轉 Lambda表達式例子
1.Java8 新特性介紹
寫java的同學對java8肯定知道 那么java8到底有哪些特性呢，總結如下：
Lambda表達式
函數式接口
Stream
Optional
Predicate
Function
Consumer
Filter
Map-Reduce
新的Date API取id 列表
List<Integer> transactionsIds = transactions.parallelStream(). filter(t -> t.getType() == Transaction.GROCERY). sorted(comparing(Transaction::getValue).reversed()). map(Transaction::getId). collect(toList());
最核心的當然是函數式編程了，寫代碼非常簡單，請看下面詳細例子介紹
2.Java8 lambda使用總結－結合實例介紹
很多同學一開始接觸Java8可能對Java8 Lambda表達式有點陌生，下面我將結合實例介紹Java8的使用 並與Java7進行比較：
基礎類
@Data
@Builder
@AllArgsConstructor
@NoArgsConstructor
public class Person {
private int id;
private String name;
private String address;
}1.List操作
public class ExampleList {
private static List<String> items = new ArrayList<>();
static {
items.add("A");
items.add("BC");
items.add("C");
items.add("BD");
items.add("E");
}
public static void main(String[] args) {
//Java8之前操作List
for(String item:items){
System.out.println(item);
}
//Java8 lambda遍歷list
items.forEach(c-> System.out.println(c));
items.forEach(item->{
if("C".equals(item)){
System.out.println(item);
}
});
System.out.println("--------");
//先過濾
items.stream().filter(s->s.contains("B")).forEach(c1-> System.out.println(c1));
}
}
2.Map操作
public class ExampleMap {
private static Map<String, Integer> items = new HashMap<>();
static {
items.put("A", 10);
items.put("B", 20);
items.put("C", 30);
items.put("D", 40);
items.put("E", 50);
items.put("F", 60);
}
public static void main(String[] args) {
//Java8之前遍歷是這樣遍歷map
for(Map.Entry<String,Integer> entry:items.entrySet()){
System.out.println("key:" + entry.getKey() + " value:" + entry.getValue());
}
//Java8遍歷map
items.forEach((key,value)-> System.out.println("key:" + key + " value:" + value));
}
}
3.Groupingby操作
/** * *Java8 Collectors.groupingBy and Collectors.mapping example */
public class ExampleMapping {
private static List<Person> personList = Lists.newArrayList();
static {
personList.add(Person.builder().id(10).address("apple").address("shanghai").build());
personList.add(Person.builder().id(12).address("apple").address("wuhan").build());
personList.add(Person.builder().id(16).address("apple").address("nanjing").build());
}
public static void main(String[] args) {
//分組
Map<String, List<Person>> collect = personList.stream().collect(Collectors.groupingBy(c -> c.getAddress()));
System.out.println(collect);
}
}
4.List轉換為Map
public class ExampleListConvertMap {
private static List<Person> personList = Lists.newArrayList();
static{
personList.add(Person.builder().id(20).name("zhangsan").address("shanghai").build());
personList.add(Person.builder().id(30).name("lisi").address("nanjing").build());
}
public static void main(String[] args) {
//Java8 List轉換Map
Map<Integer,Person> map_ = personList.stream().collect(Collectors.toMap((key->key.getId()),(value->value)));
map_.forEach((key,value)-> System.out.println(key + ":" + value));
Map<Integer, Person> mappedMovies = personList.stream().collect(
Collectors.toMap(Person::getRank, Person::getData));
}
}收集成實體本身map
代碼如下：
accounts.stream().collect(Collectors.toMap(Account::getId, account -> account));
account -> account是一個返回本身的lambda表達式，其實還可以使用Function接口中的一個默認方法代替，使整個方法更簡潔優雅：
accounts.stream().collect(Collectors.toMap(Account::getId, Function.identity()));
重復key的情況
代碼如下：
accounts.stream().collect(Collectors.toMap(Account::getUsername, Function.identity()));
這個方法可能報錯（java.lang.IllegalStateException: Duplicate key），因為name是有可能重復的。toMap有個重載方法，可以傳入一個合並的函數來解決key沖突問題：
accounts.stream().collect(Collectors.toMap(Account::getUsername, Function.identity(), (key1, key2) -> key2));
這里只是簡單的使用后者覆蓋前者來解決key重復問題。
指定具體收集的map
toMap還有另一個重載方法，可以指定一個Map的具體實現，來收集數據：
accounts.stream().collect(Collectors.toMap(Account::getUsername, Function.identity(), (key1, key2) -> key2, LinkedHashMap::new))
5.FilterMap操作
public class ExampleFilterMap {
private static Map<Integer,String> map_ = Maps.newHashMap();
static{
map_.put(1, "linode.com");
map_.put(2, "heroku.com");
map_.put(3, "digitalocean.com");
map_.put(4, "aws.amazon.com");
}
public static void main(String[] args) {
//before java iterator map
String result = null;
for(Map.Entry<Integer,String> entry:map_.entrySet()){
if("heroku.com".equals(entry.getValue())){
result = entry.getValue();
}
}
System.out.println("Before Java 8 :" + result);
//Java8 Map->Stream->Filter->String
result = map_.entrySet().stream().
