Java中的泛型类型和对象[复制](Generic Types and Object in Java [duplicate])

 
 
 这个问题在这里已经有了答案： 
 
 
 
  
 List <Dog>是List <Animal>的子类吗？ 为什么Java泛型不是隐含多态的？ 15个答案 
 
 
 
 
 我刚刚已经了解到Java中的泛型类型存在问题。 如果我想编写一个使用泛型类的方法，就会出现问题。 有了明确的类型说明，我将失去泛型类为我提供的灵活性。 我也不能使用“Object”作为Method中的通用数据类型。 但为什么不呢？ “对象”是Java中每个类的基本类，并且这种语言具有多态性，所以为什么这不起作用？ 
 
 自己的例子来演示： 
 
 Triplet.java 
 
public class Triplet <T> {
    public T a, b, c;

    Triplet(T a, T b, T c) {
        this.a = a;
        this.b = b;
        this.c = c;
    }
}
 
 Main.java 
 
public class Main {
    public static void main(String[] args) {
        Triplet<String> triplet = new Triplet<>("Hello", "Guten Tag", "Bonjour");
        printPartsOfTriplet(triplet);
    }

    static void printPartsOfTriplet(Triplet<Object> triplet) {
        System.out.println(triplet.a);
        System.out.println(triplet.b);
        System.out.println(triplet.c);
    }
}
 
 错误信息 
 
Main.java:4: error: incompatible types: Triplet<String> cannot be converted to Triplet<Object>
        printPartsOfTriplet(triplet);
                            ^
Note: Some messages have been simplified; recompile with -Xdiags:verbose to get full output
1 error

 
 
This question already has an answer here:
 
 
 
  
 Is List<Dog> a subclass of List<Animal>? Why are Java generics not implicitly polymorphic?  16 answers  
 
 
 
 
I just have learned that there is a problem with generic types in Java. The problem occurs if I want to write a method that uses a generic class. With the explicit specification of a type I will lose the flexibility that generic classes offer to me. I also cant use "Object" for the generic data type in the Method. But why not? "Object" is the basic class of every class in Java and this language has polymorphism so why this doesnt work?
 
Own Example to demonstrate:
 
Triplet.java
 
public class Triplet <T> {
    public T a, b, c;

    Triplet(T a, T b, T c) {
        this.a = a;
        this.b = b;
        this.c = c;
    }
}
 
Main.java
 
public class Main {
    public static void main(String[] args) {
        Triplet<String> triplet = new Triplet<>("Hello", "Guten Tag", "Bonjour");
        printPartsOfTriplet(triplet);
    }

    static void printPartsOfTriplet(Triplet<Object> triplet) {
        System.out.println(triplet.a);
        System.out.println(triplet.b);
        System.out.println(triplet.c);
    }
}
 
Error Message
 
Main.java:4: error: incompatible types: Triplet<String> cannot be converted to Triplet<Object>
        printPartsOfTriplet(triplet);
                            ^
Note: Some messages have been simplified; recompile with -Xdiags:verbose to get full output
1 error

原文：https://stackoverflow.com/questions/47335486