java8新特性（Lambda表達式、四大核心內置對象）

 

 

java8新特性的主要表現：

　　一、Lambda表達式

　　二、函數式接口

　　三、方法引用和構造器引用

　　四、Stream API

　　五、接口中的默認方法與靜態方法

　　六、新時間日期API

 

 

Lambda表達式：

//原來的匿名內部類  @Test public void test1(){ Comparator<String> com = new Comparator<String>(){ @Override public int compare(String o1, String o2) { return Integer.compare(o1.length(), o2.length()); } }; TreeSet<String> ts = new TreeSet<>(com); TreeSet<String> ts2 = new TreeSet<>(new Comparator<String>(){ @Override public int compare(String o1, String o2) { return Integer.compare(o1.length(), o2.length()); } }); }

 

//如今的 Lambda 表達式  @Test public void test2(){ Comparator<String> com = (x, y) -> Integer.compare(x.length(), y.length()); TreeSet<String> ts = new TreeSet<>(com); }

 

List<Employee> emps = Arrays.asList( new Employee(101, "張三", 18, 9999.99), new Employee(102, "李四", 59, 6666.66), new Employee(103, "王五", 28, 3333.33), new Employee(104, "趙六", 8, 7777.77), new Employee(105, "田七", 38, 5555.55) ); //需求：獲取公司中年齡小於 35 的員工信息 public List<Employee> filterEmployeeAge(List<Employee> emps){ List<Employee> list = new ArrayList<>(); for (Employee emp : emps) { if(emp.getAge() <= 35){ list.add(emp); } } return list; } @Test public void test3(){ List<Employee> list = filterEmployeeAge(emps); for (Employee employee : list) { System.out.println(employee); } } //需求：獲取公司中工資大於 5000 的員工信息 public List<Employee> filterEmployeeSalary(List<Employee> emps){ List<Employee> list = new ArrayList<>(); for (Employee emp : emps) { if(emp.getSalary() >= 5000){ list.add(emp); } } return list; }

 

public interface MyPredicate<T> { public boolean test(T t); } public class FilterEmployeeForAge implements MyPredicate<Employee>{ @Override public boolean test(Employee t) { return t.getAge() <= 35; } } public class FilterEmployeeForSalary implements MyPredicate<Employee> { @Override public boolean test(Employee t) { return t.getSalary() >= 5000; } }

 

//優化方式一：策略設計模式 public List<Employee> filterEmployee(List<Employee> emps, MyPredicate<Employee> mp){ List<Employee> list = new ArrayList<>(); for (Employee employee : emps) { if(mp.test(employee)){ list.add(employee); } } return list; } @Test public void test4(){ List<Employee> list = filterEmployee(emps, new FilterEmployeeForAge()); for (Employee employee : list) { System.out.println(employee); } System.out.println("------------------------------------------"); List<Employee> list2 = filterEmployee(emps, new FilterEmployeeForSalary()); for (Employee employee : list2) { System.out.println(employee); } }

 

//優化方式二：匿名內部類  @Test public void test5(){ List<Employee> list = filterEmployee(emps, new MyPredicate<Employee>() { @Override public boolean test(Employee t) { return t.getId() <= 103; } }); for (Employee employee : list) { System.out.println(employee); } }

 

//優化方式三：Lambda 表達式  @Test public void test6(){ List<Employee> list = filterEmployee(emps, (e) -> e.getAge() <= 35); list.forEach(System.out::println); System.out.println("------------------------------------------"); List<Employee> list2 = filterEmployee(emps, (e) -> e.getSalary() >= 5000); list2.forEach(System.out::println); }

 

//優化方式四：Stream API  @Test public void test7(){ emps.stream() .filter((e) -> e.getAge() <= 35) .forEach(System.out::println); System.out.println("----------------------------------------------"); emps.stream() .map(Employee::getName) .limit(3) .sorted() .forEach(System.out::println); }

 

Lambda表達式的基礎語法：

　　->：稱爲箭頭操做符，或者lambda操做符，箭頭操做符把表達式拆分紅兩份

　　　　左側：表達式的參數列表

　　　　右側：表達式須要執行的功能， 即Lambda體

 

語法格式一：無參數，無返回值     () -> System.out.println("Hello Lambda!");

@Test public void test1(){ int num = 0;//jdk 1.7 前，必須是 final Runnable r = new Runnable() { @Override public void run() { System.out.println("Hello World!" + num); } }; r.run(); System.out.println("-------------------------------"); Runnable r1 = () -> System.out.println("Hello Lambda!"); r1.run(); }

 

語法格式二：有一個參數，而且無返回值      (x) -> System.out.println(x)

@Test public void test2(){ Consumer<String> con = (x) -> System.out.println(x); con.accept("好好學習，每天向上！"); }

 

語法格式三：若只有一個參數，小括號能夠省略不寫    x -> System.out.println(x)

@Test public void test2(){ Consumer<String> con = x -> System.out.println(x); con.accept("xxx"); }

 