filter(map->"heroku.com".equals(map.getValue()))
.map(map->map.getValue())
.collect(Collectors.joining());
System.out.println("Java 8 :" + result);
Map<Integer,String> collect = map_.entrySet().stream()
.filter(c->c.getKey()==2)
.collect(Collectors.toMap(p->p.getKey(),p->p.getValue()));
System.out.println(collect);
}
}
6.Optional操作可以防止NullPointException
Optional<String> optional = Optional.of("hello");
System.out.println(optional.isPresent());//true
System.out.println(optional.get());//hello
System.out.println(optional.orElse("false"));
optional.ifPresent((s)-> System.out.println(s.charAt(0)));//h
7.給出一個詳細的例子
@Data
@Builder
@AllArgsConstructor
@NoArgsConstructor
public class Employee {
private String name;
private int salary;
private String office;
}
public class ExampleEmployee {
private static List<Employee> employeeList = Lists.newArrayList();
static{
employeeList.add(Employee.builder().name("Matt").salary(5000).office("New York").build());
employeeList.add(Employee.builder().name("Steve").salary(6000).office("London").build());
employeeList.add(Employee.builder().name("Carrie").salary(20000).office("New York").build());
employeeList.add(Employee.builder().name("Peter").salary(7000).office("New York").build());
employeeList.add(Employee.builder().name("Pat").salary(8000).office("London").build());
employeeList.add(Employee.builder().name("Tammy").salary(29000).office("Shanghai").build());
}
public static void main(String[] args) {
//anyMatch
boolean isMatch = employeeList.stream().anyMatch(employee -> employee.getOffice().equals("London"));
System.out.println(isMatch);
//返回所有salary大於6000
boolean matched = employeeList.stream().allMatch(employee -> employee.getSalary()>4000);
System.out.println(matched);
//找出工資最高
Optional<Employee> hightestSalary = employeeList.stream().max((e1,e2)->Integer.compare(e1.getSalary(),e2.getSalary()));
System.out.println(hightestSalary);
//返回姓名列表
List<String> names = employeeList.stream().map(employee -> employee.getName()).collect(Collectors.toList());
System.out.println(names);
//List轉換成Map
Map<String,Employee> employeeMap = employeeList.stream().collect(Collectors.toMap((key->key.getName()),(value->value)));
employeeMap.forEach((key,value)-> System.out.println(key + "=" + value));
//統計辦公室是New York的個數
long officeCount = employeeList.stream().filter(employee -> employee.getOffice().equals("Shanghai")).count();
System.out.println(officeCount);
//List轉換為Set
Set<String> officeSet = employeeList.stream().map(employee -> employee.getOffice()).distinct().collect(Collectors.toSet());
System.out.println(officeSet);
//查找辦公室地點是New York的員工
Optional<Employee> allMatchedEmployees = employeeList.stream().filter(employee -> employee.getOffice().equals("New York")).findAny();
System.out.println(allMatchedEmployees);
//按照工資的降序來列出員工信息
List<Employee> sortEmployeeList = employeeList.stream().sorted((e1,e2)->Integer.compare(e2.getSalary(),e1.getSalary())).collect(Collectors.toList());
//按照名字的升序列出員工信息
List<Employee> sortEmployeeByName = employeeList.stream().sorted((e1,e2)->e1.getName().compareTo(e2.getName())).collect(Collectors.toList());
System.out.println(sortEmployeeList);
System.out.println("按照名字的升序列出員工信息:" + sortEmployeeByName);
//獲取工資最高的前2條員工信息
List<Employee> top2EmployeeList= employeeList.stream()
.sorted((e1,e2)->Integer.compare(e2.getSalary(),e1.getSalary()))
.limit(2)
.collect(Collectors.toList());
System.out.println(top2EmployeeList);
