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是如何工作的。你可以在我尝试的地方看到我的代码。通用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");

    }
}

人员界面位于

之下

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。我收到了这个错误：

 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

有关泛型有意义的更好示例，请查看各种Java Collections类。例如，将Map从任何内容转换为其他内容（String to Integer，Integer to String，Integer to Integer，Person to String）是有意义的。您只需要一个类来实现所有这些映射，但是通过使用其类型实例化每个映射，

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

编译器和IDE知道必须为get()方法提供什么类型的密钥，以及结果的类型，从而防止在开始时出现大量编码错误。

另一个查找仿制药效果良好的示例的地方是Google图书馆：请参阅GuavaExplained。

对于练习练习，您可以尝试使用左右子节点实现二叉树，其中树节点都是相同的泛型类型T.

对于编译错误，Generics不能用于原始类型 - 小写的类似boolean和int-尽管你可以使用Boolean和Integer。此外，界面通常也是通用的：HashMap<K,V> implements Map<K,V>

Answer 2

对于您的示例，您似乎应该将界面更改为：

public interface Person<B,C> {
    public boolean equals(Person other);        //is this the same person?
    public B getName();
    public C getAge();
}

然后

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

But this isn't really a good use of generics。另请注意，您不能将int或其他原语用于泛型。你必须使用Integer或其他拳击课。

试图理解java中的泛型

2 个答案: