引用特定对象的实例方法 [英] Reference to an instance method of a particular object

问题描述

在下面的代码中，它在使用类名传递方法引用变量时有效，但在使用用户对象传递引用变量时会出错。

In the following code, it works when passing the method reference variable with the class name, but when passing the reference variable with a user object there is an error.

public class User {
    private String name;

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

    public void printName() {
        System.out.println(name);
    }    
}


public class Main {
    public static void main(String[] args) {
        User u1 = new User("AAA");
        User u2 = new User("BBB");
        User u3 = new User("ZZZ");

        List<User> userList = Arrays.asList(u1, u2, u3);        

        userList.forEach(User::printName); // works
        userList.forEach(u1::printName); // compile error
    }
}

推荐答案

userList.forEach 期待消费者<？扩展用户> - 换句话说，这是一种接受用户引用的方法并使用它做一些事情。

userList.forEach is expecting a Consumer<? extends User> - in other words, a method which accept a User reference and do something with it.

可能是：

• 接受用户的静态方法参数，在这种情况下，参数将在每次迭代时使用列表中的相关元素填充：

• A static method accepting a User parameter, in which case the parameter will be populated with the relevant element in the list on each iteration:

staticMethod(userFromList)

接受用户参数，提供特定的实例来调用它 - 再次，该参数将填充相关元素：

An instance method (of any class) accepting a User parameter, provided with a specific instance to call it on - again, the parameter will be populated with the relevant element:

target.instanceMethod(userFromList)

带 no 参数的用户上的实例方法，提供不带特定实例，其中方法调用的target 将是每次迭代时列表中的相关元素：

An instance method on User with no parameters, provided without a specific instance, which case the target of the method call will be the relevant element in the list on each iteration:

userFromList.instanceMethod()

因为你已经试图指定一个目标和该方法没有任何参数， forEach 方法对每个元素没有任何作用 - 它可以' t将它作为参数传递，因为该方法没有任何参数，并且它不能将它用作方法目标，因为您已经指定了一个。

Because you've tried to specify a target and the method doesn't have any parameters, the forEach method has nothing it can do with each element - it can't pass it as an argument because the method doesn't have any parameters, and it can't use it as the method target because you've already specified one.

您的工作代码显示了第三个示例。以下是另外两种方法可以让您演示前两个：

Your working code shows the third example. Here are two other methods to allow you to demonstrate the first two:

public class UserPrinter {
    private final String name;

    public UserPrinter(String name) {
        this.name;
    }

    public static void staticPrintUser(User user) {
        // Assuming you add a User.getName() method
        System.out.println("staticPrintUser: " + user.getName());
    }

    public void instancePrintUser(User user) {
        System.out.println("instancePrintUser (instance " + name + "): "
            + user.getName());
    }
}

然后：

userList.forEach(UserPrinter::staticPrintUser);    // equivalent to
//userList.forEach(p -> UserPrinter.staticPrintUser(p));
UserPrinter printer = new UserPrinter("my printer");
userList.forEach(printer::instancePrintUser);      // equivalent to
//userList.forEach(p -> printer.instancePrintUser(p));

如果确实想要致电 printUser 在同一个用户上三次，忽略列表中的用户，你可以使用：

If you really want to call printUser on the same User three times, ignoring the User in the list, you could use:

userList.forEach(ignored -> u1.printName());

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