五、JKD1.8—Lambda表達式和Stream API的簡介

Lambda 簡介：

Lambda 是一個匿名函數，我們可以把 Lambda 表達式理解爲是一段可以傳遞的代碼（將代碼像數據一樣進行傳遞）。可以寫出更簡潔、更靈活的代碼。作爲一種更緊湊的代碼風格，使Java的語言表達能力得到了提升。

public class TestLDemo {
	
	@Test
	public void test1(){
		////之前的匿名內部類示例1
		Comparator<String> com = new Comparator<String>() {
			@Override
			public int compare(String o1, String o2) {
				return o1.compareTo(o2);
			}
		};
		TreeSet<String> ts = new TreeSet<>(com);

		//之前的匿名內部類示例2
		List<Integer> list = Arrays.asList(5, 9, 6, 2, 1, 8);
		for (Integer num:list) {
			System.out.println(num);//5 9 6 2 1 8
		}

		System.out.println("-------------------------------------");
		Collections.sort(list, new Comparator<Integer>() {
			@Override
			public int compare(Integer o1, Integer o2) {
				return o1.compareTo(o2);
			}
		});
		for (Integer num : list) {
			System.out.println(num);//1 2 5 6 8 9
		}

	}

	@Test
	public void test2(){
		//Lambda表達式示例1
		Comparator<String> com = (x,y)-> x.compareTo(y);
		TreeSet<String> set = new TreeSet<>(com);
		//或者
		TreeSet<String> set1 = new TreeSet<>((x, y) -> x.compareTo(y));

		//Lambda表達式示例2
		List<Integer> list = Arrays.asList(5,9,6,2,1,8);

		for (Integer num : list) {
			System.out.println(num);//5 9 6 2 1 8
		}
		System.out.println("*******************************");
		Collections.sort(list,(x,y)-> x.compareTo(y));
		for (Integer num : list) {
			System.out.println(num);//1 2 5 6 8 9
		}
	}
}

下面通過策略設計模式優化代碼舉例子說明Lambda和Stream API（示例來自於尚硅谷課程）

1、定義實體類

      需求

            a、獲取公司中年齡小於 35 的員工信息

            b、獲取公司中工資大於 5000 的員工信息

public class Employee {

	private int id;
	private String name;
	private int age;
	private double salary;

	public Employee() {
	}

	public Employee(String name) {
		this.name = name;
	}

	public Employee(String name, int age) {
		this.name = name;
		this.age = age;
	}

	public Employee(int id, String name, int age, double salary) {
		this.id = id;
		this.name = name;
		this.age = age;
		this.salary = salary;
	}

	public int getId() {
		return id;
	}

	public void setId(int id) {
		this.id = id;
	}

	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;
	}

	public String show() {
		return "測試方法引用！";
	}

	@Override
	public int hashCode() {
		final int prime = 31;
		int result = 1;
		result = prime * result + age;
		result = prime * result + id;
		result = prime * result + ((name == null) ? 0 : name.hashCode());
		long temp;
		temp = Double.doubleToLongBits(salary);
		result = prime * result + (int) (temp ^ (temp >>> 32));
		return result;
	}

	@Override
	public boolean equals(Object obj) {
		if (this == obj)
			return true;
		if (obj == null)
			return false;
		if (getClass() != obj.getClass())
			return false;
		Employee other = (Employee) obj;
		if (age != other.age)
			return false;
		if (id != other.id)
			return false;
		if (name == null) {
			if (other.name != null)
				return false;
		} else if (!name.equals(other.name))
			return false;
		if (Double.doubleToLongBits(salary) != Double.doubleToLongBits(other.salary))
			return false;
		return true;
	}

	@Override
	public String toString() {
		return "Employee [id=" + id + ", name=" + name + ", age=" + age + ", salary=" + salary + "]";
	}

}

 

public class TestLDemo {

	List<Employee> emps = Arrays.asList(
			new Employee(101, "張三", 18, 9999.99),
			new Employee(102, "李四", 59, 6666.66),
			new Employee(103, "王五", 28, 3333.33),
			new Employee(104, "趙六", 8, 7777.77),
			new Employee(105, "田七", 38, 5555.55)
	);

	//需求：獲取公司中年齡小於 35 的員工信息
	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;
	}

	//需求：獲取公司中工資大於 5000 的員工信息
	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;
	}

	@Test
	public void test3() {
		List<Employee> list = filterEmployeeAge(emps);

		for (Employee employee : list) {
			System.out.println(employee);
		}
		System.out.println("------------------------");

		List<Employee> list1 = filterEmployeeSalary(emps);
		for (Employee employee : list1) {
			System.out.println(employee);
		}
	}
}

