1. Why does the Map interface not inherit the Collection interface?
· Set is an unordered collection and does not allow duplicate elements
· List is an ordered collection and allows duplicate elements
· Map is a key-value pair
· It is regarded as a combination
of a set of keys and a set of values · Map It is designed as a collection of key-value pairs, so there is no need to inherit the Collection interface
2. The difference between HashMap and Hashtable?
· Synchronization or thread safety
· Null key and Null value
· Iteration value
· Default capacity
3. What is the difference between comparable and comparator?
The comparable interface is actually from the java.lang package
. It has a compareTo(Object obj) method to sort objects
. The comparator interface is actually from the java.util package
. It has a compare(Object obj1, Object obj2) method to Sort objects
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4. How to sort the list of Objects?
· To sort the array of objects, we can use the Arrays.sort() method.
· If you want to sort the collection of objects, you need to use the Collections.sort() method
5. What is the difference between fail-fast and fail-safe?
· Fail fast quickly reports any failures. Whenever any of the problems will cause the fail fast system failures
· In the Java Fail fast iterator, iterating over the collection of objects sometimes causes concurrent modification exceptions. There are two reasons for this situation
· If a thread is iterating a collection and another A thread tries to modify the collection at the same time
. After calling the remove() method, how can we still try to modify the collection object?
6. What is the difference between Iterator, ListIterator and Enumeration?
· Enumeration interface is available in Java 1.2 version, so Enumeration is a legal and standardized interface
· Enumeration uses elements() method
· Iterator is implemented for all Java collection classes
· Iterator uses iterator method
· Iterator can only go in one direction
· ListIterator Only implemented for classes of type List
· ListIterator uses the listIterator() method
7. What is the difference between Set and List in Java?
· Set is a collection that does not allow duplicate elements
· Set has no index
· Set only allows one null value
· Set has classes: HashSet, LinkedHashMap, TreeSet
· List has index
· List allows N null values
· List can be displayed in the order of insertion
· List has classes: Vector, ArrayList, LinkedList
8. What is the difference between arraylist and vector?
· Vector was introduced in the first version of Java, which means that vector is a legally standardized class
· ArrayList was introduced in Java 1.2 and is part of the Java collection framework
· Vector is synchronized
· ArrayList is not synchronized of
9. What class implements the List interface?
· ArrayList
· LinkedList
· Vector
10. What class implements the Set interface?
· HashSet
· LinkedHashSet
· TreeSet
11. How to ensure the safety of a collection thread?
· Vector, Hashtable, Properties and Stack are all synchronized classes, so they are all thread-safe and can be used in a multi-threaded environment
· Use Collections.synchronizedList(list)) method to ensure that the list class is thread-safe
· Use The java.util.Collections.synchronizedSet() method can ensure that the set class is thread-safe
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12. Is it possible to add null elements to TreeSet or HashSet?
· You can add a null to the hashset
· TreeSet also allows a null value
13. Which design pattern does Iterator conform to?
· Iterator design pattern
14. Which data structure does HashSet implement?
· HashSet implements hashmap internally
15. Why can't Collection inherit Cloneable and Serializable?
· List and Set only inherit the Collection interface
· SortedMap inherits the Map interface
16. What is the difference between array and arraylist?
· Array is similar to object collection type and has a fixed size.
· Arraylist is a collection of homogeneous and heterogeneous elements
17. What is the Properties class?
· Properties is a subclass of Hashtable. It is used to maintain a list of values, where their keys and values are of String type
18.How to convert a string to an arraylist?
· Use arrayList.toArray() method
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