語法格式四：有兩個以上的參數，有返回值，而且 Lambda 體中有多條語句

@Test public void test3(){ Comparator<Integer> com = (x, y) -> { System.out.println("xxxx"); return Integer.compare(x, y); }; }

 

語法格式五：若 Lambda 體中只有一條語句， return 和 大括號均可以省略不寫

@Test public void test4(){ Comparator<Integer> com = (x, y) -> Integer.compare(x, y); }

 

語法格式六：Lambda 表達式的參數列表的數據類型能夠省略不寫，由於JVM編譯器經過上下文推斷出，數據類型，即「類型推斷」   (Integer x, Integer y) -> Integer.compare(x, y);

 

@Test public void test5(){ String[] strs = {"aaa", "bbb", "ccc"}; List<String> list = new ArrayList<>(); show(new HashMap<>()); } public void show(Map<String, Integer> map){ }

 

 

lambda表達式須要函數式接口的支持：

函數式接口：接口中只有一個抽象方法的接口，稱爲函數式接口，可使用註解 @FunctionalInterface 修飾，能夠檢查是不是函數式接口

@FunctionalInterface public interface Fun { public Integer getValue(Integer num); }

//需求：對一個數進行運算  @Test public void test6(){ Integer num = operation(100, (x) -> x * x); System.out.println(num); System.out.println(operation(200, (y) -> y + 200)); } public Integer operation(Integer num, Fun mf){ return mf.getValue(num); }

 

//比較兩個實體，先按照年齡比較，相同則按照姓名比較 List<Employee> emps = Arrays.asList( new Employee(101, "張三", 18, 9999.99), new Employee(102, "李四", 59, 6666.66), new Employee(103, "王五", 28, 3333.33), new Employee(104, "趙六", 8, 7777.77), new Employee(105, "田七", 38, 5555.55) ); @Test public void test1(){ Collections.sort(emps, (e1, e2) -> { if(e1.getAge() == e2.getAge()){ return e1.getName().compareTo(e2.getName()); }else{ return -Integer.compare(e1.getAge(), e2.getAge()); } }); for (Employee emp : emps) { System.out.println(emp); } } 

 

//字符串操做
@FunctionalInterface public interface MyFunction { public String getValue(String str); } //需求：用於處理字符串 public String strHandler(String str, MyFunction mf){ return mf.getValue(str); } @Test public void test2(){ String trimStr = strHandler("\t\t\tjava天下第一 ", (str) -> str.trim()); System.out.println(trimStr); String upper = strHandler("abcdef", (str) -> str.toUpperCase()); System.out.println(upper); String newStr = strHandler("PHP天下第一", (str) -> str.substring(2, 5)); System.out.println(newStr); }

 

public interface MyFunction2<T, R> { public R getValue(T t1, T t2); } //需求：對於兩個 Long 型數據進行處理 public void op(Long l1, Long l2, MyFunction2<Long, Long> mf){ System.out.println(mf.getValue(l1, l2)); } @Test public void test3(){ op(100L, 200L, (x, y) -> x + y); op(100L, 200L, (x, y) -> x * y); }

 

、

java8中內置的四大函數式接口

Customer<T> 消費型接口

　　void accept( T t);

//Consumer<T> 消費型接口 :  @Test public void test1(){ happy(10000, (m) -> System.out.println("這次購物消費" + m + "元")); } public void happy(double money, Consumer<Double> con){ con.accept(money); }

 

Supplier( T t )  供給型接口

　　T get();

//Supplier<T> 供給型接口 :  @Test public void test2(){ List<Integer> numList = getNumList(10, () -> (int)(Math.random() * 100)); for (Integer num : numList) { System.out.println(num); } } //需求：產生指定個數的整數，並放入集合中 public List<Integer> getNumList(int num, Supplier<Integer> sup){ List<Integer> list = new ArrayList<>(); for (int i = 0; i < num; i++) { Integer n = sup.get(); list.add(n); } return list; }

 

Function<T, R>:函數型接口

　　R apply(T t);

//Function<T, R> 函數型接口：  @Test public void test3(){ String newStr = strHandler("\t\t\t 我大尚硅谷威武 ", (str) -> str.trim()); System.out.println(newStr); String subStr = strHandler("我大尚硅谷威武", (str) -> str.substring(2, 5)); System.out.println(subStr); } //需求：用於處理字符串 public String strHandler(String str, Function<String, String> fun){ return fun.apply(str); }

 

 

Predicate<T>:斷言型接口

　　boolean test(T t);

//Predicate<T> 斷言型接口：  @Test public void test4(){ List<String> list = Arrays.asList("java", "php", "android", "ios", ".net"); List<String> strList = filterStr(list, (s) -> s.length() > 4); for (String str : strList) { System.out.println(str); } } //需求：將知足條件的字符串，放入集合中 public List<String> filterStr(List<String> list, Predicate<String> pre){ List<String> strList = new ArrayList<>(); for (String str : list) { if(pre.test(str)){ strList.add(str); } } return strList; }