java8 lambda

最近公司项目升级成了jdk8，看了一下jdk8的新特，感觉还不错，尤其是lambda的表达式，写完以后代码比较简洁，并且还有比较高深的样子，有木有，下面整理一下平常使用到的以及在网上看到的表达式方便以后使用。

1. 线程

java8 之前：
new Thread(new Runnable() {
    @Override
    public void run() {
        System.out.println("Before Java8, too much code for too little to do");
    }
}).start();

java8：
new Thread( () -> System.out.println("In Java8, Lambda expression rocks !!") ).start();

2. 事件监听代码

java8 之前：
JButton show =  new JButton("Show");
show.addActionListener(new ActionListener() {
    @Override
    public void actionPerformed(ActionEvent e) {
    System.out.println("Event handling without lambda expression is boring");
    }
});

java8：
show.addActionListener((e) -> {
    System.out.println("Light, Camera, Action !! Lambda expressions Rocks");
});

3. 操作集合

java8 之前：
List features = Arrays.asList("Lambdas", "Default Method", "Stream API", "Date and Time API");
for (String feature : features) {
    System.out.println(feature);
}

java8：
List features = Arrays.asList("Lambdas", "Default Method", "Stream API", "Date and Time API");
features.forEach(n -> System.out.println(n));

使用Java 8的方法引用更方便，方法引用由::双冒号操作符标示：
features.forEach(System.out::println);

4. 过滤集合

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 + " ");
        }
 }

输入结果：
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

更简便的方法：
public static void filter(List names, Predicate condition) {
    names.stream().filter((name) -> (condition.test(name))).forEach((name) -> {
        System.out.println(name + " ");
    });
}

表达式中加入Predicate：
例如要找到所有以J开始，长度为四个字母的名字，你可以合并两个Predicate并传入：
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));

6.Java 8中使用lambda表达式的Map和Reduce示例

不使用lambda表达式为每个订单加上12%的税：
List costBeforeTax = Arrays.asList(100, 200, 300, 400, 500);
for (Integer cost : costBeforeTax) {
    double price = cost + .12*cost;
    System.out.println(price);
}

使用lambda表达式：
List costBeforeTax = Arrays.asList(100, 200, 300, 400, 500);
costBeforeTax.stream().map((cost) -> cost + .12*cost).forEach(System.out::println);

使用lambda表达式的Map和Reduce
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);

7. 通过过滤创建一个String集合

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);

8. 对集合的每个元素应用方法

将字符串换成大写并用逗号链接起来：
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);

9. 复制不同的值，创建一个子列表

用所有不同的数字创建一个正方形列表：
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);

10. 计算集合元素的最大值、最小值、总和以及平均值

获取数字的个数、最小值、最大值、总和以及平均值：
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());