java8新特性（Lambda表达式、四大核心内置对象）

it2022-05-05 119

java8新特性的主要表现：

　　1、Lambda表达式

　　2、函数式接口

　　3、方法引用和构造器引用

　　4、Stream API

　　5、接口中的默认方法与静态方法

　　6、新时间日期API

Lambda表达式：

//原来的匿名内部类 @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()); } }); }

//现在的 Lambda 表达式 @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, "张三", 18, 9999.99), new Employee(102, "李四", 59, 6666.66), new Employee(103, "王五", 28, 3333.33), new Employee(104, "赵六", 8, 7777.77), new Employee(105, "田七", 38, 5555.55) ); //需求：获取公司中年龄小于 35 的员工信息 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：获取公司中工资大于 5000 的员工信息 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; } }

//优化方式一：策略设计模式 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); } }

//优化方式二：匿名内部类 @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); } }

//优化方式三：Lambda 表达式 @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); }

//优化方式四：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表达式的基础语法：

　　->：称为箭头操作符，或者lambda操作符，箭头操作符把表达式拆分成两份

　　　　左侧：表达式的参数列表

　　　　右侧：表达式需要执行的功能，即Lambda体

语法格式一：无参数，无返回值 () -> System.out.println("Hello Lambda!");

@Test public void test1(){ int num = 0;//jdk 1.7 前，必须是 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(); }

语法格式二：有一个参数，并且无返回值 (x) -> System.out.println(x)

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

语法格式三：若只有一个参数，小括号可以省略不写 x -> System.out.println(x)

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

语法格式四：有两个以上的参数，有返回值，并且 Lambda 体中有多条语句

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

语法格式五：若 Lambda 体中只有一条语句， return 和大括号都可以省略不写

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

语法格式六：Lambda 表达式的参数列表的数据类型可以省略不写，因为JVM编译器通过上下文推断出，数据类型，即“类型推断” (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表达式需要函数式接口的支持：

函数式接口：接口中只有一个抽象方法的接口，称为函数式接口，可以使用注解 @FunctionalInterface 修饰，可以检查是否是函数式接口

@FunctionalInterface public interface Fun { public Integer getValue(Integer num); } //需求：对一个数进行运算 @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); }

//比较两个实体，先按照年龄比较，相同则按照姓名比较 List<Employee> emps = Arrays.asList( new Employee(101, "张三", 18, 9999.99), new Employee(102, "李四", 59, 6666.66), new Employee(103, "王五", 28, 3333.33), new Employee(104, "赵六", 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); } }

//字符串操作@FunctionalInterface public interface MyFunction { public String getValue(String str); } //需求：用于处理字符串 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：对于两个 Long 型数据进行处理 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中内置的四大函数式接口

Customer<T> 消费型接口

　　void accept( T t);

//Consumer<T> 消费型接口 : @Test public void test1(){ happy(10000, (m) -> System.out.println("此次购物消费" + m + "元")); } public void happy(double money, Consumer<Double> con){ con.accept(money); }

Supplier( T t ) 供给型接口

　　T get();

//Supplier<T> 供给型接口 : @Test public void test2(){ List<Integer> numList = getNumList(10, () -> (int)(Math.random() * 100)); for (Integer num : numList) { System.out.println(num); } } //需求：产生指定个数的整数，并放入集合中 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>:函数型接口

　　R apply(T t);

//Function<T, R> 函数型接口： @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：用于处理字符串 public String strHandler(String str, Function<String, String> fun){ return fun.apply(str); }

Predicate<T>:断言型接口

　　boolean test(T t);

//Predicate<T> 断言型接口： @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：将满足条件的字符串，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; }

转载于:https://www.cnblogs.com/lzb0803/p/9064735.html

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