HashMapp为什么在Hash的时候减1

在Java的Hashmap中有如下代码：

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final Node<K,V> getNode(int hash, Object key) {
        Node<K,V>[] tab; Node<K,V> first, e; int n; K k;
        if ((tab = table) != null && (n = tab.length) > 0 &&
            (first = tab[(n - 1) & hash]) != null) {
            if (first.hash == hash && // always check first node
                ((k = first.key) == key || (key != null && key.equals(k))))
                return first;
            if ((e = first.next) != null) {
                if (first instanceof TreeNode)
                    return ((TreeNode<K,V>)first).getTreeNode(hash, key);
                do {
                    if (e.hash == hash &&
                        ((k = e.key) == key || (key != null && key.equals(k))))
                        return e;
                } while ((e = e.next) != null);
            }
        }
        return null;
    }

上面有一行是 first = tab[(n - 1) & hash]) != null

HashMap为什么在传入另一个Map时加一

首先简单介绍下TreeSet和TreeMap的两种排序：

• 自然排序
• 通过comparator排序

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  private static void compareWithCpmparator(){ TreeSet<String> treeSet =new TreeSet<>(); List<String> list =new ArrayList<>(); list.add("a"); list.add("d"); list.add("b"); treeSet.addAll(list); Iterator<String> iterator =treeSet.iterator(); while (iterator.hasNext()){ System.out.println(iterator.next()); } Comparator<String> comparator1 = (Comparator<String>) treeSet.comparator(); if (comparator1 == null){ System.out.println("comparator1是空"); }else { System.out.println("comparator1不是空"); } } public static void main(String[] args) { compareWithCpmparator(); }

  运行之后的结果如下:

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  a b d comparator1是空

  这段代码里面获取的comparator是空的，Debug一遍，发现这个方法其实调用的是NavigableMap里面的comparator

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  public Comparator<? super E> comparator() { return m.comparator(); }

  查看官网上对其的介绍：

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  Comparator<? super K> comparator() Returns the comparator used to order the keys in this map, or null if this map uses the natural ordering of its keys. Returns: the comparator used to order the keys in this map, or null if this map uses the natural ordering of its keys

  在调用这个方法的时候若是自然排序，那么会返回一个null。若是通过comparator进行排序的话当前集合采用的comparator。
  查看官网对reeSet的无参构造器的解释：

  /**

  • Constructs a new, empty tree set, sorted according to the
  • natural ordering of its elements. All elements inserted into
  • the set must implement the {@link Comparable} interface.
  • Furthermore, all such elements must be mutually
  • comparable: {@code e1.compareTo(e2)} must not throw a
  • {@code ClassCastException} for any elements {@code e1} and
  • {@code e2} in the set. If the user attempts to add an element
  • to the set that violates this constraint (for example, the user
  • attempts to add a string element to a set whose elements are
  • integers), the {@code add} call will throw a
  • {@code ClassCastException}.

在使用TreeSet的时候，插入的元素需要实现Comparable这个接口，而刚刚的元素是String，查看String的代码发现:

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public final class String implements java.io.Serializable, Comparable<String>, CharSequence {

确实实现了，再测试一个没有实现的元素：