How to initialize the ArrayList collection in java

Overview

ArrayList is a non-thread-safe collection based on dynamic arrays. The elements of ArrayList can be empty and repeated. , and at the same time it is ordered (the order of reading and storage is consistent).

ArrayList inherits AbstractList and implements List, RandomAccess (can be accessed quickly), Cloneable (can be cloned), java .io.Serializable (supports serialization)

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There are three ways to initialize ArrayList:

1. No-parameter construction, the default length is 10, which is the most commonly used initialization method:

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

At this time, we start from the source code As you can see, there is only one line of code in it: this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA, then the defined DEFAULTCAPACITY_EMPTY_ELEMENTDATA can be found in the source code:

/**
  * 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 = {};

It can be known from the comments that an empty array is defined in the source code as the default size , and determine how much to expand the array when the first element is added. This logic will be explained in the next section of adding elements.

2. Specify the initialization length:

/**
  * 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);
     }
  }

3. Use a Collection object to construct

/**
     * 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) {
            // c.toArray might (incorrectly) not return Object[] (see 6260652)
            if (elementData.getClass() != Object[].class)
                elementData = Arrays.copyOf(elementData, size, Object[].class);
        } else {
            // replace with empty array.
            this.elementData = EMPTY_ELEMENTDATA;
        }
    }

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