java常用集合浅层解析-面试必备

ArrayList

1.动态数组

2.线程不安全

3.存储空间连续

4.查询快，添加删除慢

• 构造方法

/**
 + Shared empty array instance used for default sized empty instances. We
 + distinguish this from EMPTY_ELEMENTDATA to know how much to inflate when
 + first element is added.
 */
private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};

/**
 + Constructs an empty list with an initial capacity of ten.
 */
public ArrayList() {
    this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
}

这个构造方法很简单，初始化了一个空的elementData,并没有赋予数组长度

• 元素添加

/**
 + Default initial capacity.
 */
private static final int DEFAULT_CAPACITY = 10;

/**
 + The array buffer into which the elements of the ArrayList are stored.
 + The capacity of the ArrayList is the length of this array buffer. Any
 + empty ArrayList with elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA
 + will be expanded to DEFAULT_CAPACITY when the first element is added.
 */
transient Object[] elementData; // non-private to simplify nested class access

/**
 + The size of the ArrayList (the number of elements it contains).
 *
 + @serial
 */
private int size;

/**
 + Appends the specified element to the end of this list.
 *
 + @param e element to be appended to this list
 + @return <tt>true</tt> (as specified by {@link Collection#add})
 */
public boolean add(E e) {
    // 首先进行扩充
    ensureCapacityInternal(size + 1);  // Increments modCount!! 
    // 将元素追加到最后
    elementData[size++] = e; 
    return true;
}

// 扩充
private void ensureCapacityInternal(int minCapacity) {
    ensureExplicitCapacity(calculateCapacity(elementData, minCapacity));
}

// 计算数组大小 第一次调用此处的elementData={}，所以返回值为DEFAULT_CAPACITY=10，也就是默认的数组长度是10
private static int calculateCapacity(Object[] elementData, int minCapacity) {
    if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {
        return Math.max(DEFAULT_CAPACITY, minCapacity);
    }
    return minCapacity;
}

private void ensureExplicitCapacity(int minCapacity) {
    modCount++;

    // overflow-conscious code
    if (minCapacity - elementData.length > 0) // 当加上当前元素后的集合长度（size）大于现在数组长度（elementData.length）在进行扩充
        grow(minCapacity);
}

// 真正的扩充操作
private void grow(int minCapacity) {
    // overflow-conscious code
    int oldCapacity = elementData.length; // 此处oldCapacity=0
    int newCapacity = oldCapacity + (oldCapacity >> 1); // 此处newCapacity=0 
    if (newCapacity - minCapacity < 0) // 此处minCapacity=10
        newCapacity = minCapacity; // 此处newCapacity=10
    if (newCapacity - MAX_ARRAY_SIZE > 0)
        newCapacity = hugeCapacity(minCapacity);
    // minCapacity is usually close to size, so this is a win:
    elementData = Arrays.copyOf(elementData, newCapacity); //数组拷贝
}

真正的数组长度是在第一次添加的时候进行初始化的,默认为10
最主要的消耗是在扩容(数组拷贝)
当集合长度大于数组长度的时候进行扩充，扩充的标准是1.5倍[oldCapacity + (oldCapacity >> 1)]

• 查询

public E get(int index) {
    rangeCheck(index);// 校验

    return elementData(index);
}

E elementData(int index) {
    return (E) elementData[index];
}

Vector

1.动态数组，类似于ArrayList

2.线程安全

3.消耗大

• 构造方法

public Vector() {
    this(10); // initialCapacity初始容量
}

• 元素添加

/**
 * Appends the specified element to the end of this Vector.
 *
 * @param e element to be appended to this Vector
 * @return {@code true} (as specified by {@link Collection#add})
 * @since 1.2
 */
public synchronized boolean add(E e) {
    modCount++;
    ensureCapacityHelper(elementCount + 1);
    elementData[elementCount++] = e;
    return true;
}

被synchronized修饰，线程安全，但是效率较低

• 在指定位置添加元素

public void add(int index, E element) {
    insertElementAt(element, index);
}
public synchronized void insertElementAt(E obj, int index) {
    modCount++;
    if (index > elementCount) {
        throw new ArrayIndexOutOfBoundsException(index
                                                 - " > " + elementCount);
    }
    ensureCapacityHelper(elementCount + 1);
    System.arraycopy(elementData, index, elementData, index + 1, elementCount - index);
    elementData[index] = obj;
    elementCount++;
}

LinkedList

1.双向链表:jdk1.7/8以后

2.插入快,查询慢

• 构造函数

/**
 * Constructs an empty list.
 */
public LinkedList() {
}

空的构造方法

• 元素添加

public boolean add(E e) {
    linkLast(e);
    return true;
}

void linkLast(E e) {
        final Node<E> l = last;
        final Node<E> newNode = new Node<>(l, e, null);
        last = newNode;
        if (l == null)
            first = newNode;
        else
            l.next = newNode;
        size++;
        modCount++;
    }

