java8 lambda表达式实现自定义用户组件，Don't Repeat Yourself

一、用户组件的功能  

     使用java8 lambda表达式实现实现世界的一个例子：用户组件。此用户组件有以下几个操作：获取用户列表，获取单个用户，增加用户，删除用户，更新用户。所有的操作都是使用UserRequest对象并返回一个UserResponse对象。

       在各个操作中，需要在方法执行前进行请求合法性验证，方法执行后进行特殊的响应处理，检查所有的响应特性
执行日志记录或其他操作。

      在原来的处理方式中，需要在每个操作中都把要执行的请求方法包括在至少一个try-catch语句块中。在java8中，可以创建一个的方法统一处理方法执行前后的操作，如源代码中的callComponent方法，使代码更加简单，更少的代码重复。

二、代码样例

    1.定义一个用户类User

    2.定义用户组件的接口UserComponent,UserRequest,UserResponse

    3.使用lambda表达式实现用户组件的接口UserComponentImpl

    4.实现Adapter类UserComponentAdapter

    5.创建测试类UserComponentTest

public class User {
	Integer id;
	String name;
	Integer age;

	User() {
	}

	User(int id, String name, int age) {
		this.id = id;
		this.name=name;
		this.age = age;
	}

	public String toString() {
		return "" + id + "-" + name + "-" + age;
	}
}

public interface UserComponent {
	UserResponse fetchAllUsers(UserRequest req);

	UserResponse fetchUser(UserRequest req);

	UserResponse deleteUser(UserRequest req);

	UserResponse updateUser(UserRequest req);

	UserResponse insertUser(UserRequest req);
}

import java.util.ArrayList;
import java.util.List;
import java.util.stream.Collectors;

public class UserComponentImpl implements UserComponent {
	List<User> users = new ArrayList<User>();

	public UserResponse fetchAllUsers(UserRequest req) {
		return new UserResponse(users);
	}

	public UserResponse fetchUser(UserRequest req) {
		return new UserResponse(users.stream()
				.filter(u -> u.id.equals(req.user.id))
				.collect(Collectors.toList()));
	}

	public UserResponse deleteUser(UserRequest req) {
		users = users.stream().filter(u -> (!u.id.equals(req.user.id)))
				.collect(Collectors.toList());
		return new UserResponse(true);
	}

	public UserResponse updateUser(UserRequest req) {
		List<User> list = users.stream().filter(u -> u.id.equals(req.user.id))
				.collect(Collectors.toList());
		if (list.size() > 0) {
			User user = list.get(0);
			user.name = req.user.name;
			user.age = req.user.age;
			return new UserResponse(true);
		}
		return new UserResponse(false);
	}

	public UserResponse insertUser(UserRequest req) {
		users.add(req.user);
		return new UserResponse(true);
	}
}

import java.io.IOException;
import java.util.List;
import java.util.function.Function;

public class UserComponentAdapter {
	UserComponent uc = new UserComponentImpl();

	UserResponse callComponent(UserRequest request,
			Function<UserRequest, UserResponse> func) {
		try {
			// 验证请求的合法性
			UserResponse response = func.apply(request);
			// 特殊的响应处理，检查所有的响应特性
			// 执行日志记录和/或其他操作
			return response;
		} catch (Exception e) {
			e.printStackTrace();
			// 进行异常处理
			return new UserResponse(true);
		}
	}

	List<User> fetchAllUsers() throws IOException {
		return callComponent(new UserRequest(), uc::fetchAllUsers).users;
	}

	User fetchUser(Integer id) throws IOException {
		return callComponent(new UserRequest(new User(id, "", 0)),
				uc::fetchUser).users.get(0);
	}

	boolean deleteUser(Integer id) throws IOException {
		return callComponent(new UserRequest(new User(id, "", 0)),
				uc::deleteUser).success;
	}

	boolean updateUser(User user) throws IOException {
		return callComponent(new UserRequest(user), uc::updateUser).success;
	}

	boolean insertUser(User user) throws IOException {
		return callComponent(new UserRequest(user), uc::insertUser).success;
	}
}

import java.io.IOException;

public class UserComponentTest {
	public static void main(String[] args) throws IOException {
		UserComponentAdapter uca = new UserComponentAdapter();
		uca.insertUser(new User(1, "张三", 12));
		uca.insertUser(new User(2, "李四", 21));
		uca.insertUser(new User(3, "王五", 32));
		uca.insertUser(new User(4, "赵六", 43));
		
		System.out.println(uca.fetchAllUsers());
		
		System.out.println(uca.fetchUser(2));
		
		System.out.println(uca.updateUser(new User(3, "王五五", 55)));
		
		System.out.println(uca.deleteUser(4));
		
		System.out.println(uca.fetchAllUsers());
	}
}

public class UserRequest {
	User user;

	public UserRequest() {

	}

	public UserRequest(User user) {
		this.user = user;
	}
}

import java.util.List;

public class UserResponse {
	List<User> users;
	boolean success = true;

	public UserResponse(boolean success) {
		this.success=success;
	}

	public UserResponse(List<User> users) {
		this.users = users;
	}
}

三、参考资料

http://docs.oracle.com/javase/tutorial/java/javaOO/lambdaexpressions.html.

http://www.dreamsyssoft.com/java-8-lambda-tutorial/