why null value is not allowed in hashtable?
121506
The reason for allowing only one null key is because keys in a HashMap has to be unique. On the other hand Hashtable does not allow null keys. This is because the objects used as keys in a Hashtable implements the hashCode() and equals() methods for their storage and retrieval.
321141
A null pointer is a pointer which points nothing. Some uses of the null pointer are: a) To initialize a pointer variable when that pointer variable isn't assigned any valid memory address yet. b) To pass a null pointer to a function argument when we don't want to pass any valid memory address.
461743
HashMap allows a single null key and multiple null values. TreeMap does not allow null keys but can have multiple null values. HashMap allows heterogeneous elements because it does not perform sorting on keys. TreeMap allows homogeneous values as a key because of sorting.
831795
Lists (like Java arrays) are zero based. Note that these operations may execute in time proportional to the index value for some implementations (the LinkedList class, for example). Thus, iterating over the elements in a list is typically preferable to indexing through it if the caller does not know the implementation.
1088359
HashMap does not allow duplicate keys however it allows to have duplicate values. HashSet permits to have a single null value. HashMap permits single null key and any number of null values.
1441524
NullPointerException is a RuntimeException . In Java, a special null value can be assigned to an object reference. NullPointerException is thrown when an application attempts to use an object reference that has the null value. These include: Calling an instance method on the object referred by a null reference.
1196699
Nullable types represent value-type variables that can be assigned the value of null. You cannot create a nullable type based on a reference type. (Reference types already support the null value.) So, no they're not reference types.
300656
An empty string, however, is a value - it is a string of no characters. Null is essentially 'nothing' - it's the default 'value' (to use the term loosely) that Java assigns to any Object variable that was not initialized. Null isn't really a value - and as such, doesn't have properties.
944439
['null.', 'undefined.', 'NaN.', 'empty.', 'string ("")', 'false.']
1169324
The null key is handled specially in HashMap using 2 separate methods putForNullKey(V value) and getForNullKey(). Null key is mapped always to 0th Index. This null key scenario is treated in isolated methods to eliminate null key code check and improve performance of get and put methods.
1021860
HashMap doesn't allow duplicate keys but allows duplicate values. That means A single key can't contain more than 1 value but more than 1 key can contain a single value. HashMap allows null key also but only once and multiple null values.
1435420
If several references point to same String without even knowing it, it would be bad if one of the references modified that String value. That's why String objects are immutable.
463962
An ArrayList behaves just like an array, except that it will dynamically expand and contract when you add/remove elements to it. A HashTable is useful when you want to lookup objects indexed by a 'key' value. It also dynamically expands and contracts but doesn't impose an order on the elements like an array does.
107708
Null is the value of reference variable. But empty string is blank.It gives the length=0 . Empty string is a blank value,means the string does not have any thing. No method can be invoked on a object which is assigned a NULL value.
260574
C# | IsNullOrEmpty() Method It is used to check whether the specified string is null or an Empty string. A string will be null if it has not been assigned a value. A string will be empty if it is assigned “” or String. Empty (A constant for empty strings).
967312
null. Null means having no value; in other words null is zero, like if you put so little sugar in your coffee that it's practically null. Null also means invalid, or having no binding force.
642665
Null Keys and Null Values TreeMap sorts elements in natural order and doesn't allow null keys because compareTo() method throws NullPointerException if compared with null. So if we try to run the next code: TreeMap<String, Integer> treeMap = new TreeMap<>(); treeMap.