Java 8十个lambda表达式案例

矫情吗；* 2022-07-15 07:14 96阅读 0赞

http://www.jdon.com/idea/java/10-example-of-lambda-expressions-in-java8.html

### 1. 实现Runnable线程案例 ###

使用() -> \{\} 替代匿名类：

//Before Java 8:
    new Thread(new Runnable() {
        @Override
        public void run() {
            System.out.println("Before Java8 ");
        }
    }).start();
    
    //Java 8 way:
    new Thread( () -> System.out.println("In Java8!") ).start();
         
    Output:
    too much code, for too little to do
    Lambda expression rocks !!

你可以使用 下面语法实现Lambda:

(params) -> expression  
(params) -> statement  
(params) -> \{ statements \}

如果你的方法并不改变任何方法参数，比如只是输出，那么可以简写如下：

() -> System.out.println("Hello Lambda Expressions");

如果你的方法接受两个方法参数，如下：

(int even, int odd) -> even + odd

### 2.实现事件处理 ###

如果你曾经做过Swing 编程，你将永远不会忘记编写事件侦听器代码。使用lambda表达式如下所示写出更好的事件侦听器的代码。

// Before Java 8:
    JButton show =  new JButton("Show");
    show.addActionListener(new ActionListener() {
         @Override
         public void actionPerformed(ActionEvent e) {
               System.out.println("without lambda expression is boring");
            }
         });
    
    
    // Java 8 way:
    show.addActionListener((e) -> {
        System.out.println("Action !! Lambda expressions Rocks");
    });

在java 8中你可以使用Lambda表达式替代丑陋的匿名类。

### 3.使用Lambda表达式遍历List集合 ###

//Prior Java 8 :
    List features = Arrays.asList("Lambdas", "Default Method", 
    "Stream API", "Date and Time API");
    for (String feature : features) {
       System.out.println(feature);
    }
    
    //In Java 8:
    List features = Arrays.asList("Lambdas", "Default Method", "Stream API",
     "Date and Time API");
    features.forEach(n -> System.out.println(n));
    
    // Even better use Method reference feature of Java 8
    // method reference is denoted by :: (double colon) operator
    // looks similar to score resolution operator of C++
    features.forEach(System.out::println);
    
    Output:
    Lambdas
    Default Method
    Stream API
    Date and Time API

方法引用是使用两个冒号::这个操作符号。

### 4.使用Lambda表达式和函数接口 ###

为了支持函数编程，Java 8加入了一个新的包java.util.function，其中有一个接口java.util.function.Predicate是支持Lambda函数编程：

public static void main(args[]){
      List languages = Arrays.asList("Java", "Scala", "C++", "Haskell", "Lisp");
    
      System.out.println("Languages which starts with J :");
      filter(languages, (str)->str.startsWith("J"));
    
      System.out.println("Languages which ends with a ");
      filter(languages, (str)->str.endsWith("a"));
    
      System.out.println("Print all languages :");
      filter(languages, (str)->true);
    
       System.out.println("Print no language : ");
       filter(languages, (str)->false);
    
       System.out.println("Print language whose length greater than 4:");
       filter(languages, (str)->str.length() > 4);
    }
    
     public static void filter(List names, Predicate condition) {
        for(String name: names)  {
           if(condition.test(name)) {
              System.out.println(name + " ");
           }
        }
      }
    }
    
    Output:
    Languages which starts with J :
    Java
    Languages which ends with a
    Java
    Scala
    Print all languages :
    Java
    Scala
    C++
    Haskell
    Lisp
    Print no language :
    Print language whose length greater than 4:
    Scala
    Haskell
    
    //Even better
     public static void filter(List names, Predicate condition) {
        names.stream().filter((name) -> (condition.test(name)))
            .forEach((name) -> {System.out.println(name + " ");
        });
     }

你能看到来自Stream API 的filter方法能够接受 Predicate参数, 能够允许测试多个条件。

### 5.复杂的结合**Predicate **使用 ###

java.util.function.Predicate提供and(), or() 和 xor()可以进行逻辑操作，比如为了得到一串字符串中以"J"开头的4个长度：

// We can even combine Predicate using and(), or() And xor() logical functions
     // for example to find names, which starts with J and four letters long, you
     // can pass combination of two Predicate
     Predicate<String> startsWithJ = (n) -> n.startsWith("J");
     Predicate<String> fourLetterLong = (n) -> n.length() == 4;
       
     names.stream()
          .filter(startsWithJ.and(fourLetterLong))
          .forEach((n) -> System.out.print("\nName, which starts with
                'J' and four letter long is : " + n));

其中startsWithJ.***and***(fourLetterLong)是使用了AND逻辑操作。

### 6.使用Lambda实现Map 和 Reduce ###

最流行的函数编程概念是map，它允许你改变你的对象，在这个案例中，我们将costBeforeTeax集合中每个元素改变了增加一定的数值，我们将Lambda表达式 x -> x\*x传送map()方法，这将应用到stream中所有元素。然后我们使用 forEach() 打印出这个集合的元素.

