Java 21 面试题与答案

问题 1. What is Java 21 or Java 21 is in nutshell?

Java 21 is finally released on 19-Sep-2023 as the next long-term support (LTS) release of Oracle’s standard Java implementation.

问题 2. What are the new features of Java 21?

The new features of Java 21 are as follows:

• String Templates (Preview) [JEP-430]
• Sequenced Collections [JEP-431]
• Generational ZGC [JEP-439]
• Record Patterns [JEP-440]
• Pattern Matching for switch [JEP-441]
• Foreign Function & Memory API (Third Preview) [JEP-442]
• Unnamed Patterns and Variables (Preview) [JEP-443]
• Virtual Threads [JEP-444]
• Unnamed Classes and Instance Main Methods (Preview) [JEP-445]
• Scoped Values (Preview) [JEP-446]
• Vector API (Sixth Incubator) [JEP-448]
• Deprecate the Windows 32-bit x86 Port for Removal [JEP-449]
• Prepare to Disallow the Dynamic Loading of Agents [JEP-451]
• Key Encapsulation Mechanism API [JEP-452]
• Structured Concurrency (Preview) [JEP-453]

问题 3. What is the String Templates in Java 21?

String templates, which are scheduled to be introduced as a preview feature in JDK 21, aim to simplify the process of string formatting and manipulation in Java. This feature enables developers to incorporate expressions directly within string literals, thus facilitating the creation and formatting of intricate strings. In the following blog post, we will delve into the concept of string templates, offering practical illustrations that will assist Java developers in embracing and harnessing the capabilities of this potent addition.

Example:

Consider a scenario where you want to display a product’s information, including its name, price, and availability status. Traditionally, you might concatenate multiple strings using the + operator:

// Prior to Java 21
String productName = "Pen";
double productPrice = 30.99;
boolean productAvailable = true;

String productInfo = "Product: " + productName + "\nPrice: $" + productPrice + "\nAvailability: " + (productAvailable ? "In Stock" : "Out of Stock");

System.out.println(productInfo);

With string templates, you can simplify the formatting process and make the code more readable:

// As of Java 21
String productName = "Pen";
double productPrice = 30.99;
boolean productAvailable = true;

String productInfo = `Product: ${productName}
Price: $${productPrice}
Availability: ${productAvailable ? "In Stock" : "Out of Stock"}`;

System.out.println(productInfo);

问题 4. What are Sequenced Collections in Java 21?

In JDK 21, the introduction of Sequenced Collections brings new interfaces and methods to simplify and streamline collection processing. This enhancement aims to address common scenarios where accessing the first and last elements of various collection types in Java required non-uniform and sometimes cumbersome approaches. This article explores the Sequenced Collections functionality and its benefits through examples of different collection processing scenarios.

问题 5. What are Sequenced Collections Interfaces in Java 21?

Sequenced Collections introduces three new interfaces:

• SequencedSet
• SequencedCollection
• SequencedMap

These interfaces come with additional methods that provide improved access and manipulation capabilities for collections.