默认添加到链表结尾，prev指向原结尾元素，原结尾元素next指针指向新添加元素，并记录结尾元素为新添加元素。只有指针移动，并没有数组拷贝，所以插入效率较快

• 查询

public E get(int index) {
    checkElementIndex(index);
    return node(index).item;
}

Node<E> node(int index) {
    // assert isElementIndex(index);

    if (index < (size >> 1)) {
        Node<E> x = first;
        for (int i = 0; i < index; i++)
            x = x.next;
        return x;
    } else {
        Node<E> x = last;
        for (int i = size - 1; i > index; i--)
            x = x.prev;
        return x;
    }
}

查询采用二分法查找，先将数组拆分成一半，然后进行遍历。所以查询较慢。当index值接近二分之一size时，更慢。

HashMap

1.存储结构：数组+链表/数组+红黑树

2.线程不安全

• 构造方法

static final float DEFAULT_LOAD_FACTOR = 0.75f;
public HashMap() {
    this.loadFactor = DEFAULT_LOAD_FACTOR; // all other fields defaulted
}

没有初始化数组，负载因子为0.75

• 添加元素

public V put(K key, V value) {
    return putVal(hash(key), key, value, false, true);
}

final V putVal(int hash, K key, V value, boolean onlyIfAbsent,
               boolean evict) {
    Node<K,V>[] tab; Node<K,V> p; int n, i;
    if ((tab = table) == null || (n = tab.length) == 0) //[1]
        n = (tab = resize()).length; // [2]
    if ((p = tab[i = (n - 1) & hash]) == null) [// [3]
        tab[i] = newNode(hash, key, value, null); // [4]
    else {
        Node<K,V> e; K k;
        if (p.hash == hash &&
            ((k = p.key) == key || (key != null && key.equals(k))))// [5]
            e = p; // [6]
        else if (p instanceof TreeNode) // [7]
            e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);  //[8]
        else { //[9]
            for (int binCount = 0; ; ++binCount) { 
                if ((e = p.next) == null) { // [10]
                    p.next = newNode(hash, key, value, null); // [11]
                    if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st [12]
                        treeifyBin(tab, hash); // [13]
                    break;
                }
                if (e.hash == hash &&
                    ((k = e.key) == key || (key != null && key.equals(k)))) // [14]
                    break;
                p = e;
            }
        }
        if (e != null) { // existing mapping for key  [15]
            V oldValue = e.value;
            if (!onlyIfAbsent || oldValue == null) // [16]
                e.value = value;
            afterNodeAccess(e);
            return oldValue;
        }
    }
    ++modCount;
    if (++size > threshold) // [17]
        resize();
    afterNodeInsertion(evict);
    return null;
}

[1]判断table是否为null,长度是否为0，table用于扩充时记录扩充后的新数组
[2]进行数组扩充，将新数组赋值给tab,n为新数组的长度
[3]判断新key需要存储的数组节点是否有值
[4]如果没有值，直接存储于该节点，如果当前数组节点有值
[5]判断新key与当前存储的key是否相同
[6]记录当前存储元素到e
[7]判断当前节点是否为数节点
[8]进行树节点操作
[9]当前节点存储的key与新key不同，并且不是树形结构（链表结构）
[10]循环遍历，找到链表的尾节点
[11]将新元素追加到链表的末尾，即原尾节点的next指针指向新元素
[12]当链表的长度达到8时，转为树形结构[13]
[14]循环过程中如果发现存储的key与新key相同，则中断循环
[15]如果存在匹配的key，则替换value
[16]返回旧值
[17]能走到这里说明是新增元素，并不是更新元素，判断当前集合长度是否大于threshold（threshold=当前集合长度*0.75）,如果大于需要进行扩充

