java - 试图理解 Java 中的泛型

Question

我正在修改一本书中的示例类，并尝试将其转换为使用泛型来帮助我了解它的工作原理。您可以在下面我尝试的地方看到我的代码。泛型 A、B 和 C 曾经是 String String 和 int。

public class Student<A,B,C> implements Person {
    A id;
    B name;
    C age;

    public Student(A i, B n, C a) {
        id = i;
        name = n;
        age = a;
    }

    protected int studyHours() {return age/2;}
    public A getID() {return id;}
    public B getName() {return name;}
    public C getAge() {return age;}
    public boolean equals(Person other) {
        if(!(other instanceof Student)) return false;
        Student s = (Student) other;
        return id.equals(s.id);
    }
    public String toString() {
        return "Student(ID: " + id + ", Name: " + name + ", Age: " + age + ")";
    }
    public static void main(String[] var0) {
        Student<String,String,int> studentOne = new Student("123", "Guillermo", 34);
        Student<String,String,int> studentTwo = new Student("345", "Cheryl", 35);
        int numberOfStudyHours;

        numberOfStudyHours = studentOne.studyHours();
        System.out.println("Guillermo studies " +numberOfStudyHours+ " hours");

    }
}

人机界面如下

public interface Person {
    public boolean equals(Person other);        //is this the same person?
    public String getName();
    public int getAge();
}

非常感谢您的专家指导。我收到此错误:

 Student.java:4: error: Student is not abstract and does not override abstract method getAge() in Person
        public class Student<A,B,C> implements Person {
               ^
    Student.java:18: error: getAge() in Student cannot implement getAge() in Person
            public C getAge() {return age;}
                     ^
      return type C is not compatible with int
      where C is a type-variable:
        C extends Object declared in class Student
    Student.java:17: error: getName() in Student cannot implement getName() in Person
            public B getName() {return name;}
                     ^
      return type B is not compatible with String
      where B is a type-variable:
        B extends Object declared in class Student
    Student.java:15: error: bad operand types for binary operator '/'
            protected int studyHours() {return age/2;}
                                                  ^
      first type:  C
      second type: int
      where C is a type-variable:
        C extends Object declared in class Student
    Student.java:28: error: unexpected type
                Student<String,String,int> studentOne = new Student("123", "Guillermo", 34);
                                      ^
      required: reference
      found:    int
    Student.java:29: error: unexpected type
                Student<String,String,int> studentTwo = new Student("345", "Cheryl", 35);
                                      ^
      required: reference
      found:    int

Answer 1

有关泛型有意义的一些更好的示例，请查看各种 Java Collections类。例如，从任何事物到任何事物(字符串到整数、整数到字符串、整数到整数、人到字符串)创建一个映射是有意义的。您只需要一个类来实现所有这些映射，但是通过实例化每个映射及其类型，

Map<Person,Person> motherMap = new HashMap<>();

编译器和 IDE 知道必须给 get() 什么样的 key 方法以及结果的类型，从而在一开始就防止大量编码错误。

寻找使用泛型产生良好效果的示例的另一个地方是 google 库:参见 GuavaExplained .

作为练习，您可以尝试实现具有左右子节点的二叉树，其中树节点都是相同的泛型类型 T。

对于编译错误，泛型不能用于基本类型——小写类型，如 boolean 和 int——尽管你可以使用 Boolean 和 Integer。此外，接口(interface)通常也是通用的:HashMap<K,V> implements Map<K,V>