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ImmutableList是一个不可变、线程安全的列表集合，它只会获取传入对象的一个副本，而不会影响到原来的变量或者对象，如下代码：

        int a = 23;ImmutableList<Integer> list = ImmutableList.of(a, 12);System.out.println(list);a = 232;System.out.println(list);

输出结果：‘

[23, 12]
[23, 12]

ImmutableList创建不可变对象有两种方法，一种是使用静态of方法，另外一种是使用静态内部类Builder。

• 静态内部类构建方法
  先看下源码，他是一个静态内部类，使用static和final修饰符，来辅助外部类ImmutableList构建不可变的集合对象

•   public static final class Builder<E>extends ImmutableCollection.Builder<E>{private Object[] contents;private int size;public Builder(){this(4);}Builder(int capacity){this.contents = new Object[capacity];this.size = 0;}Builder<E> ensureCapacity(int minCapacity){if (this.contents.length < minCapacity) {this.contents = ObjectArrays.arraysCopyOf(this.contents, expandedCapacity(this.contents.length, minCapacity));}return this;}public Builder<E> add(E element){Preconditions.checkNotNull(element);ensureCapacity(this.size + 1);this.contents[(this.size++)] = element;return this;}public Builder<E> addAll(Iterable<? extends E> elements){if ((elements instanceof Collection)) {Collection<?> collection = (Collection)elements;ensureCapacity(this.size + collection.size());}super.addAll(elements);return this;}public Builder<E> add(E... elements){for (int i = 0; i < elements.length; i++) {ObjectArrays.checkElementNotNull(elements[i], i);}ensureCapacity(this.size + elements.length);System.arraycopy(elements, 0, this.contents, this.size, elements.length);this.size += elements.length;return this;}public Builder<E> addAll(Iterator<? extends E> elements){super.addAll(elements);return this;}public ImmutableList<E> build(){switch (this.size) {case 0: return ImmutableList.of();case 1: E singleElement = this.contents[0];return ImmutableList.of(singleElement);}if (this.size == this.contents.length){return new RegularImmutableList(this.contents);}return new RegularImmutableList(ObjectArrays.arraysCopyOf(this.contents, this.size));}}

  使用的具体示例：

•         //使用内部类的方式ImmutableList<Integer> list = ImmutableList .<Integer>builder().add(12).add(23).add(34).build();

• 静态方法
  静态方法提供了多个参数的of静态方法

•         //获取一个空的不可变集合对象ImmutableList<String> list1 = ImmutableList .<String>of();//获取一个有一个元素的不可变集合对象ImmutableList<String> list2 = ImmutableList .<String>of("12");//获取一个有两个元素的不可变集合对象ImmutableList<String> list3 = ImmutableList .<String>of("12","23");

• 复制数组中的元素到ImmutableList中

•         List<String> list4 = new ArrayList<String>();list4.add("1");list4.add("2");list4.add("3");//copy数组list4的一个副本List<String> list5 = ImmutableList .<String>copyOf(list4);