Interpretation and application practice of new features of Java 8

1. Introduction

Java 8 brings many major improvements and new features. These new features make Java programming more convenient and efficient, and increase the readability and maintainability of code.

2. Lambda expression

1. A lambda expression is an anonymous function that can be used as an instance of a functional interface.

   (parameters) -> expression
   

   In the above syntax, the parameter list and expression are separated by arrows " -> ".

2. A functional interface is an interface that contains only one abstract method. Lambda expressions can be assigned to variables of functional interface types.

   // 定义函数式接口
   interface MyInterface {
         
         
       void myMethod();
   }
   
   // 使用Lambda表达式创建MyInterface接口的实例
   MyInterface myInterface = () -> System.out.println("Hello, world!");
   

3. Lambda expressions can be used in various application scenarios, such as collection operations, event processing, etc.

3. Streaming programming

1. A stream is a sequence of elements generated from a source, supporting sequential and parallel aggregation operations.

2. Streams can be used to operate data sources such as collections and arrays.

   // 创建一个字符串list
   List<String> list = Arrays.asList("Java", "Python", "JavaScript");
   
   // 通过list创建一个流
   Stream<String> stream = list.stream();
   

3. Streams support intermediate operations and termination operations. The intermediate operation returns a new stream, and multiple intermediate operations can be called in a chain; the termination operation is the calculation of the final result and can only be called once.

   // 中间操作：过滤出长度大于4的字符串
   Stream<String> filteredStream = stream.filter(e -> e.length() > 4);
   
   // 中间操作：对长度大于4的字符串进行排序
   Stream<String> sortedStream = filteredStream.sorted();
   
   // 终止操作：将流转换为数组，并输出
   String[] resultArray = sortedStream.toArray(String[]::new);
   Arrays.stream(resultArray).forEach(System.out::println);
   

4. Streaming programming can improve the simplicity and readability of code, while also automating optimization operations such as parallel processing.

4. Date/Time API

1 Overview

Java 8 introduces a completely new date/time API that replaces the old Date and Calendar classes with elegant and easy-to-remember API methods. The new API provides many new features, such as more precise time representation (nanosecond level), immutability and thread safety.

2. Use of LocalDate, LocalTime, LocalDateTime and other classes

Java 8 provides multiple date and time related classes, the commonly used ones are:

• LocalDate: represents the date, for example: 2019-12-31.
• LocalTime: indicates time, for example: 23:59:59.999.
• LocalDateTime: represents date and time, for example: 2019-12-31T23:59:59.999.
• Instant: Represents a timestamp, which can be accurate to the nanosecond level.
• ZonedDateTime: Represents date and time with time zone.

These classes are immutable and therefore thread-safe, and objects can be created via factory methods. For example:

LocalDateTime now = LocalDateTime.now(); // 当前时间
LocalDate date = LocalDate.of(2023, Month.MAY, 25); // 特定日期
LocalTime time = LocalTime.of(21, 8, 28, 0); // 特定时间

3. Formatting and parsing

The new date/time API in Java 8 also provides formatting and parsing functions, which can format date/time into strings or parse strings into date/time objects.

LocalDateTime dateTime = LocalDateTime.now();
DateTimeFormatter formatter = DateTimeFormatter.ofPattern("yyyy-MM-dd HH:mm:ss");
String formattedDateTime = dateTime.format(formatter); // 格式化为字符串
LocalDateTime parsedDateTime = LocalDateTime.parse(formattedDateTime, formatter); // 解析为日期时间对象

4. Application examples
   The date/time API of Java 8 can be applied to various scenarios, such as:

// 计算两个日期之间的天数
LocalDate date1 = LocalDate.of(2023, Month.MAY, 25);
LocalDate date2 = LocalDate.of(2024, Month.MAY, 25);
long daysBetween = ChronoUnit.DAYS.between(date1, date2);

// 打印出每个月包含多少天
YearMonth yearMonth = YearMonth.of(2023, Month.FEBRUARY);
int daysInMonth = yearMonth.lengthOfMonth();
System.out.printf("Days in month: %d%n", daysInMonth);

// 判断一个特定的时间是否在另一个时间之前或之后
LocalDateTime dateTime1 = LocalDateTime.of(2023, Month.MAY, 25, 21, 8, 28);
LocalDateTime dateTime2 = LocalDateTime.of(2023, Month.MAY, 26, 21, 8, 28);
boolean isBefore = dateTime1.isBefore(dateTime2);
boolean isAfter = dateTime1.isAfter(dateTime2);

5. Duplicate annotations and type annotations

1. Concept and function

Java 8 introduced the functionality of repeated annotations and type annotations.
Repeated annotations allow the same annotation to be used multiple times on the same element, for example:

@Author(name = "Alice")
@Author(name = "Bob")
public class Book {
    
    
    // ...
}

Type annotations allow the use of annotations on elements such as types, methods, parameters and variables, for example:

void process(@NonNull String param) {
    
    
    // ...
}

class Example<@TypeParameter T> {
    
    
    // ...
}

2. Repeat the annotation instance

Create a @Tag annotation, which can be used to add tags to classes or methods.

@Repeatable(Tags.class)
public @interface Tag {
    
    
    String value();
}

@Retention(RetentionPolicy.RUNTIME)
public @interface Tags {
    
    
    Tag[] value();
}

@Tags({
    
    @Tag("java"), @Tag("programming")})
public class MyClass {
    
    
    // ...
}

3. Type annotation examples

Create a @NonNull annotation to indicate that the element cannot be null.

@Target({
    
    ElementType.TYPE_USE, ElementType.PARAMETER, ElementType.FIELD})
@Retention(RetentionPolicy.RUNTIME)
public @interface NonNull {
    
    
}

public class Example {
    
    
    private @NonNull String name;

    public Example(@NonNull String name) {
    
    
        this.name = Objects.requireNonNull(name);
    }

    public void process(@NonNull String param) {
    
    
        // ...
    }
}

6. Summary review

1. Review of new features in Java 8
   Java 8 introduces many new features, including Lambda expressions, stream processing API, date/time API, default methods and interface enhancements, etc. These new features enrich the functions of the Java language itself and also improve the readability, maintainability and reusability of the code.

2. Application suggestions
   When using the new features of Java 8, you should pay attention to the following points:

• Make sure the JDK version is 1.8 or higher.
• Familiarize yourself with the new APIs and use them appropriately to simplify your code.
• Refactor existing code to take advantage of new features.