Interview Questions-java Basics-collections and arrays

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What are the basic interfaces of the 1.Java collection class framework?

The collection class interface specifies a set of objects called elements. Each specific implementation class of the collection class interface can optionally save and sort the elements in its own way. Some collection classes allow duplicate keys, some are not allowed.
The Java Collection class provides a set of well-designed interfaces and classes that support the manipulation of a set of objects. The most basic interfaces within the Java Collection class are:
Collection: Represents a set of objects, each of which is its child element.
Set: A collection that does not contain duplicate elements.
List: A sequence of collection, and can contain repeating elements.
Map: You can map a key (key) to an object with a value, and the key cannot be duplicated.

2. Why does the collection class not implement the cloneable and serializable interfaces?

The semantics and meanings of cloning (cloning) or serialization (serialization) are related to specific implementations. Therefore, the specific implementation of the collection class should determine how to be cloned or serialized.

3. What is an iterator (Iterator)?

The iterator interface provides many ways to iterate over a collection element. Each collection class contains an instance of the iterator that can be returned

Iterative methods. Iterators can delete elements of the underlying collection during the iteration, but they cannot call the collection's remove (Object Obj) directly, and can be removed through the Remove () method of the iterator. What is the difference between 4.Iterator and listiterator? Their differences are listed below:
The iterator can be used to traverse the set and list collection, but Listiterator can only traverse the list.
Iterator to a collection can only be forward traversal, listiterator can be either forward or back.
Listiterator implements the iterator interface and includes other functions such as adding elements, replacing elements, getting the index of the previous and subsequent elements, and so on. 5. What is the difference between fast failure (fail-fast) and security failure (fail-safe)? Iterator's security failure is based on a copy of the underlying collection, so it is not affected by modifications on the source collection. All of the collection classes below the Java.util package are fast failures, and all classes under the Java.util.concurrent package fail safely. A fast-failing iterator throws an concurrentmodificationexception exception, and a security-failed iterator never throws such an exception. How does the HashMap in 6.Java work? HashMap in Java is a key-value pair (Key-value) that stores elements. HashMap requires a hash function that uses the hashcode () and Equals () methods to add and retrieve elements to the collection/collection. When the put () method is called, HashMap calculates the hash value of the key and stores the key-value pair on the appropriate index in the collection. If the key already exists, value is updated to the new value. Some of the important features of HashMap are its capacity (capacity), load factor (payload factor) and expansion limit (threshold resizing). Where does the importance of the 7.hashCode () and Equals () methods manifest? HashMap in Java uses the hashcode () and Equals () methods to determine the index of key-value pairs, which are used when the values are obtained by key. If these two methods are not implemented correctly, two different keys may have the same hash value, and therefore may be considered equal by the collection. Moreover, these two methods are also used to discover duplicate elements. So the realization of these two methods is very important to the accuracy and correctness of hashmap.

What is the difference between 8.HashMap and Hashtable?

HashMap and Hashtable both implement the map interface, so many features are very similar. However, they have the following different points:
HashMap allows keys and values to be null, while Hashtable does not allow keys or values to be null.
The Hashtable is synchronous, while HashMap is not. Therefore, HashMap is more suitable for single-threaded environments, while Hashtable is suitable for multithreaded environments.
HashMap provides a collection of keys that can be used for iteration, so HashMap is a quick failure. On the other hand, Hashtable provides an enumeration of keys (enumeration).
Hashtable is generally considered to be a legacy class.

9. What is the difference between arrays (array) and list (ArrayList)? When should I use array instead of ArrayList?

The different points of the array and ArrayList are listed below:
An array can contain primitive types and object types, and ArrayList can only contain object types.
The array size is fixed, and the size of the ArrayList is dynamically changing.
ArrayList provides more methods and features, such as AddAll (), RemoveAll (), iterator (), and so on.
For basic type data, the collection uses automatic boxing to reduce the coding effort. However, this approach is relatively slow when dealing with fixed-size base data types.

What is the difference between 10.ArrayList and LinkedList?