//獲取平均工資
OptionalDouble averageSalary = employeeList.stream().mapToInt(employee->employee.getSalary()).average();
System.out.println("平均工資:" + averageSalary);
//查找New York
OptionalDouble averageSalaryByOffice = employeeList.stream().filter(employee -> employee.getOffice()
.equals("New York"))
.mapToInt(employee->employee.getSalary())
.average();
System.out.println("New York辦公室平均工資:" + averageSalaryByOffice);
}
}
8.Java8常見操作
public class EmployeeTest {
public static List<Employee> generateData() {
return Arrays.asList(new Employee("Matt", 5000, "New York"),
new Employee("Steve", 6000, "London"),
new Employee("Carrie", 10000, "New York"),
new Employee("Peter", 7000, "New York"),
new Employee("Alec", 6000, "London"),
new Employee("Sarah", 8000, "London"),
new Employee("Rebecca", 4000, "New York"),
new Employee("Pat", 20000, "New York"),
new Employee("Tammy", 9000, "New York"),
new Employee("Fred", 15000, "Tokyo"));
}
public static Map<String, Integer> generateMapData() {
Map<String, Integer> items = Maps.newHashMap();
items.put("A", 10);
items.put("B", 20);
items.put("C", 30);
items.put("D", 40);
items.put("E", 50);
items.put("F", 60);
return items;
}
@Test
public void testEmployee() {
List<Employee> results = generateData();
//打印出名字是Steve的員工信息
results.forEach(c -> {
if (c.getName().equals("Steve")) {
System.out.println(c);
}
});
System.out.println("---------");
//找出年薪超過6000的員工
results.stream().filter(c -> c.getSalary() >= 60000).forEach(c -> System.out.println(c));
System.out.println("--->>>>>>----");
//java8遍歷map
Map<String, Integer> map_ = generateMapData();
map_.forEach((key, value) -> System.out.println("key:" + key + "," + "value:" + value));
System.out.println("---->>>>分組>>>-----");
//java8 分組操作
Map<String, List<Employee>> groupMap = results.stream().collect(Collectors.groupingBy(c -> c.getOffice()));
System.out.println(groupMap);
System.out.println("---->>>>List轉化為Map>>>----");
//List轉化Map
Map<String, Object> map = results.stream().collect(Collectors.toMap(Employee::getName, Employee::getOffice));
map.forEach((key, value) -> System.out.println("key:" + key + "," + "value:" + value));
System.out.println("---->>>>>>>----");
Map<Integer, Employee> employeeMap = results.stream().collect(Collectors.toMap((key -> key.getSalary()), (value -> value)));
employeeMap.forEach((key, value) -> System.out.println(key + "," + value));
System.out.println("---->>遍歷map>>>----");
//打印出值大於30的map
Map<String, Integer> resultMap = map_.entrySet().stream().filter(c -> c.getValue() > 30).collect(Collectors.toMap(p -> p.getKey(), p -> p.getValue()));
resultMap.forEach((key, value) -> System.out.println(key + "=" + value));
System.out.println(">>>>>>>>>>>>>>>");
//打印key=D的map
Map<String, Integer> mapResults = map_.entrySet().stream().filter(c -> c.getKey().equals("D")).collect(Collectors.toMap(p -> p.getKey(), p -> p.getValue()));
mapResults.forEach((key, value) -> System.out.println(key + ">>>>" + value));
System.out.println(">>>>>>>Optional>>>>>>>");
Optional<String> optional = Optional.of("hello");
System.out.println(optional.isPresent());
}
@Test
public void testEmployeeExample() {
//anyMatch
List<Employee> employeeList = generateData();
boolean isMatch = employeeList.stream().anyMatch(employee -> employee.getOffice().equals("London"));
System.out.println(isMatch);
//allMatch
boolean matched = employeeList.stream().allMatch(employee -> employee.getOffice().equals("London"));
