Collection of classic Java interview questions (8)

1. The difference between Collection and Collections

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java.util.Collection is the parent interface of a collection framework. It provides common interface methods for basic operations on collection objects. The Collection interface has many specific implementations in the Java class library. The significance of the Collection interface is to provide a maximum unified operation method for various specific collections.

java.util.Collections is a wrapper class. It contains various static polymorphic methods related to collection operations. This class cannot be instantiated and is like a utility class that serves Java's Collection framework. It provides a series of static methods to implement operations such as searching, sorting, and thread safety on various collections.

2. The elements in Set cannot be repeated, so what method is used to distinguish duplicates? Should I use == or equals()? What is the difference between them?

==：

Basic type: Compare whether the value is the same

Reference type: Compare whether the address value is the same

equals ():

Reference type: By default, the address value is compared, which can be overridden. The comparison is whether the member variable values ​​of the object are the same.

If a class does not define its own equals method, its default equals method (inherited from the Object class) uses the == operator, which also compares whether the objects pointed to by the two variables are the same object. At this time Using equals will get the same result as using ==. If two independent objects are compared, false will always be returned.

If the class you write hopes to be able to compare whether the contents of two instance objects created by the class are the same, then you must override the equals method and write your own code to decide under what circumstances two objects can be considered The content is the same.

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3. What is the role of Iterator

is used to traverse collections Elements in have advantages over for and for each:

will not report an exception due to node deletion, and you can also implement it with your own data structure.

4. What is the difference between HashSet and TreeSet, and when to use them?

Basic:

1. TreeSet is implemented as a binary tree. The data in Treeset is automatically sorted and null values ​​are not allowed.

2. HashSet is implemented by a hash table. The data in HashSet is unordered. You can put nulls, but you can only put one null. The values ​​in both cannot be repeated, just like in the database. the only constraint.

3. HashSet requires that the object put in must implement the HashCode() method. The object put in is identified by the hashcode code. String objects with the same content have the same hashcode, so the put in Content cannot be repeated. But objects of the same class can be placed into different instances.

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Time complexity:

HashSet is implemented by a hash table, therefore, it The elements are unordered. The time complexity of add(), remove(), and contains() methods is O(1).

TreeSet is implemented by a tree structure, and the elements in it are ordered. Therefore, the time complexity of the add(), remove(), and contains() methods is O(logn).

When using TreeSet to save custom class objects, the class where the customization is located must implement the Comparable interface. If this interface is not implemented, the size relationship cannot be distinguished, and if you want to sort in the TreeSet, then To compare all fields, that is to say, in TreeSet, we rely on whether the compare() method returns 0 to determine whether there are duplicate elements.

TreeSet relies on Comparable to distinguish duplicate data;

HashSet relies on hashCode() and equals() to distinguish duplicate data.

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