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1-Lambda表达式
为什么使用 Lambda 表达式
Lambda是一个匿名函数，我们可以把 Lambda表达式理解为是一段可以传递的代码（将代码像数据一样进行传递）。可以写出更简洁、更灵活的代码。作为一种更紧凑的代码风格，使Java的语言表达能力得到了提升。
package day01.com.lm.java8;
import java.util.Objects;
public class Employee {
private int id;
private String name;
private Integer age;
private double salary;
private Status status;
public Employee(String name, Integer age, double salary, Status status) {
this.name = name;
this.age = age;
this.salary = salary;
this.status = status;
}
public Employee() {
super();
}
public Employee(int id) {
this.id = id;
}
public Employee(String name, Integer age, double salary) {
this.name = name;
this.age = age;
this.salary = salary;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public Integer getAge() {
return age;
}
public void setAge(Integer age) {
this.age = age;
}
public double getSalary() {
return salary;
}
public void setSalary(double salary) {
this.salary = salary;
}
public Status getStatus() {
return status;
}
public void setStatus(Status status) {
this.status = status;
}
@Override
public String toString() {
return "Employee{" +
"id=" + id +
", name='" + name + '\'' +
", age=" + age +
", salary=" + salary +
", status=" + status +
'}';
}
@Override
public boolean equals(Object o) {
if (this == o) return true;
if (o == null || getClass() != o.getClass()) return false;
Employee employee = (Employee) o;
return id == employee.id &&
age == employee.age &&
Double.compare(employee.salary, salary) == 0 &&
Objects.equals(name, employee.name);
}
@Override
public int hashCode() {
return Objects.hash(id, name, age, salary);
}
public enum Status {
FREE,
BUSY,
VOCATION
}
}
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package day01.com.lm.java8;
public interface MyPredicate {
public boolean test(T t);
}
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package day01.com.lm.java8;
public class FilterEmployeeByAge implements MyPredicate {
@Override
public boolean test(Employee t) {
return t.getAge() >= 35;
}
}
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package day01.com.lm.java8;
public class FilterEmployeeBySalary implements MyPredicate {
@Override
public boolean test(Employee t) {
return t.getSalary() >= 5000;
}
}
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package day01.com.lm.java8;
import org.junit.Test;
import java.util.*;
public class TestLambda1 {
//原来的匿名内部类
public void test1() {
Comparator<Integer> com = new Comparator<Integer>() {
@Override
public int compare(Integer o1, Integer o2) {
return Integer.compare(o1, o2);
}
};
TreeSet<Integer> ts = new TreeSet<>(com);
}
//Lambda 表达式
public void test2() {
Comparator<Integer> com = (x, y) -> Integer.compare(x, y);
TreeSet<Integer> ts = new TreeSet<>(com);
}
List<Employee> employees = Arrays.asList(
new Employee("张三", 18, 9999.99),
new Employee("李四", 38, 5555.99),
new Employee("王五", 50, 6666.66),
new Employee("赵六", 16, 3333.33),
new Employee("田七", 8, 7777.77)
);
//需求：获取当前公司中员工年龄大于35的员工信息
@Test
public void test3() {
List<Employee> list = filterEmployees(this.employees);
for (Employee employee : list) {
System.out.println(employee);
}
}
public List<Employee> filterEmployees(List<Employee> list) {
List<Employee> emps = new ArrayList<>();
for (Employee emp : list) {
if (emp.getAge() >= 35) {
emps.add(emp);
}
}
return emps;
}
//需求：获取当前公司中员工工资大于5000的员工信息
public List<Employee> filterEmployees2(List<Employee> list) {
List<Employee> emps = new ArrayList<>();
for (Employee emp : list) {
if (emp.getSalary() >= 5000) {
emps.add(emp);
}
}
return emps;
}
//优化方式一：策略设计模式
@Test
public void test4() {
List<Employee> list = filterEmployee(employees, new FilterEmployeeByAge());
for (Employee employee : list) {
System.out.println(employee);
}
System.out.println("-----------------------------------");
List<Employee> list2 = filterEmployee(employees, new FilterEmployeeBySalary());
for (Employee employee : list2) {
System.out.println(employee);
}
}
public List<Employee> filterEmployee(List<Employee> list, MyPredicate<Employee> mp) {
List<Employee> emps = new ArrayList<>();
for (Employee employee : list) {
if (mp.test(employee)) {
emps.add(employee);
}
}
return emps;
}
//优化方式二：匿名内部类
@Test
public void test5() {
List<Employee> list = filterEmployee(employees, new MyPredicate<Employee>() {
@Override
public boolean test(Employee t) {
return t.getSalary() <= 5000;
}
});
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(employees, (e) -> e.getSalary() <= 5000);
list.forEach(System.out::println);
}
//优化方式四：Stream API
@Test
public void test7() {
employees.stream()
.filter((e) -> e.getSalary() >= 5000)
.limit(2)
.forEach(System.out::println);
System.out.println("----------------------------------------");
employees.stream()
.map(Employee::getName)
.forEach(System.out::println);
}
}
cs