// 扩容
final Node<K,V>[] resize() {
    Node<K,V>[] oldTab = table; // [1]
    int oldCap = (oldTab == null) ? 0 : oldTab.length; //[2]
    int oldThr = threshold; // [3]
    int newCap, newThr = 0;
    if (oldCap > 0) {
        if (oldCap >= MAXIMUM_CAPACITY) { // [4]
            threshold = Integer.MAX_VALUE;
            return oldTab;
        }
        else if ((newCap = oldCap << 1) < MAXIMUM_CAPACITY &&
                 oldCap >= DEFAULT_INITIAL_CAPACITY) // [5]
            newThr = oldThr << 1; 
    }
    else if (oldThr > 0) // [6]
        newCap = oldThr;
    else { // [7]              
        newCap = DEFAULT_INITIAL_CAPACITY; 
        newThr = (int)(DEFAULT_LOAD_FACTOR * DEFAULT_INITIAL_CAPACITY); 
    }
    if (newThr == 0) { // [8]
        float ft = (float)newCap * loadFactor;
        newThr = (newCap < MAXIMUM_CAPACITY && ft < (float)MAXIMUM_CAPACITY ?
                  (int)ft : Integer.MAX_VALUE);
    }
    threshold = newThr;  // [9]
    @SuppressWarnings({"rawtypes","unchecked"})
        Node<K,V>[] newTab = (Node<K,V>[])new Node[newCap]; 
    table = newTab;  // [10]
    if (oldTab != null) {
        for (int j = 0; j < oldCap; ++j) {// [11]
            Node<K,V> e;
            if ((e = oldTab[j]) != null) {// [12]
                oldTab[j] = null;
                if (e.next == null) 
                    newTab[e.hash & (newCap - 1)] = e;
                else if (e instanceof TreeNode) // [13]
                    ((TreeNode<K,V>)e).split(this, newTab, j, oldCap);
                else { // preserve order  [14]
                    Node<K,V> loHead = null, loTail = null;
                    Node<K,V> hiHead = null, hiTail = null;
                    Node<K,V> next;
                    do {
                        next = e.next;
                        if ((e.hash & oldCap) == 0) {
                            if (loTail == null)
                                loHead = e;
                            else
                                loTail.next = e;
                            loTail = e;
                        }
                        else {
                            if (hiTail == null)
                                hiHead = e;
                            else
                                hiTail.next = e;
                            hiTail = e;
                        }
                    } while ((e = next) != null);
                    if (loTail != null) {
                        loTail.next = null;
                        newTab[j] = loHead;
                    }
                    if (hiTail != null) {
                        hiTail.next = null;
                        newTab[j + oldCap] = hiHead;
                    }
                }
            }
        }
    }
    return newTab;
}

[1]oldTab用来记录上次扩充的table
[2]oldCap用来记录上次扩充table的长度
[3]oldThr用来记录上次扩充的阈值
[4]如果oldCap大于等于最大值（2^{30）,threshold等于2}30-1,直接返回，不在进行扩充
[5]newCap等于（oldCap*2），如果newCap小于最大值（2^{30）并且oldCap大于初始值（2}4）,则newThr=oldThr*2
[6]如果oldThr大于0，则newCap等于oldThr,上次扩充的阈值
[7]如果oldCap和oldThr都不大于0，则newCap等于2^{4,newThr等于2}4*0.75(首次扩充)
[8]当oldCap小于2^4的时，newThr等于0，newThr=2*oldCap*0.75
[9]threshold等于newThr，记录下次需要扩充的阈值
[10]创建新的Node数字，长度为newCap
[11]如果oldTab不为空，则遍历这个数组
[12]将原数组的元素散列到新数组中
[13]以红黑树的结构重新散列元素
[14]以链表的结构重新散列元素

• get方法，先根据key计算出对应的数组指针位置，然后遍历链表或者红黑树获取相同key的元素

Iterator<Map.Entry<String, Integer>> entryIterator = map.entrySet().iterator();
    while (entryIterator.hasNext()) {
        Map.Entry<String, Integer> next = entryIterator.next();
        System.out.println("key=" + next.getKey() + " value=" + next.getValue());
    }

Iterator<String> iterator = map.keySet().iterator();
    while (iterator.hasNext()){
        String key = iterator.next();
        System.out.println("key=" + key + " value=" + map.get(key));

    }

map.forEach((key,value)->{
    System.out.println("key=" + key + " value=" + value);
});

hashmap只能在单线程中使用，并尽量减少扩容，循环链表的时间复杂度是O(n),O(logn)

多线程场景下推荐使用ConcurrentHashMap

ConcurrentHashMap

Object put(Object key, int hash, Object value, boolean onlyIfAbsent) {
    lock();
    try {
        int c = count;
        if (c++ > threshold) // ensure capacity
            rehash();
        HashEntry[] tab = table;
        int index = hash & (tab.length - 1);
        HashEntry first = tab[index];
        HashEntry e = first;
        while (e != null && (e.hash != hash || !key.equals(e.key)))
            e = e.next;

        Object oldValue;
        if (e != null) {
            oldValue = e.value;
            if (!onlyIfAbsent)
                e.value = value;
        }
        else {
            oldValue = null;
            ++modCount;
            tab[index] = new HashEntry(key, hash, first, value);
            count = c; // write-volatile
        }
        return oldValue;
    } finally {
        unlock();
    }
}

ConcurrentHashMap之所以是线程安全的是因为在添加元素的时候先上了一个锁，操作完成在解锁。

HashSet

1.hashmap存储数据
2.不允许存储重复元素的集合

• 构造方法

public HashSet() {
    map = new HashMap<>();
}

• 添加元素

private static final Object PRESENT = new Object();

public boolean add(E e) {
    return map.put(e, PRESENT)==null;
}

此方法将添加的元素e作为hashmap的key,value都是相同的PRESENT,因为hashmap的key是不允许重复的，所以相同的元素添加进来，后添加的会覆盖先添加的，这就是不允许重复的原因