Java通用 - 对象方法无法找到(Java Generic - object method cannot be found)

 我有一个接口，看起来像： 
 
package com.sample.service;

import com.sample.model.Request;

public interface Service<T> {
    void add(T domain);
    <E> boolean update(Request<E> request);
}
 
 Request.java 
 
package com.sample.model;

public class Request<E> {
    private final E entity;

    public Request(E entity) {
        this.entity = entity;
    }

    public E getEntity() {
        return entity;
    }
}
 
 以下是我的实施 
 
package com.sample.service;

import com.sample.domain.User;
import com.sample.model.Form;
import com.sample.model.Request;

public class UserServiceImpl implements Service<User> {

    @Override
    public  void add(User user) {
        System.out.println("add: " + user);
    }

    @Override
    public <Form> boolean update(Request<Form> request) {
        System.out.println("update: " + request);
        Form form = request.getEntity();
        //form.setUsername("some_username");//This line caused compile error
        return true;
    }
}
 
 这里是Form.java 
 
package com.sample.model;

public class Form {
    private String username;

    public String getUsername() {
        return username;
    }

    public void setUsername(String username) {
        this.username = username;
    }

    @Override
    public String toString() {
        return username;
    }
}
 
 和User.java 
 
package com.sample.domain;

public class User {
    private String name;

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    @Override
    public String toString() {
        return name;
    }
}
 
 当我编译并执行这些代码时，一切都很好。 但是，当我取消注释UserServiceImpl中的注释行时，出现编译错误 
 
UserServiceImpl.java:19: error: cannot find symbol
        form.setUsername("some_username");
 
 我的问题是为什么无法找到表单对象中的setUsername方法？ 表单对象是可变的。 

I've an interface that looks like: 
 
package com.sample.service;

import com.sample.model.Request;

public interface Service<T> {
    void add(T domain);
    <E> boolean update(Request<E> request);
}
 
Where the Request.java 
 
package com.sample.model;

public class Request<E> {
    private final E entity;

    public Request(E entity) {
        this.entity = entity;
    }

    public E getEntity() {
        return entity;
    }
}
 
The following is my implementation
 
package com.sample.service;

import com.sample.domain.User;
import com.sample.model.Form;
import com.sample.model.Request;

public class UserServiceImpl implements Service<User> {

    @Override
    public  void add(User user) {
        System.out.println("add: " + user);
    }

    @Override
    public <Form> boolean update(Request<Form> request) {
        System.out.println("update: " + request);
        Form form = request.getEntity();
        //form.setUsername("some_username");//This line caused compile error
        return true;
    }
}
 
And here are the Form.java 
 
package com.sample.model;

public class Form {
    private String username;

    public String getUsername() {
        return username;
    }

    public void setUsername(String username) {
        this.username = username;
    }

    @Override
    public String toString() {
        return username;
    }
}
 
and User.java
 
package com.sample.domain;

public class User {
    private String name;

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    @Override
    public String toString() {
        return name;
    }
}
 
When I compiled and execute these codes everything is fine. However, when I uncomment the line commented in UserServiceImpl, I got compile error 
 
UserServiceImpl.java:19: error: cannot find symbol
        form.setUsername("some_username");
 
My question is why the setUsername method in form object cannot be found? The form object is mutable.

原文：https://stackoverflow.com/questions/33084131