Below is a glossary of some of the most common terms used in the Java world.
Every Java programmer is expected to know most of these terminology.
member declared transient is not serialized and thus does not
contribute to the persistent state of an object.
variable marked volatile is not optimized during compilation. This
usually happens for variables used in multithreading environment
where the value of the variable may be changed by more than one
threads. Hence, it's not ok to optimize them by just considering one
hashmap whose keys are allowed to be garbage collected if they have
no reference to them other than the hash-map. This property of the
weak hashmap makes is vulnerable to random changes. Keys may
disappear while the map is in operation. Size() method may return
different values even if invoked twice immediately and so on.
Clearly, WeakHashMaps are used to prevent memory leaks. For exmaple,
if a hash-map is expected to be in use for a long time and a normal
hash-map is used instead of a WeakHashMap, then due the presence of
the strong hash-map, all the keys will remain in memory forever. They
will never be garbage collected and to prevent memory leak, the
programmer has to remember to remove them from the hash-map.
WeakHashMap automates this for the programmer. Also, if the value
referred by a key has no other references, then in ordinary maps,
even that value will remain in memory forever, but not so in
Weak, Soft and
is a reference to an object such that it can be cleared anytime by
the garbage collector if that object has no other strong reference
is like weak reference, but the object it refers to is garbage
collected only the JVM is not having enough memory. So, an object
held by SoftReference sticks a little while longer than those held by
WeakReference. Accordingly, WeakReference objects can be used to hold
extremely large objects in cache so that they can be cleared as soon
as their use is over. While SoftReference can be used to hold medium
size objects in cache.
is a reference to an object which has been garbage collected. Since
its been already collected, the get method of a PhantomReference
always returns null.
as iterator, just has bigger function names and used by some older
version of Enumeration, has extra function remove()
iterator meant only for Lists. It can move forward as well as
backward.It can also add an element in the list and return the next
or previous index to current position.
by ArrayList, LinkedList and Vector
and Vector use a growing array in the background while LinkedList
uses a linked list.
is synchronized while ArrayList is not.
Primitive Wrapper Classes
class (like Integer, Double etc) objects can store an additional
value of NULL
classes work with Objects and not primitives.
to write functions like equals, hashCode etc and encapsulate any
functions have fixed signatures where they can accept only objects.
So if we pass primitives there, first of all they will not accept.
Even if they accept somehow, it would be impossible to distinguish
between different primitives because they will not have
methods/fields like <primitive>.class.
since objects are passed by reference, passing an object allows us
to pass different sized primitives like Double and Integer by the
same object argument interface. Primitive arguments cannot handle
primitives allows us to add methods to primitives types. Plain
primitives cannot have any behavior defined by methods.
Useful methods in class Object
protected void finalize()
Finalize() method is called when the GC (Garbage Collector) decides that an object is
not referenced from anywhere in the code and is available for clean-up.
It is called at most once per object.
Objective of this method is to allow the object to do a clean-up (like close
file-handles, free system resources etc.) before being garbage collected.
The finalize() method of base class Object does nothing.
It is intended for sub-classes to override this method to perform clean-up.
Note 1: It is legal for an object to become reachable again in its
finalize method (by assigning "this" to some global variable for example).
In this case, the GC will not garbage collect that object unless it
becomes unreachable again. However, in the second case, its
finalize() would not be called again
Note 2: Any exception thrown by finalize() is ignored.
public final void wait()
wait() method provides 3 flavors:
public final void wait() throws InterruptedException
public final void wait(long timeout) throws InterruptedException
public final void wait(long timeout, int nanos) throws InterruptedException
wait() methods are used by a thread owning lock on an object (monitor)
to give up the lock and wait to become live again when some other thread calls notify() or notifyAll(). timeout in the second method is meant to specify the maximum time a thread will wait before becoming live again.
The last method provides a fine control by allowing user to specify timeout in nanoseconds alongwith milliseconds.
Note: The waiting thread gives up the monitor before beginning to wait and so must acquire the monitor before it can start running again.
public String toString()
This method is called whenever a string representation of the object is desired, example in System.out.println()
Object.toString() by default returns: getClass().getName() + '@' + Integer.toHexString(hashCode())
Difference between Hashmap and Hashtable
1) Hashtable is synchronized while hashmap is not. So hashmap performs better where synchronization is not required.
2) Hashmap allows null keys and null values while hashtable does not.
3) Most code which deals with Hashmap interacts through the Map interface.
Map interface is implemented by LinkedHashMap, TreeMap etc. classes too which help to maintain a particular order of elements.
So its easy to substitute TreeMap or LinkedHashMap in place of HashMap. There is no such substitute available for Hashtable.
4) Synchronization in a HashMap is easy to achieve by using Collections.synchronizedMap(myMap)
5) So in practice, Hashmap is better than Hashtable for all the use cases and should be preferred.
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