Map
描述的是一种映射关系,一个 key 对应一个 value,可以添加,删除,修改和获取 key/value,util 提供了多种 Map
HashMap
: hash 表实现的 map,插入删除查找性能都是 O(1),key 没有顺序 TreeMap
: 红黑树实现的 map,插入删除查找都是 O(lgn),key 按从大到小顺序排列 Hashtable
: hash 实现,线程安全,key 和 value 都不能为空,key 没有顺序 LinkedHashMap
: hash + 链表实现,按插入顺序排序 IdentityHashMap
: 判断 key 相等的条件是,两个引用指向同一个对象,即 key == e.key
WeakHashMap
: 弱引用 map,不会获取数据的强引用,当数据被 GC 清理时,数据将被删除 Map
的主要接口如下:
isEmpty
: 判断是否没有元素 size
: 获取元素个数 get
: 获取指定 key 的 value getOrDefault
: 获取指定 key 的 value,如果没有 key,返回默认值 containsKey
: 判断字典是否包含 key containsValue
: 判断字典是否包含 value keySet
: key 的集合 values
: value 的集合 entrySet
: 包含 key/value 的集合,主要用于遍历 put
: 添加一个 key/value putIfAbsent
: key 不存在才添加,如果 key 存在,返回 value,如果 key 不存在,返回 null putAll
: 合并 map,不存在的 key 添加,已存在的 key 覆盖 remove(key)
: 删除,返回老 value remove(key, val)
: 存在 map[key] = val
才删除,返回是否有元素删除 replace(key, newVal)
: 替换,返回老 value replace(key, val, newVal
: 存在 map[key] = val
才替换,返回是否有元素替换 repalceAll
: 对所有的 key/value 执行 BiFounction
替换原来的 value compute
: 所选的 key/oldValue 执行 BiFounction
替换原来的 value;如果 key 不存在,则 oldValue 为 null computeIfPresent
: key 存在才执行 BiFounction
替换原来的 value computeIfAbsent
: key 不存在才对 key 执行 Founction
作为 value 插入 merge
: 用 oldValue 和 newValue 执行 BiFounction
替换原来的 value;如果 key 不存在,则 oldValue 为 null { Map<String, String> map = new HashMap<>(Map.of( "key0", "val0", "key1", "val1", "key2", "val2", "key3", "val3" )); assertEquals(map.size(), 4); assertFalse(map.isEmpty()); assertTrue(map.containsKey("key3")); assertTrue(map.containsValue("val3")); assertEquals(map.get("key3"), "val3"); assertEquals(map.get("key6"), null); assertEquals(map.getOrDefault("key3", "defaultValue"), "val3"); assertEquals(map.getOrDefault("key6", "defaultValue"), "defaultValue"); assertThat(map.keySet(), equalTo(Set.of("key0", "key1", "key2", "key3"))); assertThat(map.values(), hasItems("val0", "val1", "val2", "val3")); for (Map.Entry<String, String> entry : map.entrySet()) { System.out.println(entry.getKey() + " => " + entry.getValue()); } map.forEach((k, v) -> System.out.println(k + " => " + v)); map.clear(); assertTrue(map.isEmpty()); } { Map<String, String> map = new HashMap<>(); map.put("key0", "val0"); map.putAll(Map.of("key1", "val1", "key2", "val2")); assertEquals(map.putIfAbsent("key3", "val3"), null); assertEquals(map.putIfAbsent("key3", "val33"), "val3"); assertThat(map, equalTo(Map.of( "key0", "val0", "key1", "val1", "key2", "val2", "key3", "val3" ))); } { Map<String, String> map = new HashMap<>(Map.of( "key0", "val0", "key1", "val1", "key2", "val2", "key3", "val3" )); assertEquals(map.remove("errorKey"), null); assertEquals(map.remove("key0"), "val0"); assertFalse(map.remove("key1", "errorValue")); assertTrue(map.remove("key1", "val1")); assertThat(map, equalTo(Map.of( "key2", "val2", "key3", "val3" ))); } { Map<String, String> map = new HashMap<>(Map.of( "key0", "val0", "key1", "val1", "key2", "val2", "key3", "val3" )); assertEquals(map.replace("errorKey", "replaceValue"), null); assertEquals(map.replace("key0", "replaceValue"), "val0"); assertFalse(map.replace("key1", "errorValue", "replaceValue")); assertTrue(map.replace("key1", "val1", "replaceValue")); assertThat(map, equalTo(Map.of( "key0", "replaceValue", "key1", "replaceValue", "key2", "val2", "key3", "val3" ))); } { Map<String, String> map = new HashMap<>(Map.of( "key0", "val0", "key1", "val1", "key2", "val2", "key3", "val3" )); map.replaceAll((k, v) -> k + v); assertThat(map, equalTo(Map.of( "key0", "key0val0", "key1", "key1val1", "key2", "key2val2", "key3", "key3val3" ))); } { Map<String, String> map = new HashMap<>(Map.of( "key0", "val0", "key1", "val1", "key2", "val2" )); assertEquals(map.compute("key0", (k, v) -> k + v), "key0val0"); assertEquals(map.computeIfPresent("key1", (k, v) -> k + v), "key1val1"); assertEquals(map.computeIfPresent("key6", (k, v) -> k + v), null); assertEquals(map.computeIfAbsent("key2", k -> k + k.replace("key", "val")), "val2"); assertEquals(map.computeIfAbsent("key3", k -> k + k.replace("key", "val")), "key3val3"); assertThat(map, equalTo(Map.of( "key0", "key0val0", "key1", "key1val1", "key2", "val2", "key3", "key3val3" ))); } { Map<String, String> map = new HashMap<>(Map.of( "key0", "val0", "key1", "val1", "key2", "val2" )); assertEquals(map.merge("key0", "newVal", (oldValue, newValue) -> (oldValue + "->" + newValue)), "val0->newVal"); assertEquals(map.merge("key3", "newVal", (oldValue, newValue) -> (oldValue + "->" + newValue)), "newVal"); assertThat(map, equalTo(Map.of( "key0", "val0->newVal", "key1", "val1", "key2", "val2", "key3", "newVal" ))); }