System.out.println(matched);
//找出工資最高的
Optional<Employee> employeeOptional = employeeList.stream().max((e1,e2)->Integer.compare(e1.getSalary(),e2.getSalary()));
System.out.println(employeeOptional);
//找出工資最少的
Optional<Employee> employee = employeeList.stream().min((e1,e2)->Integer.compare(e1.getSalary(),e2.getSalary()));
System.out.println(employee);
//返回姓名列表
List<String> names = employeeList.stream().map(c->c.getName()).collect(Collectors.toList());
System.out.println(names);
System.out.println(">>>>>>>>>>>");
//List轉化Map
Map<String,Employee> employeeMap = employeeList.stream().collect(Collectors.toMap((key->key.getName()),(value->value)));
employeeMap.forEach((key,value)-> System.out.println(key + "=" + value));
//統計辦公室是New York的個數
long officeCount = employeeList.stream().filter(c->c.getOffice().equals("New York")).count();
System.out.println(officeCount);
long salaryCount = employeeList.stream().filter(c->c.getSalary()>60000).count();
System.out.println(salaryCount);
//List轉化為Set
Set<String> officeSet = employeeList.stream().map(c->c.getOffice()).distinct().collect(Collectors.toSet());
System.out.println(officeSet);
Set<Integer> salarySet = employeeList.stream().map(c->c.getSalary()).distinct().collect(Collectors.toSet());
System.out.println(salarySet);
//查找辦公室地點是New York的員工
Optional<Employee> optionals = employeeList.stream().filter(c->c.getOffice().equals("New York")).findAny();
System.out.println(optionals);
System.out.println(">>>>>工資降序排序>>>>>");
//按照工資的降序來列出員工信息
List<Employee> sortSalaryEmployeeList = employeeList.stream().sorted((e1,e2)->Integer.compare(e2.getSalary(),e1.getSalary())).collect(Collectors.toList());
System.out.println(sortSalaryEmployeeList);
System.out.println(">>>>>姓名升序排序>>>>>");
List<Employee> sortNameEmployeeList = employeeList.stream().sorted((e1,e2)->e1.getName().compareTo(e2.getName())).collect(Collectors.toList());
System.out.println(sortNameEmployeeList);
System.out.println(">>>>獲取工資最高的前2條員工信息");
List<Employee> dispaly2EmployeeList = employeeList.stream().sorted((e1,e2)->Integer.compare(e2.getSalary(),e1.getSalary())).limit(2).collect(Collectors.toList());
System.out.println(dispaly2EmployeeList);
System.out.println(">>>>獲取平均工資");
OptionalDouble averageSalary = employeeList.stream().mapToInt(c->c.getSalary()).average();
System.out.println(averageSalary);
System.out.println(">>>>獲取工作地點的平均工資");
OptionalDouble optionalDouble = employeeList.stream().filter(c->c.getOffice().equals("New York")).mapToInt(c->c.getSalary()).average();
System.out.println(optionalDouble);
System.out.println(">>>>>>Java8 Optional用法>>>>>>");
Optional<String> stringOptional = Optional.of("test");
System.out.println(stringOptional.get());
Optional<String> isOptional = Optional.ofNullable("hello");
System.out.println(isOptional.isPresent());
System.out.println(isOptional.get());
System.out.println(isOptional.orElse("0"));
System.out.println(">>>>>>>>>>>>");
//Optional<String> optionalVal = Optional.of(null);
// System.out.println(optionalVal);
Optional<String> optional = Optional.ofNullable("optional");
System.out.println(optional);
System.out.println(optional.isPresent());
System.out.println(optional.get());
System.out.println(optional.orElse("haha"));
System.out.println(">>>>>>>>>>>>");
Optional<Employee> employeeOptional_ = employeeList.stream().filter(c->c.getOffice().equals("New York")).findFirst();
System.out.println(employeeOptional_);
}
}