简介
“Lambda 表达式”(lambda expression)是一个匿名函数，Lambda表达式基于数学中的λ演算得名，直接对应于其中的lambda抽象(lambda abstraction)，是一个匿名函数，即没有函数名的函数。Lambda表达式可以表示闭包（注意和数学传统意义上的不同）。
基本语句
Runnable noArguments = () -> System.out.println("Hello World");
ActionListener oneArgument = event -> System.out.println("button clicked");
Runnable multiStatement = () -> {
System.out.print("Hello");
System.out.println(" World");
};
BinaryOperator<Long> add = (x, y) -> x + y; q
BinaryOperator<Long> addExplicit = (Long x, Long y) -> x + y;
编写线程
Runnable runnable = () -> {
System.out.println("Lambda 创建线程");
};
Thread thread = new Thread(runnable);
thread.start();
利用Lambda运算
BinaryOperator<Integer> add = (x, y) -> x + y;
Integer x = add.apply(20, 30);
统计数组中某元素出现的个数
String name[] = {"张三", "李四", "王五", "孙刘", "赵强", "李明", "赵强", "汪汪"};
long num = Arrays.stream(name).filter(x -> x.equals("赵强")).count();
数组去重并转化成集合存储
String name[] = {"张三", "李四", "王五", "孙刘", "赵强", "李明", "赵强", "汪汪"};
List<String> stringList = Arrays.stream(name).filter(x -> !x.equals("赵强")).collect(toList());
数组去重，并对元素加后缀
String name[] = {"张三", "李四", "王五", "孙刘", "赵强", "李明", "赵强", "汪汪"};
List<String> stringList2 = Arrays.stream(name).filter(x -> !x.equals("赵强")).map(y -> y + "加后缀：").collect(toList());
数组求和
int[] a = {1, 2, 3, 4, 5, 6, 7, 8, 3, 4, 5, 6, 7, 8, 9, 10 };
int count = 0;
// for循环
for (int i = 0; i < a.length; i++) {
count += a[i];
}
// foreach
count = 0;
for (int i = 0; i < a.length; i++) {
count += a[i];
}
// Lambda
int num = Arrays.stream(a).reduce(0, (b, c) -> b + c);
Lambda 的提取对象字段，并拼接
class User{
private String username;
private int age;
public User(){}
public User(String username, int age) {
this.username = username;
this.age = age;
}
public String getUsername() {
return username;
}
public void setUsername(String username) {
this.username = username;
}
public int getAge() {
return age;
}
public void setAge(int age) {
this.age = age;
}
}
List<User> userList = asList(new User("张三" , 10) , new User("李四" , 10) , new User("王五" , 13));
String result = userList.stream().map(User::getUsername).collect(Collectors.joining("," , "[" , "]"));
输出结果：[张三,李四,王五]
Lambda groupBy
List<User> userList = asList(new User("张三" , 10) , new User("李四" , 10) , new User("王五" , 13));
Stream<User> userStream = userList.stream();
Map<Integer , List<String>> integerStringMap = userStream.collect(groupingBy(user->user.getAge() , mapping(User::getUsername , toList())));
System.out.println(integerStringMap.toString());
输出结果：{10=[张三, 李四], 13=[王五]}
String result2 =
userList.stream().map(User::getUsername)
.reduce(new StringJoiner(",1 ", "[", "]"),
StringJoiner::add,
StringJoiner::merge)
.toString();
System.out.println(result2);
输出结果：[张三,1 李四,1 王五]