Hashtable
的 key/value 都不允许为空
{ Map<Integer, Integer> map = new HashMap<>(); assertDoesNotThrow(() -> map.put(null, 1)); assertDoesNotThrow(() -> map.put(1, null)); } { Map<Integer, Integer> map = new Hashtable(); assertThrows(NullPointerException.class, () -> map.put(null, 1)); assertThrows(NullPointerException.class, () -> map.put(1, null)); }
IdentityHashMap
判断相等的条件是 key 和 entry.key 是否为同一个引用对象
Map<String, String> map = new IdentityHashMap<>(); String key1 = new String("key1"); map.put(key1, "val1"); assertFalse(key1 == "key1"); assertTrue(key1.equals("key1")); assertEquals(map.get(key1), "val1"); assertEquals(map.get("key1"), null);
WeakHashMap
的 key 为弱引用,当原对象被 GC 回收时,这个 key 也会被自动删除
{ Map<String, String> map = new WeakHashMap<>(); String key1 = new String("key1"); map.put(key1, "val1"); assertEquals(map.get("key1"), "val1"); key1 = null; System.gc(); assertEquals(map.get("key1"), null); } { Map<String, String> map = new WeakHashMap<>(); String val1 = new String("val1"); map.put("key1", val1); assertEquals(map.get("key1"), "val1"); val1 = null; System.gc(); assertEquals(map.get("key1"), "val1"); }
SortedMap
继承自 Map
,key 是有序的,提供了顺序相关的几个接口
firstKey lastKey headMap tailMap subMap
SortedMap<String, String> map = new TreeMap<>(Map.of( "key0", "val0", "key1", "val1", "key2", "val2", "key3", "val3", "key4", "val4" )); assertEquals(map.firstKey(), "key0"); assertEquals(map.lastKey(), "key4"); assertThat(map.headMap("key2").keySet(), equalTo(Set.of("key0", "key1"))); assertThat(map.tailMap("key3").keySet(), equalTo(Set.of("key3", "key4"))); assertThat(map.subMap("key2", "key3").keySet(), equalTo(Set.of("key2")));
NavigableMap
继承自 SortedMap
,提供了如下几个接口
lowerKey higherKey floorKey ceilingKey lowerEntry higherEntry floorEntry ceilingEntry pollFirstEntry pollLastEntry headSet tailSet subSet
{ NavigableMap<String, String> map = new TreeMap<>(Map.of( "key0", "val0", "key1", "val1", "key2", "val2", "key3", "val3", "key4", "val4" )); assertEquals(map.lowerKey("key3"), "key2"); assertEquals(map.higherKey("key3"), "key4"); assertEquals(map.floorKey("key3"), "key3"); assertEquals(map.ceilingKey("key3"), "key3"); assertEquals(map.lowerEntry("key3").getKey(), "key2"); assertEquals(map.higherEntry("key3").getKey(), "key4"); assertEquals(map.floorEntry("key3").getKey(), "key3"); assertEquals(map.ceilingEntry("key3").getKey(), "key3"); map.remove("key3"); assertEquals(map.floorKey("key3"), "key2"); assertEquals(map.ceilingKey("key3"), "key4"); assertEquals(map.floorEntry("key3").getKey(), "key2"); assertEquals(map.ceilingEntry("key3").getKey(), "key4"); } { NavigableMap<String, String> map = new TreeMap<>(Map.of( "key0", "val0", "key1", "val1", "key2", "val2", "key3", "val3", "key4", "val4" )); assertEquals(map.pollFirstEntry().getKey(), "key0"); assertArrayEquals(map.keySet().toArray(), new String[]{"key1", "key2", "key3", "key4"}); assertEquals(map.pollLastEntry().getKey(), "key4"); assertArrayEquals(map.keySet().toArray(), new String[]{"key1", "key2", "key3"}); } { NavigableMap<String, String> map = new TreeMap<>(Map.of( "key0", "val0", "key1", "val1", "key2", "val2", "key3", "val3", "key4", "val4" )); assertArrayEquals(map.headMap("key2", true).keySet().toArray(), new String[]{"key0", "key1", "key2"}); assertArrayEquals(map.tailMap("key3", false).keySet().toArray(), new String[]{"key4"}); assertArrayEquals(map.subMap("key2", false, "key3", true).keySet().toArray(), new String[]{"key3"}); }