Java 有两万个字符串要存放到 arrayList 里面，怎么操作效率最高？

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1. 问题

• 有两万个字符串要存放到 arrayList 里面，怎么操作效率最高？

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2. 解答

• ArrayList 底层是数组实现的，扩容会耗费时间，这里用了 ArrayList 三个构造方法中的 public ArrayList(int initialCapacity) 构造了一个指定初始容量的空列表，再用 Collection 提供的 addAll(Collection c) 批量增加元素，方法如下：

import java.util.ArrayList;
import java.util.Collections;

public class Test {
    public static void main(String[] args) {
        ArrayList list = new ArrayList(20000);
        Collections.addAll(list, "1", "2", "...", "20000");
    }
}

• ArrayList 源码中的构造方法部分：

public class ArrayList<E> extends AbstractList<E>
        implements List<E>, RandomAccess, Cloneable, java.io.Serializable {
    private static final long serialVersionUID = 8683452581122892189L;

    /**
     * Default initial capacity.
     */
    private static final int DEFAULT_CAPACITY = 10;

    /**
     * Shared empty array instance used for empty instances.
     */
    private static final Object[] EMPTY_ELEMENTDATA = {};

    /**
     * Shared empty array instance used for default sized empty instances. We
     * distinguish this from EMPTY_ELEMENTDATA to know how much to inflate when
     * first element is added.
     */
    private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};

    /**
     * The array buffer into which the elements of the ArrayList are stored.
     * The capacity of the ArrayList is the length of this array buffer. Any
     * empty ArrayList with elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA
     * will be expanded to DEFAULT_CAPACITY when the first element is added.
     */
    transient Object[] elementData; // non-private to simplify nested class access

    /**
     * The size of the ArrayList (the number of elements it contains).
     *
     * @serial
     */
    private int size;

    /**
     * Constructs an empty list with the specified initial capacity.
     *
     * @param initialCapacity the initial capacity of the list
     * @throws IllegalArgumentException if the specified initial capacity
     *                                  is negative
     */
    public ArrayList(int initialCapacity) {
        if (initialCapacity > 0) {
            this.elementData = new Object[initialCapacity];
        } else if (initialCapacity == 0) {
            this.elementData = EMPTY_ELEMENTDATA;
        } else {
            throw new IllegalArgumentException("Illegal Capacity: " +
                    initialCapacity);
        }
    }

    /**
     * Constructs an empty list with an initial capacity of ten.
     */
    public ArrayList() {
        this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
    }

    /**
     * Constructs a list containing the elements of the specified
     * collection, in the order they are returned by the collection's
     * iterator.
     *
     * @param c the collection whose elements are to be placed into this list
     * @throws NullPointerException if the specified collection is null
     */
    public ArrayList(Collection<? extends E> c) {
        elementData = c.toArray();
        if ((size = elementData.length) != 0) {
            // defend against c.toArray (incorrectly) not returning Object[]
            // (see e.g. https://bugs.openjdk.java.net/browse/JDK-6260652)
            if (elementData.getClass() != Object[].class)
                elementData = Arrays.copyOf(elementData, size, Object[].class);
        } else {
            // replace with empty array.
            this.elementData = EMPTY_ELEMENTDATA;
        }
    }
}