public static void main(String[] args) {
// 创建一个集合
ArrayList<String> one = new ArrayList<>();
one.add("a");
one.add("b");
one.add("c");
System.out.println("ArrayList 1: " + one);
// 创建另一个集合
ArrayList<String> other = new ArrayList<>();
other.add("a");
other.add("b");
other.add("d");
System.out.println("ArrayList 2: " + other);
// 前者是否包含后者某个元素
boolean isContains = isContainsOne(one, other);
System.out.println("ArrayList 1: " + one + " ; isContains:" + isContains);
// 使用retainAll运算后
boolean isRetainAll = one.retainAll(other);
System.out.println("ArrayList 1: " + one + " ; isRetainAll:" + isRetainAll);
}
/**
* 前者是否包含后者某个元素
*
* @param oneList 第一个集合
* @param otherList 第二个集合
* @return 是否包含
*/
private static boolean isContainsOne(ArrayList<String> oneList, ArrayList<String> otherList) {
for (String one : otherList) {
boolean contains = oneList.contains(one);
if (contains) {
return true;
}
}
return false;
}
}
运行结果：
ArrayList 1: [a, b, c] ArrayList 2: [a, b, d] ArrayList 1: [a, b, c] ; isContains:true ArrayList 1: [a, b] ; isRetainAll:true
### 3.总结
retainAll 方法：
>
> Retains only the elements in this list that are contained in the specified collection. In other words, removes from this list all of its elements that are not contained in the specified collection.
>
>
>
只保留此列表中包含在指定集合中的元素。换句话说，从该列表中删除指定集合中不包含的所有元素。我看一下源码：
public boolean retainAll(Collection<?> c) {
Objects.requireNonNull(c);
return batchRemove(c, true);
}
private boolean batchRemove(Collection<?> c, boolean complement) {
final Object[] elementData = this.elementData;
int r = 0, w = 0;
boolean modified = false;
try {
for (; r < size; r++)
if (c.contains(elementData[r]) == complement)
elementData[w++] = elementData[r];
} finally {
// Preserve behavioral compatibility with AbstractCollection,
// even if c.contains() throws.
if (r != size) {
System.arraycopy(elementData, r,
elementData, w,
size - r);
w += size - r;
}
if (w != size) {
// clear to let GC do its work
for (int i = w; i < size; i++)
elementData[i] = null;
modCount += size - w;
size = w;
modified = true;
}
}
return modified;
}
实际上就是求`交集`，但是交集的结果放在了第一个集合里，如果后续还要使用第一个集合就会有影响，而且 `retainAll` 的返回值说明的是 `是否删除了元素` 我们可以看下边的两个例子：