ArrayList and LinkedList all implement the list interface, they have the following different points:
ArrayList is an index-based data interface, and its underlying is an array. It can randomly access elements with an O (1) time complexity. In contrast, LinkedList stores its data as a list of elements, with each element linked to its previous and subsequent elements, in which case the time complexity of finding an element is O (n).
It is faster to insert, add, and delete than Arraylist,linkedlist, because when an element is added anywhere in the collection, it is not necessary to recalculate the size as an array or to update the index.
LinkedList accounts for more memory than ArrayList because LinkedList stores two references for each node, one to the previous element, and one to the next.
You can also refer to ArrayList vs. LinkedList.

What are the 11.Comparable and comparator interfaces for? List the differences between them.

Java provides a comparable interface that contains only one CompareTo () method. This method can be used to sort two objects in a single order. Specifically, it returns a negative number, 0, a positive number to indicate that the input object is less than, equal to, greater than the already existing object.
Java provides a comparator interface that contains two methods of compare () and Equals (). The Compare () method is used to sort two input parameters, return a negative number, 0, and a positive number indicates that the first parameter is less than, equal to, greater than the second argument. The Equals () method requires an object as a parameter that determines whether the input parameters are equal to comparator. This method returns true only if the input parameter is also a comparator and the input parameter is the same as the current comparator's ordering result.

12. What is the Java priority queue?

Priorityqueue is an unbounded queue based on a priority heap whose elements are sorted in natural order (natural orders). At the time of creation, we can provide it with a comparator that is responsible for ordering the elements. Priorityqueue do not allow null values because they do not have a natural order, or they do not have any associated comparators. Finally, Priorityqueue is not thread-safe, and the time complexity of the queue and the queue is O (log (n)).

13. Do you know the big O symbol (big-o notation)? Can you give examples of different data structures?

The large O symbol describes how well the size of the algorithm or performance is in the worst scenario when the elements in the data structure increase.
The large o symbol can also be used to describe other behaviors, such as memory consumption. Because the collection class is actually a data structure, we generally use the large O notation to choose the best implementation based on time, memory, and performance. The large o symbol provides a good indication of the performance of a large amount of data.

14. How do I weigh the use of unordered arrays or ordered arrays?

The greatest benefit of an ordered array is that the time complexity of the lookup is O (log n), and the unordered array is O (n). The disadvantage of an ordered array is that the time complexity of the insert operation is O (n), because elements with large values need to be moved backward to make the new element space. Instead, the insertion time complexity of an unordered array is constant O (1).

What are the best practices for the 15.Java collection class framework?

It is important for performance to correctly select the type of collection to use according to the needs of the application, such as: if the size of the element is fixed and can be known beforehand, we should use array instead of ArrayList.
Some collection classes allow you to specify the initial capacity. Therefore, if we can estimate the number of elements stored, we can set the initial capacity to avoid recalculation of hash values or expansion.
For type safety, the reason for readability and robustness is always to use generics. Also, using generics avoids classcastexception at run time.
Using the invariant Class (immutable Class) provided by the JDK as the key to the map avoids implementing the hashcode () and Equals () methods for our own classes.
The interface is better than the implementation when programming.
The underlying collection is actually empty, the return length is a collection of 0 or an array, do not return null.

What are the differences between the 16.Enumeration interface and the iterator interface?

The enumeration is twice times faster than iterator and consumes less memory at the same time. However, iterator is far more secure than enumeration because other threads are not able to modify objects in the collection that is being traversed by iterator. At the same time, iterator allows the caller to delete elements from the underlying collection, which is not possible for enumeration.

What is the difference between 17.HashSet and TreeSet?

HashSet is implemented by a hash table, so its elements are unordered. The time complexity of the add (), remove (), contains () method is O (1).
On the other hand, TreeSet is implemented by a tree-shaped structure in which the elements are ordered. Therefore, the time complexity of the add (), remove (), contains () method is O (Logn).

Interview Questions-java Basics-collections and arrays