Lambda表達式的優勢

Lambda表達式的優勢

1. 匿名內部類和Lambda的對比

//原來的匿名內部類
    @Test
    public void test1(){
        Comparator<Integer> comparator = new Comparator<Integer>() {

            @Override
            public int compare(Integer o1, Integer o2) {
                return Integer.compare(o1, o2);
            }
        };

        TreeSet<Integer> ts = new TreeSet<>(comparator);
    }

//Lambda表達式
    @Test
    public void test2(){
        Comparator<Integer> comparator = (x ,y) -> Integer.compare(x, y);
        TreeSet<Integer> ts = new TreeSet<>(comparator);

    }

2. 用Lambda表達式優化代碼

• 需求1：獲取當前公司中員工年齡大於35的員工信息

• 需求2：獲取當前公司中員工工資大於5000的員工信息

  public class Employee {
  private String name;
  private int age;
  private double salary;
  
  public Employee() {
      super();
  }
  
  public Employee(String name, int age, double salary) {
      super();
      this.name = name;
      this.age = age;
      this.salary = salary;
  }
  
  public String getName() {
      return name;
  }
  
  public void setName(String name) {
      this.name = name;
  }
  
  public int getAge() {
      return age;
  }
  
  public void setAge(int age) {
      this.age = age;
  }
  
  public double getSalary() {
      return salary;
  }
  
  public void setSalary(double salary) {
      this.salary = salary;
  }
  
  @Override
  public String toString() {
      return "Employee{" +
              "name='" + name + '\'' +
              ", age=" + age +
              ", salary=" + salary +
              '}';
  }
  }

  List<Employee> employees = Arrays.asList(
          new Employee("張三", 18 ,9999.99),
          new Employee("李四", 38, 5555.99),
          new Employee("王五", 50, 6666.66),
          new Employee("趙六", 16, 3333.33),
          new Employee("田七", 8, 7777.77)
  );

  實現方式

  @Test
  public void test3(){
              //獲取年齡大於35歲的員工信息
      List<Employee> employeeList = filterEmployee(employees);
      System.out.println("年齡大於35歲的員工信息：");
      for (Employee employee : employeeList){
          System.out.println(employee);
      }
  
      //獲取當前公司中員工工資大於5000的員工信息
      List<Employee> employeeList1 = filterEmployees2(employees);
      System.out.println("工資大於5000的員工信息：");
      for (Employee employee : employeeList1){
          System.out.println(employee);
      }
  }
  
  獲取當前公司中員工年齡大於35的員工信息
  public List<Employee> filterEmployee(List<Employee> employeeList){
      List<Employee> emps = new ArrayList<>();
  
      for (Employee emp : employeeList){
          if (emp.getAge() >= 35){
              emps.add(emp);
          }
      }
      return emps;
  }
  
  獲取當前公司中員工工資大於5000的員工信息
  public List<Employee> filterEmployees2(List<Employee> employeeList){
  
      ArrayList<Employee> employees = new ArrayList<>();
      for (Employee employee : employeeList){
          if (employee.getSalary() >= 5000){
              employees.add(employee);
          }
      }
      return employees;
  }

  優化方案一：策略設計模式

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

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

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

  @Test
  public void test4(){
      List<Employee> employeeList = filterEmployee(employees, new FilterEmployeeByAge());
      System.out.println("年齡大於35歲的員工信息：");
      for (Employee employee : employeeList){
          System.out.println(employee);
      }
  
      System.out.println("工資大於5000的員工信息：");
      List<Employee> employeeList1 = filterEmployee(this.employees, new FilterEmployeeBySalary());
  
      for (Employee employee : employeeList1){
          System.out.println(employee);
      }
  
  }
  public List<Employee> filterEmployee(List<Employee> employeeList, MyPredicate<Employee> mp){
      ArrayList<Employee> employees = new ArrayList<>();
  
      for (Employee employee : employeeList){
          if (mp.test(employee)){
              employees.add(employee);
          }
      }
      return employees;
  }

  優化方案二：匿名內部類

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

  @Test
  public void test5(){
      List<Employee> employeeList = filterEmployee(employees, new MyPredicate<Employee>() {
          @Override
          public boolean test(Employee employee) {
              return employee.getAge() >= 35;
          }
      });
      System.out.println("年齡大於35歲的員工信息：");
      for (Employee employee : employeeList){
          System.out.println(employee);
      }
  
      List<Employee> employeeList1 = filterEmployee(employees, new MyPredicate<Employee>() {
          @Override
          public boolean test(Employee employee) {
              return employee.getSalary() >= 5000;
          }
      });
      System.out.println("工資大於5000的員工信息：");
      for (Employee employee : employeeList1){
          System.out.println(employee);
      }
  }

  優化方案三：Lambda表達式

  public List<Employee> filterEmployee(List<Employee> employeeList, MyPredicate<Employee> mp){
      ArrayList<Employee> employees = new ArrayList<>();
  
      for (Employee employee : employeeList){
          if (mp.test(employee)){
              employees.add(employee);
          }
      }
      return employees;
  }

  @Test
  public void test6(){
      System.out.println("年齡大於35歲的員工信息：");
      List<Employee> employeeList = filterEmployee(employees, (e) -> e.getAge() >= 35);
      employeeList.forEach(System.out::println);
  
      System.out.println("工資大於5000的員工信息：");
      List<Employee> employeeList1 = filterEmployee(employees, (e) -> e.getSalary() >= 5000);
      employeeList1.forEach(System.out::println);
  }

  優化方案四：Lambda表達式和Stream API

  @Test
  public void test7(){
      System.out.println("年齡大於35歲的員工信息：");
      employees.stream()
               .filter((e) -> e.getAge() >= 35)
               .forEach(System.out::println);
  
      System.out.println("工資大於5000的員工信息：");
      employees.stream()
               .filter((e) -> e.getSalary() >= 5000)
               .forEach(System.out::println);
  }