Easily confusing differences in Java

The difference between HashMap and Hashtable in Java

① Historical reasons: Hashtable is given to the old Dictonary class, HashMap is an implementation of the map interface introduced by Java1.2

②HASHMAP allows null key-value pairs, while Hashtable does not allow

③hashtable Synchronous, HashMap non-synchronous, higher efficiency than hashtable

Please outline the difference between throw and throws in an anomaly.

①throw represents an action that throws an abnormal action; throws represents a state that represents a method that may have an exception thrown
②throw is used in method implementations, while throws is used in method declarations
③throw can only be used to throw an exception, while throws may throw multiple exceptions
differences in memory overflow and memory leaks
Memory overflow out of memory, refers to the program in the application of the RAM, there is not enough memory space for its use, there is an out-of-memory; for example, an integer is applied, but a long can be stored to save it.

Memory leak memories leak, refers to the program after the application of memory, can not release the requested memory space, a memory leak damage may be ignored, but the memory leak accumulation of serious consequences, no matter how much memory, sooner or later will be occupied.

Memory leak will eventually result in out of memory!

Memory overflow is the memory you require to allocate more than the system can give you, the system can not meet the demand, so overflow.
A memory leak is when you request a system to allocate memory for use (new), but you don't return it after you've used it (delete), and you can't access the memory you're applying to (maybe you lost the address), and the system can't assign it to the required program again. A plate with all kinds of methods can only Pack 4 fruit, you installed 5, the result fell on the ground can not eat. This is overflow! such as the stack, the stack when the stack is bound to create a space overflow, called overflow, stack empty and then do the fallback stack also produces a space overflow, called underflow. Is that the allocated memory is not enough to drop the sequence of data items called a memory overflow.

In the way that happens, memory leaks can be categorized into 4 categories:

① an often-issued memory leak. The code that occurs in memory leaks is executed multiple times, causing a memory leak each time it is executed.
② accidental memory leaks. Code that occurs with a memory leak occurs only under certain circumstances or during operation. The occurrence and the incidental sex are opposite. For a given environment, the occasional may become a frequent occurrence. So test environments and test methods are critical to detecting memory leaks.
③ disposable memory leaks. The code that occurs with a memory leak is only executed once, or because of an algorithmic flaw, there is always a piece of memory that leaks. For example, allocating memory in the class's constructor does not release the memory in the destructor, so a memory leak occurs only once.
④ an implicit memory leak. The program keeps allocating memory while it is running, but it does not release memory until the end. Strictly speaking, there is no memory leak, because the final program frees up all the requested memory. But for a server program that needs to run for days, weeks, or months, not releasing memory in time can also result in the eventual exhaustion of all of the system's memory. So, we call this kind of memory leak as an implicit memory leak.

From the user's point of view of using the program, the memory leak itself does not have any harm, as a general user, there is no sense of memory leaks. What is really harmful is the accumulation of memory leaks, which eventually consumes all the memory of the system. From this point of view, a one-time memory leak is harmless, because it does not accumulate, and the implicit memory leak is very harmful because it is more difficult to detect than the usual and sporadic memory leaks.

Easily confusing differences in Java