java8:Lambda表达式、函数式接口

怀梦想,致远方 2020-11-11 13:49:17
java java8 Lambda 表达式 表达


1、

Comparator接口:

@FunctionalInterface
public interface Comparator<T> {
int compare(T o1, T o2);

(1)匿名内部类

 @Test
public void test1() {
Comparator<Integer> com = new Comparator<Integer>() {
@Override
public int compare(Integer o1, Integer o2) {
return Integer.compare(o1, o2);
}
};
}

(2)Lambda表达式

 @Test
public void test2(){
Comparator<Integer> com=(o1,o2)->Integer.compare(o1,o2);
}

(3)案例(获取年龄大于35的员工信息)

常规方式:

定义一个员工类,遍历所有员工的信息,将年龄大于35的员工的信息存储到新的集合中。但是,如果把需求改为获取工资大于5000的员工信息,只需要改变的是程序的判断条件,这就造成了代码的冗余

设计模式:策略设计模式

新建一个员工类

@Data
@AllArgsConstructor
public class Employee {
private int id;
private String name;
private int age;
private double salary;
}

创建一个接口,该接口定义一个用于过滤的方法

@FunctionalInterface
public interface MyPredicate<T> {
public boolean test(T t);
}

创建两个接口的实现类:实现类重写接口的方法分别过滤年龄和工资

工资

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

年龄

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

测试类:

添加员工的信息

定义过滤方法,该方法的第一个参数是一个Employee类型的List集合,第二个参数是一个接口类型的,也就是说可以传递该接口的所有实现类,也就是说没有将方法写死,方法的灵活度更高了

第三个方法是测试类,传递员工的信息和接口的实现类,返回的是过滤后的结果

public class TestLambda {
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)
);
public List<Employee> filterEmployee(List<Employee> list, MyPredicate<Employee> myPredicate) {
List<Employee> employees = new ArrayList<>();
for (Employee employee : list) {
if (myPredicate.test(employee)) {
employees.add(employee);
}
}
return employees;
}
@Test
public void test() {
List<Employee> employeeList = filterEmployee(emps, new FilterEmployeeForAge());
for (Employee employee : employeeList) {
System.out.println(employee);
}
System.out.println("-------------------");
List<Employee> list = filterEmployee(emps, new FilterEmployeeForSalary());
for (Employee employee : list) {
System.out.println(employee);
}
}

测试结果:

Employee(id=101, name=张三, age=18, salary=9999.99)
Employee(id=103, name=王五, age=28, salary=3333.33)
Employee(id=104, name=赵六, age=8, salary=7777.77)
-------------------
Employee(id=101, name=张三, age=18, salary=9999.99)
Employee(id=102, name=李四, age=59, salary=6666.66)
Employee(id=104, name=赵六, age=8, salary=7777.77)
Employee(id=105, name=田七, age=38, salary=5555.55)

匿名内部类方式

public List<Employee> filterEmployee(List<Employee> list, MyPredicate<Employee> myPredicate) {
List<Employee> employees = new ArrayList<>();
for (Employee employee : list) {
if (myPredicate.test(employee)) {
employees.add(employee);
}
}
return employees;
}
@Test
public void test() {
List<Employee> list = filterEmployee(emps, new MyPredicate<Employee>() {
@Override
public boolean test(Employee employee) {
return employee.getSalary() < 7000;
}
});
for (Employee employee : list) {
System.out.println(employee);
}
}

匿名内部类方式与策略模式相比,少了对实现类的书写,但是匿名内部类的方式还是不够简洁

Lambda表达式

 public List<Employee> filterEmployee(List<Employee> list, MyPredicate<Employee> myPredicate) {
List<Employee> employees = new ArrayList<>();
for (Employee employee : list) {
if (myPredicate.test(employee)) {
employees.add(employee);
}
}
return employees;
}
@Test
public void test() {
List<Employee> list = filterEmployee(emps,(e)->e.getSalary()<7000);
for (Employee employee : list) {
System.out.println(employee);
}
}

与函数时接口的方式相比,Lambda表达式的方式使得代码更加简洁

最简洁的方式:Stream API

 @Test
public void test() {
emps.stream()
.filter((e)->e.getSalary()>=5000)
.forEach(System.out::println);
}

此种方式只需要一个实体类

 

2、函数式接口

(1)定义一个函数式接口

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

(2)定义方法并测试

public class TestLambda {
public Integer operation(Integer num, MyFun mf) {
return mf.getValue(num);
}
@Test
public void test() {
Integer num = operation(100, (x) -> x * x);
System.out.println(num);
System.out.println(operation(200, (y) -> y + 200));
}
}

方法一的参数是一个接口类型的,该方法调用了接口内的方法

方法二是测试方法,Lambda表达式需要传递一个参数

 

3、Lambda表达式的应用

(1)排序

函数式接口

int compare(T o1, T o2);

调用函数式接口的方法

 public static <T> void sort(List<T> list, Comparator<? super T> c) {
list.sort(c);
}

测试类

 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);
}
}

(2)处理字符串

定义一个函数式接口

@FunctionalInterface
public interface MyFunction {
public String getValue(String str);
}

定义字符串处理方法和测试方法

public class TestLambda {
public String strHandler(String str, MyFunction mf){
return mf.getValue(str);
}
@Test
public void test2(){
String trimStr = strHandler(" 你好 ", (str) -> str.trim());
System.out.println(trimStr);
String upper = strHandler("qqqqeeeabc", (str) -> str.toUpperCase());
System.out.println(upper);
String newStr = strHandler("1234556", (str) -> str.substring(2, 5));
System.out.println(newStr);
}
}

(3)数据处理

函数值接口

public interface MyFunction2<T, R> {
public R getValue(T t1, T t2);
}

测试类

public class TestLambda {
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);
}
}

 

版权声明
本文为[怀梦想,致远方]所创,转载请带上原文链接,感谢
https://www.cnblogs.com/zhai1997/p/13958240.html

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