Java Map.containsKey() 有用吗？
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Is Map.containsKey() useful?
提问by dhblah
I'm wondering, does it make any sense to check for a particular key before trying to access it. Example:
我想知道，在尝试访问特定密钥之前检查它是否有意义。例子：
Map myMap ....
if myMap.containsKey(key) {
Object value = myMap.get(key);
.....
}
Example of not using containsKey:
不使用的例子containsKey：
Object value = myMap.get(key);
if (value != null) {
......
}
EDIT: to clarify on null keys and values. Let's say, that map doesn't allow null keys and null values, so this two examples be identical.
编辑：澄清空键和值。假设该映射不允许空键和空值，因此这两个示例是相同的。
采纳答案by gustafc
Yes - keys can have nullvalues:
是 - 键可以有null值：
Map myMap = ...;
myMap.put("foo", null);
if (myMap.containsKey("foo")) {
Object value = myMap.get(key); // value is null
}
Object value = myMap.get("foo");
if (value != null) {
// you won't get here
}
You could argue (and I'd agree) that not distinguishing a nullvalue from a non-existent entry was a pretty bad design decision when they first made Java's collection API.
你可能会争辩说（我同意），null当他们第一次制作 Java 的集合 API 时，不区分值和不存在的条目是一个非常糟糕的设计决定。
(Some maps - Hashtableand ConcurrentHashMap, at least - don't allow nullvalues, which makes containsKeyless important there, but it's still a nice readability improvement over m.get(k) == null.)
（一些地图 -Hashtable并且ConcurrentHashMap，至少 - 不允许null值，这在containsKey那里变得不那么重要，但它仍然是一个很好的可读性改进m.get(k) == null。）
回答by vertti
In case your map allows for nullvalues, with just get(key)you can't differentiate between key/value pair that has value = nulland "no matching key/value pair exists".
如果您的地图允许null值，只是get(key)您无法区分具有 value = 的null键/值对和“不存在匹配的键/值对”。
回答by Marco Forberg
Yes it is useful - especially if your Map allows nullvalues...
是的，它很有用 - 特别是如果您的地图允许null值...
@Test
public void testContainsKey() {
Map<String, Object> strObjMap = new HashMap<String, Object>();
strObjMap.put("null", null);
assertTrue(strObjMap.containsKey("null"));
assertNull(strObjMap.get("null"));
}
