KeySet
我们在遍历HashMap的时候会用到keySet()获取到当前HashMap的key值的一个Set集合
public Set keySet() {
Set ks = keySet;
if (ks == null) {
ks = new KeySet();
keySet = ks;
}
return ks;
}
可以看到直接创建了一个KeySet对象,final class KeySet extends AbstractSet,KeySet继承于AbstractSet
KeySet有几个方法:
size返回当前HashMap的元素数量
clear情况当前HashMap的元素
iterator返回一个KeyIterator对象,用于遍历key值
contains判断当前HashMap中是否包含有指定的key值
remove删除指定key值
forEach遍历集合，每遍历一次返回一个key值
我们先来看iterator()
HashMap#KeySet#iterator():
public final Iterator iterator() {
return new KeyIterator();
}
final class KeyIterator extends HashIterator implements Iterator {
public final K next() { return nextNode().key; }
}
KeyIterator继承于HashIterator和Iterator,只有一个next()
KeyIterator
KeyIterator中的属性:
next代表当前节点的下一个节点
current代表当前节点
expectedModCount代表修改次数 遍历期间不能对集合进行操作,否则expectedModeCount不一致会抛出异常
index当前节点的下标
keyIterator中的方法
hasNext()判断当前节点是否还有下一节点，即当前节点是否是尾节点
nextNode()返回下一节点的node值
remove()删除当前节点
我们先来看构造器
HashMap#HashIterator#HashIterator():
HashIterator() {
expectedModCount = modCount;
Node[] t = table;
current = next = null;
index = 0;
if (t != null && size > 0) { // advance to first entry
do {} while (index < t.length && (next = t[index++]) == null);
}
}
构造器中对属性进行了初始化,并通过while循环,得到数组中的第一个不为空的元素下标以及值,并将此元素值赋给next
HashMap#HashIterator#hasNext():
public final boolean hasNext() {
return next != null;
}
hasNext()判断当前元素是否有下个节点值,首次调用的话,next为构造器中首次遍历到的值
HashMap#HashIterator#nextNode():
final Node nextNode() {
Node[] t;
Node e = next;
if (modCount != expectedModCount)
throw new ConcurrentModificationException();
if (e == null)
throw new NoSuchElementException();
if ((next = (current = e).next) == null && (t = table) != null) {
do {} while (index < t.length && (next = t[index++]) == null);
}
return e;
}
nextNode()获取遍历到的当前节点,也可以理解为hasNext()所判断的下一节点,主要是看最后一个判断,e为当前要返回的节点,获取e的下一节点,并赋值给next,如果此时的next为空,则说明当前链表已经遍历完毕，那么进入判断体,开始遍历下一个数组桶中的链表,并将此链表的头结点赋值给next;
remove()比较简单这里就不多说了。
说到这里我们结合KeySet整体总结一遍流程:
HashMap hashMap = new HashMap<>();
hashMap.put("w","c");
hashMap.put("a","h");
hashMap.put("n","u");
hashMap.put("g","a");
hashMap.put("c","n");
Iterator iterator = hashMap.keySet().iterator();
while (iterator.hasNext()){
Log.i("chuan", "onResume: " + iterator.next());
}
上面是通过KeySet对集合进行的遍历
首先我们通过keySet()获取到一个Set对象(虽然Set内部也是通过HashMap实现的,但是这里只是用到了HashMap的存储功能,并没有用到迭代器,否则就陷入了先有鸡还是先有蛋的死循环中)
public Set keySet() {
Set ks = keySet;
if (ks == null) {
ks = new KeySet();
keySet = ks;
}
return ks;
}
public final Iterator iterator() { return new KeyIterator(); }
然后通过KeySet中的iterator()获取到Iterator对象,
final class KeyIterator extends HashIterator
implements Iterator {
public final K next() { return nextNode().key; }
}
public final boolean hasNext() {
return next != null;
}
final Node nextNode() {
Node[] t;
Node e = next;
if (modCount != expectedModCount)
throw new ConcurrentModificationException();
if (e == null)
throw new NoSuchElementException();
if ((next = (current = e).next) == null && (t = table) != null) {
do {} while (index < t.length && (next = t[index++]) == null);
}
return e;
}
最后通过HashIterator的hashNext()和nextNode()获取到下一节点
其实继承HashIterator的还有两个类,分别是获取value和node的ValueIterator和EntryIterator,逻辑都一样,都是先获取到指定的节点node,然后在分别获取key,value或是node
final class KeyIterator extends HashIterator
implements Iterator {
public final K next() { return nextNode().key; }
}
final class ValueIterator extends HashIterator
implements Iterator {
public final V next() { return nextNode().value; }
}
final class EntryIterator extends HashIterator
implements Iterator> {
public final Map.Entry next() { return nextNode(); }
}