apache License, Version 2.0 (the
 * "License"); you may not use this file except in compliance
 * with the License.  You may obtain a copy of the License at
 *
 *   http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing,
 * software distributed under the License is distributed on an
 * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
 * KIND, either express or implied.  See the License for the
 * specific language governing permissions and limitations
 * under the License.
 */
package org.eclipse.aether.util.graph.visitor;

import java.util.AbstractList;
import java.util.NoSuchElementException;
import java.util.RandomAccess;

/**
 * A non-synchronized stack with a non-modifiable list view which starts at the top of the stack. While
 * {@code LinkedList} can provide the same behavior, it creates many temp objects upon frequent pushes/pops.
 */
class Stack<E> extends AbstractList<E> implements RandomAccess {

    @SuppressWarnings("unchecked")
    // CHECKSTYLE_OFF: MagicNumber
    private E[] elements = (E[]) new Object[96];
    // CHECKSTYLE_ON: MagicNumber

    private int size;

    public void push(E element) {
        if (size >= elements.length) {
            @SuppressWarnings("unchecked")
            E[] tmp = (E[]) new Object[size + 64];
            System.arraycopy(elements, 0, tmp, 0, elements.length);
            elements = tmp;
        }
        elements[size++] = element;
    }

    public E pop() {
        if (size <= 0) {
            throw new NoSuchElementException();
        }
        return elements[--size];
    }

    public E peek() {
        if (size <= 0) {
            return null;
        }
        return elements[size - 1];
    }

    @Override
    public E get(int index) {
        if (index < 0 || index >= size) {
            throw new IndexOutOfBoundsException("Index: " + index + ", Size: " + size);
        }
        return elements[size - index - 1];
    }

    @Override
    public int size() {
        return size;
    }
}