java - "Object"对象类型转换和对 "Some"类对象的引用

Question

为什么我无法引用类型转换并引用某些类的对象的“Object”类的对象。下面的代码解释了它。很难用言语表达。意思是，父类(super class)Object的对象应该能够引用任何类型的Class对象。

public class ChildClass {

public static void main(String[]args){
    Simple obj1=new Simple();
    Object obj2=(Simple)obj1;
    System.out.println("1-obj1.a is "+ obj1.a+" obj1.name is "+obj1.name);
    System.out.println("2-obj2.a is "+ obj2.a+" obj2.name is "+obj2.name);/*a cannot be resolved or is not a 
    field*/
    doSomething(obj2);
    System.out.println("3-obj2.a is "+ obj2.a+" obj2.name is "+obj2.name);/*a cannot be resolved or is not a 
    field*/
    System.out.println("4-obj1.a is "+ obj1.a+" obj1.name is "+obj1.name);
}

private static void doSomething(Object obj2) {
    obj2.a=99;//a cannot be resolved or is not a field
    obj2.name="new name";//name cannot be resolved or is not a field

}
class Simple {

    int a=9;
    String name="something";

}

}

Answer 1

如果您确实希望参数的类型为 Object，则需要将 Object 转换为 Simple:

private static void doSomething(Object obj2) {
    ((Simple)obj2).a = 99;
    ((Simple)obj2).name = "new name";
}

或者，为了更安全一点:

private static void doSomething(Object obj2) {
    if (obj2 instanceof Simple) {
        Simple simple = (Simple) obj2;  
        simple.a = 99;
        simple.name = "new name";
    }
}

你好像搞反了。 Simple 类型的引用可用于调用 Object 的方法，但反之则不然。