// applying 12% VAT on each purchase
    // Without lambda expressions:
    List costBeforeTax = Arrays.asList(100, 200, 300, 400, 500);
    for (Integer cost : costBeforeTax) {
          double price = cost + .12*cost;
          System.out.println(price);
    }
    
    // With Lambda expression:
    List costBeforeTax = Arrays.asList(100, 200, 300, 400, 500);
    costBeforeTax.stream().map((cost) -> cost + .12*cost)
                          .forEach(System.out::println);
    
    Output
    112.0
    224.0
    336.0
    448.0
    560.0
    112.0
    224.0
    336.0
    448.0
    560.0

reduce() 是将集合中所有值结合进一个，Reduce类似SQL语句中的sum(), avg() 或count(),

// Applying 12% VAT on each purchase
    // Old way:
    List costBeforeTax = Arrays.asList(100, 200, 300, 400, 500);
    double total = 0;
    for (Integer cost : costBeforeTax) {
     double price = cost + .12*cost;
     total = total + price;
     
    }
    System.out.println("Total : " + total);
    
    // New way:
    List costBeforeTax = Arrays.asList(100, 200, 300, 400, 500);
    double bill = costBeforeTax.stream().map((cost) -> cost + .12*cost)
                                        .reduce((sum, cost) -> sum + cost)
                                        .get();
    System.out.println("Total : " + bill);
    
    Output
    Total : 1680.0 Total : 1680.0

### 7.通过**filtering **创建一个字符串String的集合 ###

Filtering是对大型Collection操作的一个通用操作，Stream提供filter()方法，接受一个Predicate对象，意味着你能传送lambda表达式作为一个过滤逻辑进入这个方法：

// Create a List with String more than 2 characters
    List<String> filtered = strList.stream().filter(x -> x.length()> 2)
                                            .collect(Collectors.toList());
    System.out.printf("Original List : %s, filtered list : %s %n", 
                      strList, filtered);
    
    Output :
    Original List : [abc, , bcd, , defg, jk], filtered list : [abc, bcd, defg]

### 8.对集合中每个元素应用函数 ###

我们经常需要对集合中元素运用一定的功能，如表中的每个元素乘以或除以一个值等等.

// Convert String to Uppercase and join them using coma
    List<String> G7 = Arrays.asList("USA", "Japan", "France", "Germany", 
                                    "Italy", "U.K.","Canada");
    String G7Countries = G7.stream().map(x -> x.toUpperCase())
                                    .collect(Collectors.joining(", "));
    System.out.println(G7Countries);
    
    Output : 
    USA, JAPAN, FRANCE, GERMANY, ITALY, U.K., CANADA

上面是将字符串转换为大写，然后使用逗号串起来。

### 9.通过复制不同的值创建一个子列表 ###

使用Stream的distinct()方法过滤集合中重复元素。

// Create List of square of all distinct numbers
    List<Integer> numbers = Arrays.asList(9, 10, 3, 4, 7, 3, 4);
    List<Integer> distinct = numbers.stream().map( i -> i*i).distinct()
                                             .collect(Collectors.toList());
    System.out.printf("Original List : %s,  Square Without duplicates :
                       %s %n", numbers, distinct);
    
    Output :
    Original List : [9, 10, 3, 4, 7, 3, 4],  Square Without 
                                             duplicates : [81, 100, 9, 16, 49]

### 10.计算List中的元素的最大值，最小值，总和及平均值    ###

//Get count, min, max, sum, and average for numbers
    List<Integer> primes = Arrays.asList(2, 3, 5, 7, 11, 13, 17, 19, 23, 29);
    IntSummaryStatistics stats = primes.stream().mapToInt((x) -> x)
                                                .summaryStatistics();
    System.out.println("Highest prime number in List : " + stats.getMax());
    System.out.println("Lowest prime number in List : " + stats.getMin());
    System.out.println("Sum of all prime numbers : " + stats.getSum());
    System.out.println("Average of all prime numbers : " + stats.getAverage());
    
    Output : 
    Highest prime number in List : 29
    Lowest prime number in List : 2
    Sum of all prime numbers : 129
    Average of all prime numbers : 12.9