What is polymorphism in Java? How to achieve polymorphism?
Polymorphism in Java is about performing different operations at runtime based on the actual type of an object. The way to achieve polymorphism is through inheritance and interfaces. In Java, a parent class reference can point to a subclass object, and a reference to an interface can point to an object of a class that implements the interface. This polymorphism can be achieved through method overloading and method overriding.
What is the difference between an abstract class and an interface in Java?
Both abstract classes and interfaces can be used to achieve polymorphism, but there are some important differences between them. An abstract class can contain the implementation of non-abstract methods, while the methods in an interface are abstract and require the implementation class to provide a concrete implementation. A class can only inherit from one abstract class, but can implement multiple interfaces. In addition, the fields in the interface are all public static constants, while the abstract class can have non-static member variables.
What is Java's reflection mechanism? In what situation is it used?
Java's reflection mechanism refers to dynamically obtaining class information and operating class members at runtime. You can use the reflection mechanism to obtain information such as methods, fields, and constructors of a class, and call them at runtime. The reflection mechanism can be used to dynamically create objects, call private methods, obtain annotation information, etc. In some cases, such as in framework development, it is necessary to use reflection mechanisms to implement generic, dynamic operations.
What is exception handling in Java? How does the exception handling mechanism in Java work?
Exception handling in Java refers to how to catch and handle these errors when errors occur during the running of the program. The exception handling mechanism includes three steps: throwing an exception, catching an exception, and handling an exception. When an exception occurs in the program, an exception object is thrown. If the exception is not caught, the program will terminate. You can use the try-catch statement to catch the exception and process it, or use the throws keyword to throw the exception to the upper caller.
What is Java's Collections Framework? What data structures does it include?
Java's collection framework is a set of classes and interfaces for storing and manipulating data. The collection framework includes data structures such as List, Set, Map, and Queue. List is an ordered collection that can store duplicate elements; Set is an unordered collection that does not allow to store duplicate elements; Map is a key-value pair mapping that allows to store duplicate keys; Queue is a queue data structure, Elements are allowed to enter at one end of the queue and dequeue at the other end.
What are threads in Java? How to create and start threads? How to synchronize threads?
A thread in Java refers to a code segment that is executed independently in a program and can be executed concurrently with other threads. Threads can be created and started using the Thread class in Java. A thread object can be created by inheriting the Thread class and overriding the run() method, or implementing the Runnable interface. Thread synchronization refers to a technology that ensures thread safety in a multi-threaded environment. You can use the synchronized keyword to ensure thread synchronization.
What are locks in Java? What kind of locks are there in Java?
Locks in Java are a synchronization mechanism used to control access to shared resources. There are many kinds of locks in Java, including the synchronized keyword, ReentrantLock, ReadWriteLock, etc. The synchronized keyword is the most commonly used lock in Java, which can ensure that only one thread accesses the synchronized code block or method at the same time. ReentrantLock is a reentrant lock that can be acquired multiple times in the same thread. ReadWriteLock is a read-write lock that allows multiple threads to read shared resources at the same time, but only allows one thread to write to shared resources.
What are annotations in Java? How to use annotations?
Annotations in Java are metadata that can be used to provide other information in a program, such as descriptions and tags of classes, methods, fields, etc. Annotations in Java start with the @ symbol, which can be used to mark an element, and can also contain some parameters. Annotations can be used to generate documentation, validate data, specify dependencies, and more. Annotation information can be obtained using the reflection mechanism.
What are IO operations in Java? What classes do IO operations in Java include?
IO operations in Java refer to input-output operations, which are used to read and write data. IO operations in Java include byte streams and character streams. Byte streams are used to read and write binary data, including InputStream, OutputStream, DataInputStream, DataOutputStream and other classes. Character streams are used to read and write text data, including Reader, Writer, BufferedReader, BufferedWriter and other classes.
What is JDBC in Java? How to connect to database and execute query?
JDBC in Java is a Java database connection technology that can be used to connect and operate various relational databases. You can use a JDBC driver to connect to the database and execute SQL queries using Statement or PreparedStatement objects. The steps of connecting to the database include loading the driver, establishing a connection, creating a Statement or PreparedStatement object, executing a query, processing the query result, and so on. When using JDBC, you need to pay attention to the release of resources. You can use the try-with-resources statement to automatically release resources.