查看entrySet()源码
/** * Returns a {@link Set} view of the mappings contained in this map. * * <p>The set's iterator returns the entries in ascending key order. The * sets's spliterator is * <em><a href="Spliterator.html#binding">late-binding</a></em>, * <em>fail-fast</em>, and additionally reports {@link Spliterator#SORTED} and * {@link Spliterator#ORDERED} with an encounter order that is ascending key * order. * * <p>The set is backed by the map, so changes to the map are * reflected in the set, and vice-versa. If the map is modified * while an iteration over the set is in progress (except through * the iterator's own {@code remove} operation, or through the * {@code setValue} operation on a map entry returned by the * iterator) the results of the iteration are undefined. The set * supports element removal, which removes the corresponding * mapping from the map, via the {@code Iterator.remove}, * {@code Set.remove}, {@code removeAll}, {@code retainAll} and * {@code clear} operations. It does not support the * {@code add} or {@code addAll} operations. */ public Set<Map.Entry<K,V>> entrySet() { EntrySet es = entrySet; return (es != null) ? es : (entrySet = new EntrySet()); }
而从 EntrySet es = entrySet; 和三目运算中可以知道实际返回的是entrySet，在TreeMap类中可查看entrySet的源码
private transient EntrySet entrySet; //java语言的关键字，变量修饰符，如果用transient声明一个实例变量，当对象存储时，它的值不需要维持。换句话来说就是，用transient关键字标记的成员变量不参与序列化过程。
entrySet是一个临时变量，不参与序列化（关于序列化，我的另一篇博客会有略微提及），这里只是一个声明，没有传值
同理也可以知道返回的是一个EntrySet数组，此EntrySet中存放的是Map.Entry<K,V>接口所指向的子类对象，查找源码
static final class Entry<K,V> implements Map.Entry<K,V> { K key; V value; Entry<K,V> left; Entry<K,V> right; Entry<K,V> parent; boolean color = BLACK; /** * Make a new cell with given key, value, and parent, and with * {@code null} child links, and BLACK color. */ Entry(K key, V value, Entry<K,V> parent) { this.key = key; this.value = value; this.parent = parent; } /** * Returns the key. * * @return the key */ public K getKey() { return key; } /** * Returns the value associated with the key. * * @return the value associated with the key */ public V getValue() { return value; } /** * Replaces the value currently associated with the key with the given * value. * * @return the value associated with the key before this method was * called */ public V setValue(V value) { V oldValue = this.value; this.value = value; return oldValue; } public boolean equals(Object o) { if (!(o instanceof Map.Entry)) return false; Map.Entry<?,?> e = (Map.Entry<?,?>)o; return valEquals(key,e.getKey()) && valEquals(value,e.getValue()); } public int hashCode() { int keyHash = (key==null ? 0 : key.hashCode()); int valueHash = (value==null ? 0 : value.hashCode()); return keyHash ^ valueHash; } public String toString() { return key + "=" + value; } }
可以看见TreeMap中的内部类Entry<K,V>实现了 Map.Entry<K,V>接口，那么Entry<K,V>是 Map.Entry<K,V>的一个实现类，但是Entry<K,V>中的值是如何从外界添加进来的，继续查看源码中put方法
public V put(K key, V value) { Entry<K,V> t = root; if (t == null) { compare(key, key); // type (and possibly null) check root = new Entry<>(key, value, null); size = 1; modCount++; return null; } int cmp; Entry<K,V> parent; // split comparator and comparable paths Comparator<? super K> cpr = comparator; if (cpr != null) { do { parent = t; cmp = cpr.compare(key, t.key); if (cmp < 0) t = t.left; else if (cmp > 0) t = t.right; else return t.setValue(value); } while (t != null); } else { if (key == null) throw new NullPointerException(); @SuppressWarnings("unchecked") Comparable<? super K> k = (Comparable<? super K>) key; do { parent = t; cmp = k.compareTo(t.key); if (cmp < 0) t = t.left; else if (cmp > 0) t = t.right; else return t.setValue(value); } while (t != null); } Entry<K,V> e = new Entry<>(key, value, parent); if (cmp < 0) parent.left = e; else parent.right = e; fixAfterInsertion(e); size++; modCount++; return null; }
原来put方法将值存入了一个名为root的Entry<K,V>中，root是一个临时变量
private transient Entry<K,V> root;
那么到目前为止还是不知道Entry<K,V>的值如何传入entrySet中，继续查看源码，
class EntrySet extends AbstractSet<Map.Entry<K,V>> { public Iterator<Map.Entry<K,V>> iterator() { return new EntryIterator(getFirstEntry()); } public boolean contains(Object o) { if (!(o instanceof Map.Entry)) return false; Map.Entry<?,?> entry = (Map.Entry<?,?>) o; Object value = entry.getValue(); Entry<K,V> p = getEntry(entry.getKey()); return p != null && valEquals(p.getValue(), value); } public boolean remove(Object o) { if (!(o instanceof Map.Entry)) return false; Map.Entry<?,?> entry = (Map.Entry<?,?>) o; Object value = entry.getValue(); Entry<K,V> p = getEntry(entry.getKey()); if (p != null && valEquals(p.getValue(), value)) { deleteEntry(p); return true; } return false; } public int size() { return TreeMap.this.size(); } public void clear() { TreeMap.this.clear(); } public Spliterator<Map.Entry<K,V>> spliterator() { return new EntrySpliterator<K,V>(TreeMap.this, null, null, 0, -1, 0); } }
final class EntryIterator extends PrivateEntryIterator<Map.Entry<K,V>> { EntryIterator(Entry<K,V> first) { super(first); } public Map.Entry<K,V> next() { return nextEntry(); } }
final Entry<K,V> nextEntry() { Entry<K,V> e = next; if (e == null) throw new NoSuchElementException(); if (modCount != expectedModCount) throw new ConcurrentModificationException(); next = successor(e); lastReturned = e; return e; }