java8新特性（Lambda表达式、四大核心内置对象）

 

 

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("			java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("			 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;
    }