2019独角兽企业重金招聘Python工程师标准>>> 
//在jdkapi中的方法，说明返回值为boolean类型，
boolean retainAll(Collection c) ；
//api中给的注释
//Retains only the elements in this list that are contained in the specified collection
//只保留在此集合中存在的元素，
//A.retainAll(B),A调用这个方法之后，集合A中只剩下存在于B中的元素,返回值为false表示集合A没改变，返
回true集合A发生改变
//jdk中实现的源码
Boolean removeAll(Collection c)
public boolean retainAll(Collection c) { //返回值是否发生改变
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) {
for (int i = w; i < size; i++)
elementData[i] = null;
modCount += size - w;
size = w;
modified = true;
}
}
return modified;
}
example:
public class collection_test {
public static void main(String args[]) {
Collection c=new ArrayList();
Collection c1=new ArrayList(); c.add("a1")； c.add("a2");
c1.add("a3"); c1.add("a1"); c1.add("a2");
System.out.println(c.retainAll(c1)); System.out.println(c.toString()); System.out.println(c1.toString());
} }
结果： false [a1, a2] [a3, a1, a2]