oldList
List<String> oldList = new ArrayList<>();
oldList.add("a");
oldList.add("b");
oldList.add("c");
System.out.println("oldList ## " + oldList);
newList
List<String> newList = new ArrayList<>();
newList.add("a");
newList.add("1");
newList.add("2");
System.out.println("newList ## " + newList);
1. oldList 与 newList 的交集
oldList.retainAll(newList);
System.out.println("求交集 ## " + oldList);
响应
oldList ## [a, b, c]
newList ## [a, 1, 2]
求交集 ## [a]
2. newList 与 oldList 的交集
newList.retainAll(oldList);
System.out.println("求交集 ## " + newList);
响应
oldList ## [a, b, c]
newList ## [a, 1, 2]
求交集 ## [a]
1. oldList 与 newList 的差集
oldList.removeAll(newList);
System.out.println("oldList.removeAll(newList); --> 求差集（ 在 oldList 集合中，删掉 oldList 与 newList 中都有的元素）： ## " + oldList);
响应
oldList ## [a, b, c]
newList ## [a, 1, 2]
oldList.removeAll(newList); --> 求差集（ 在 oldList 集合中，删掉 oldList 与 newList 中都有的元素）： ## [b, c]
2. newList与 oldList 的差集
newList.removeAll(oldList);
System.out.println("newList.removeAll(oldList); --> 求差集（ 在 newList 集合中，删掉 oldList 与 newList 中都有的元素）： ## " + newList);
响应
oldList ## [a, b, c]
newList ## [a, 1, 2]
newList.removeAll(oldList); --> 求差集（ 在 newList 集合中，删掉 oldList 与 newList 中都有的元素）： ## [1, 2]
3. 源码解析
removeAll() 的源码
public boolean removeAll(Collection<?> c) {
Objects.requireNonNull(c);
return batchRemove(c, false);
}
batchRemove的源码
(如果传的第二个参数是false，保留差集;如果传的是true，保留的是交集)
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;
}
1. oldList 与 newList 的并集
oldList.addAll(newList);
System.out.println("求并集 ##" + oldList);
响应
oldList ## [a, b, c]
newList ## [a, 1, 2]
求并集 ##[a, b, c, a, 1, 2]
2. newList 与 oldList 的并集
newList.addAll(oldList);
System.out.println("求并集 ##" + newList);
响应
oldList ## [a, b, c]
newList ## [a, 1, 2]
求并集 ##[a, 1, 2, a, b, c]
1. oldList 与 newList 去重后的并集
oldList.removeAll(newList); // 先求差集得到去除相同项后的 oldList
oldList.addAll(newList); // 再求并集
System.out.println("求并集 ## " + oldList);
响应
oldList ## [a, b, c]
newList ## [a, 1, 2]
求并集 ## [b, c, a, 1, 2]
2. newList与 oldList 去重后的并集
newList.removeAll(oldList);
newList.addAll(oldList);
System.out.println("求并集 ## " + newList);
响应
oldList ## [a, b, c]
newList ## [a, 1, 2]
求并集 ## [1, 2, a, b, c]
