java - java中从List中随机选择一个对象

Question

在你开始给这个问题打-1之前，我想说我已经阅读了至少4个关于此类问题的问题，但每个问题都没有完全帮助我。

我有这个类，其中包含以下方法:

class OperationDispatcher {

    OperationDispatcher(ManagementService.Client managementClient) {
        this.managementClient = managementClient;
    }

    ManagementService.Client managementClient;

    public long executeOperation(String opName, int num1, int num2) throws TException {

        List<ServiceProvider> providers = new ArrayList<ServiceProvider>();
        providers = managementClient.getProvidersForService(opName);

        long result = 0;

        if(providers.isEmpty())
            System.out.println("No Service Available");
        else
            switch(opName) {
                case "addition":
                    result = arithmeticClient.add(num1, num2);
                    break;
                case "multiplication":
                    result = arithmeticClient.multiply(num1, num2);
                    break;
                case "substraction":
                    result = arithmeticClient.substract(num1, num2);
                    break;
                case "division":
                    result = arithmeticClient.divide(num1, num2);
                    break;  
        }   
        return result;
    }

}

我正在尝试实现此行为:

1. 两个或多个对象“节点”注册到我在测试中创建的本地中央服务器；
2. 每个节点都有一个服务列表，并向其他节点发出请求或提供服务；
3. 当发出请求时，会返回一个“ServiceProviders”列表，其中每个服务提供商都是可能的“应答节点”；
4. 从列表中随机选择一个服务提供商；
5. 所选的 ServiceProvider 执行请求的算术运算。

我对第 1、2、3 和 5 点没有任何问题。我现在正在尝试修改该方法，使其从列表中存在的 ServiceProvider 中随机选择一个 ServiceProvider。

我不确定应该将代码放在哪里(在我的类(class)中)，以便从列表中随机选取对象并使其执行请求的操作。你能帮我吗？

编辑所以，我修改了我的代码如下:

class OperationDispatcher {

    private List<ServiceProvider> providers = new ArrayList<ServiceProvider>();

    OperationDispatcher(ManagementService.Client managementClient) {
        this.managementClient = managementClient;
    }

    //tiro su un management client
    ManagementService.Client managementClient;

    public long executeOperation(String opName, int num1, int num2) throws TException {

        ServiceProvider randomProvider = new ServiceProvider();
        providers = managementClient.getProvidersForService(opName);

        long result = 0;

        if(providers.isEmpty())
            System.out.println("No Service Available");

        else if(providers.size() == 1) {
            switch(opName) {
                case "addition":
                    result = arithmeticClient.add(num1, num2);
                    break;
                case "multiplication":
                    result = arithmeticClient.multiply(num1, num2);
                    break;
                case "substraction":
                    result = arithmeticClient.substract(num1, num2);
                    break;
                case "division":
                    result = arithmeticClient.divide(num1, num2);
                    break;
            }
        }

        else if (providers.size() >= 2) {
            randomProvider = providers.get(random.nextInt(providers.size()));
            if(randomProvider != null) {
                switch(opName) {
                case "addition":
                    result = arithmeticClient.add(num1, num2);
                    break;
                case "multiplication":
                    result = arithmeticClient.multiply(num1, num2);
                    break;
                case "substraction":
                    result = arithmeticClient.substract(num1, num2);
                    break;
                case "division":
                    result = arithmeticClient.divide(num1, num2);
                    break;
                }
            }
        }
        return result;
    }

}

我的测试是这样的:

@Before
public void initializeManagementServer() throws Exception {

    managementServer = new ManagementServer(7911);
    managementServer.start();

    Thread.sleep(200);
    managementServerTransport = new TSocket("localhost", 7911);
    managementServerProtocol = new TBinaryProtocol(managementServerTransport);
    managementClient = new ManagementService.Client(managementServerProtocol);
    managementServerTransport.open();
}

@Test
public void testArithmeticServer() throws Exception {

    ServiceProvider randomProvider = new ServiceProvider();

    List<String> node1Svc = new ArrayList<>();
    node1Svc.add("addition");

    List<String> node2Svc = new ArrayList<>();
    node2Svc.add("substraction");
    node2Svc.add("multiplication");

    List<String> node3Svc = new ArrayList<>();
    node3Svc.add("addition");
    node3Svc.add("multiplication");
    node3Svc.add("division");

    List<String> node4Svc = new ArrayList<>();
    node4Svc.add("addition");
    node4Svc.add("division");

    NodeManifest node1 = new NodeManifest("Node 1", node1Svc);
    NodeManifest node2 = new NodeManifest("Node 2", node2Svc);
    NodeManifest node3 = new NodeManifest("Node 3", node3Svc);
    NodeManifest node4 = new NodeManifest("Node 4", node4Svc);

    int port = managementClient.registerNode(node1);
    assertEquals(1026, port);
    port = managementClient.registerNode(node2);
    assertEquals(1027, port);
    port = managementClient.registerNode(node3);
    assertEquals(1028, port);
    port = managementClient.registerNode(node4);
    assertEquals(1029, port);



    arithmeticServer = new ArithmeticServer(port);
    arithmeticServer.start();

    arithmeticTransport = new TSocket("localhost", port);
    arithmeticProtocol = new TBinaryProtocol(arithmeticTransport);
    arithmeticClient = new ArithmeticService.Client(arithmeticProtocol);
    arithmeticTransport.open();

    OperationDispatcher dispatcher = new OperationDispatcher(managementClient);
    long result = dispatcher.executeOperation("addition", 1, 2);
    assertEquals(3, result);
    assertEquals(3, dispatcher.providers.size());

    arithmeticTransport.close();
    arithmeticServer.stop();

@After
public void teardownManagementServer() throws Exception {
    managementServerTransport.close();
    managementServer.stop();            
}
}

我已经注册了4个节点。节点#1 是请求“添加”服务的节点，节点#2、#3 和#4 都有该服务。然后我创建一个对象OperationDispatcher，请求“添加”服务，它返回一个包含三个ServiceProvider 的列表。现在我想从该列表中随机选择一项，最后由 ServiceProvider 执行该操作。现在，始终选择节点 4。怎么了？

Answer 1

您可以使用Java的Random类如下:

ServiceProvider provider = providers.get(new Random().nextInt(providers.size()));

请注意，初始化新的Random对象是一个稍微昂贵的操作，因此您可能希望创建一次，然后继续重用它:

private static final Random RANDOM = new Random();

// ...

ServiceProvider provider = providers.get(RANDOM.nextInt(providers.size()));