在Java语言中,给ConcurrentHashMap和Hashtable这些线程安全的集合中的Key或者Value插入 null(空) 值的会报空指针异常,但是单线程操作的HashMap又允许 Key 或者 Value 插入 null(空) 值。这到底是为什么呢? 1、探寻源码 为了找到原因,我们先来看这样一段源码片段,打开ConcurrentHashMap的putVal()方法,源码中第一句就...
在Java 语言中,HashMap 这种单线程下使用的集合是可以设置 null 值的,而并发集合如 ConcurrentHashMap 或 Hashtable 是不允许给 key 或 value 设置 null 值的,这是 JDK 源码层面直接实现的,这样设计的目的主要是为了防止并发场景下的歧义问题。 参考文档 www.cnblogs.com/fanguangdexiaoyuer/p/12335921.html ...
大概意思是,在并发下,如果 map.get(key) = null,ConcurrentMap 无法判断 key 的 value 为null,还是 key 不存在。 但是 HashMap 只考虑在非并发下运行,可以用 map.contains(key) 来做判断。 大师还说 I personally think that allowing nulls in Maps (also Sets) is an open invitation for programs to c...
The#removeEldestEntry(Map.Entry)method may be overridden to impose a policy for removing stale mappings automatically when new mappings are added to the map. Alternatively, since the "eldest" entry is the first entry in encounter order, programs can inspect and remove stale mappings through use of...
Learn to createclone of a HashMapin Java. We will see the java programs to createshallow copyanddeep copyof a HashMap. 1. Creating a Shallow Copy ofMap We can create a shallow copy of a givenHashMapin two ways. The first uses theclone()method, and the second is by iterating over...
program's capabilities. There are far more than I have listed here; I strongly encourage you to look up the others, figure out how to use them, and then try to apply them into your own programs. Expect further examples of other things you can do with Java collections in the near ...
Methods declared in class java.lang.Object finalize, getClass, notify, notifyAll, wait, wait, waitMethods declared in interface java.util.concurrent.ConcurrentMap forEach, replaceAllConstructor Details ConcurrentHashMap public ConcurrentHashMap() Creates a new, empty map with the default initial table...
In a non-concurrent map, you can check this via map.contains(key),but in a concurrent one, the map might have changed between calls. Further digressing: I personally think that allowing nulls in Maps (also Sets) is an open invitation for programs to contain errors that remain undetected ...
nulls in Maps (also Sets) is an open invitation for programs to contain errors that remain ...
ConcurrentHashMap的 key 和 value 不能为 null 主要是为了避免二义性。null 是一个特殊的值，表示没...