2.優化方式一：策略設計模式

不難看出上面完成了需求但是我們發現根據條件過濾員工的時候沒增加一個條件都需要增加一個方法

    a.首先定義一個策略接口

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

     b.定義兩個實現類去實現接口的策略

//策略1：獲取公司中年齡小於 35 的員工信息
public class FilterEmployeeForAge implements MyFilter<Employee>{

	@Override
	public boolean test(Employee t) {
		return t.getAge() <= 35;
	}

}

//策略2：獲取公司中工資大於 5000 的員工信息
public class FilterEmployeeForSalary implements MyFilter<Employee> {

	@Override
	public boolean test(Employee t) {
		return t.getSalary() >= 5000;
	}

}

public class TestLDemo {

	List<Employee> emps = Arrays.asList(
			new Employee(101, "張三", 18, 9999.99),
			new Employee(102, "李四", 59, 6666.66),
			new Employee(103, "王五", 28, 3333.33),
			new Employee(104, "趙六", 8, 7777.77),
			new Employee(105, "田七", 38, 5555.55)
	);

	//優化方式一：策略設計模式
	public List<Employee> filterEmployee(List<Employee> emps, MyFilter<Employee> mp) {
		List<Employee> list = new ArrayList<>();
		for (Employee employee : emps) {
			if (mp.test(employee)) {
				list.add(employee);
			}
		}
		return list;
	}

	@Test
	public void test() {
		//傳入策略1
		List<Employee> list = filterEmployee(emps,new FilterEmployeeForAge());
		for (Employee employee : list) {
			System.out.println(employee);
		}

		System.out.println("------------------------------------------");

		//傳入策略2
		List<Employee> list2 = filterEmployee(emps, new FilterEmployeeForSalary());
		for (Employee employee : list2) {
			System.out.println(employee);
		}
	}
}

 

3.優化方式二：匿名內部類

問題又來了，我沒當定製一個策略的同時，我需要去寫一個策略實現類去實現這個策略，那我們可以使用匿名內部類

public class TestLDemo {

	List<Employee> emps = Arrays.asList(
			new Employee(101, "張三", 18, 9999.99),
			new Employee(102, "李四", 59, 6666.66),
			new Employee(103, "王五", 28, 3333.33),
			new Employee(104, "趙六", 8, 7777.77),
			new Employee(105, "田七", 38, 5555.55)
	);

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

	@Test
	public void test() {
		//匿名內部類實現策略1
		List<Employee> list = filterEmployee(emps, new MyFilter<Employee>() {
			@Override
			public boolean test(Employee employee) {
				return employee.getAge() <= 35;
			}
		});

		for (Employee employee : list) {
			System.out.println(employee);
		}

		System.out.println("------------------------------------------");

		//匿名內部類實現策略2
		List<Employee> list2 = filterEmployee(emps, new MyFilter<Employee>() {
			@Override
			public boolean test(Employee employee) {
				return employee.getSalary() >= 5000;
			}
		});
		for (Employee employee : list2) {
			System.out.println(employee);
		}
	}
}

4.優化方式三：Lambda表達式

public class TestLDemo {

	List<Employee> emps = Arrays.asList(
			new Employee(101, "張三", 18, 9999.99),
			new Employee(102, "李四", 59, 6666.66),
			new Employee(103, "王五", 28, 3333.33),
			new Employee(104, "趙六", 8, 7777.77),
			new Employee(105, "田七", 38, 5555.55)
	);

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

	@Test
	public void test() {
		//Lambda匿名內部類實現策略1
		List<Employee> list = filterEmployee(emps, (e)->  e.getAge() <= 35);
		list.forEach(System.out::println);//Stream API

		System.out.println("------------------------------------------");

		//Lambda匿名內部類實現策略2
		List<Employee> list2 = filterEmployee(emps, (e) -> e.getSalary() >= 5000);
		list2.forEach(System.out::println);//Stream API
	}
}

5.優化方式4：Stream API簡化代碼

public class TestLDemo {

	List<Employee> emps = Arrays.asList(
			new Employee(101, "張三", 18, 9999.99),
			new Employee(102, "李四", 59, 6666.66),
			new Employee(103, "王五", 28, 3333.33),
			new Employee(104, "趙六", 8, 7777.77),
			new Employee(105, "田七", 38, 5555.55)
	);

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

	@Test
	public void test() {
		emps.stream()
			.filter((e) -> e.getAge() <= 35)
			.forEach(System.out::println);

		System.out.println("----------------------------------------------");

		emps.stream()
			.filter((e) -> e.getSalary() >= 5000)
			.forEach(System.out::println);

		System.out.println("----------------------------------------------");
		emps.stream()
			.map(Employee::getName)
			.limit(3)
			.sorted()
			.forEach(System.out::println);
	}
}