Collection中的之retainAll()方法的理解

傷城~ 2022-01-13 14:17 236阅读 0赞

[2019独角兽企业重金招聘Python工程师标准>>> ][2019_Python_] ![hot3.png][]

//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 \[a1, a2\] \[a3, a1, a2\]

转载于:https://my.oschina.net/u/2511906/blog/3002440

[2019_Python_]: https://my.oschina.net/u/2663968/blog/3061697
[hot3.png]: /images/20220113/1e03ec97368840edbee71f658108a3ab.png