hibernate的Java类型转换

是的，我知道我们可以在Java中向上或向下转换。但实例的类型似乎并没有真正改变，这给了我一个问题

例如

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在hibernate中，保存“父对象”时，仍然会认为它是DTO对象，无法保存。我真的可以将子实例转换为父实例吗？

不，你不能用这种方式将子实例转换为父实例。您已经为父对象创建了特定的对象。例如，

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Foo父对象新Foo（dto.getA（），dto.getB（））

您可以使用hibernate的

save（entityName，object）

方法保存“父对象”。在这种情况下，entityName是“父级”的完全限定类名。

对象的类型在创建后无法更改。如果创建FooDTO对象，它将始终是FooDTO对象

当您强制转换时，您告诉JVM您将使用类型为X的引用来指向您知道的类型为X的对象

class Parent {}
class Child extends Parent {}

class Test {
    public void stuff() {
        Parent p = new Parent(); // Parent reference, Parent object
        Parent p2 = new Child(); // Parent reference, Child object
        Child c = new Child();   // Child reference, Child object 


        Parent p2 = c; // no explicit cast required as you are up-casting
        Child c2 = (Child)p; // explicit cast required as you are down-casting. Throws ClassCastException as p does not point at a Child object
        Child c3 = (Child)p2; // explicit cast required as you are down-casting. Runs fine as p2 is pointong at a Child object
        String s1 = (String)p; // Does not compile as the compiler knows there is no way a Parent reference could be pointing at a String object            

    }
}

class Parent {}
class Child extends Parent {}

class Test {
    public void stuff() {
        Parent p = new Parent(); // Parent reference, Parent object
        Parent p2 = new Child(); // Parent reference, Child object
        Child c = new Child();   // Child reference, Child object 


        Parent p2 = c; // no explicit cast required as you are up-casting
        Child c2 = (Child)p; // explicit cast required as you are down-casting. Throws ClassCastException as p does not point at a Child object
        Child c3 = (Child)p2; // explicit cast required as you are down-casting. Runs fine as p2 is pointong at a Child object
        String s1 = (String)p; // Does not compile as the compiler knows there is no way a Parent reference could be pointing at a String object            

    }
}