用Subclass vs Superclass引用引用Subclass对象
在Java继承中，一些基本规则包括-
父类与父类之间的对象关系不存在，而子类与父对象之间的对象关系不存在，这意味着父类的引用可以保存子对象，而子引用不能保存父对象。
在重写非静态方法的情况下，运行时对象将评估子类或超类将执行哪种方法。静态方法的执行取决于对象所持有的引用的类型。
继承的其他基本规则与静态方法和非静态方法有关，即Java中的静态方法不能被覆盖，而非静态方法可以被覆盖。但是子类可以定义与超类具有相同静态方法签名的静态方法，而不必考虑作为覆盖，而称为隐藏超类的静态方法。
根据上述规则，我们将在程序的帮助下观察不同的情况。
示例
class Parent {
public void parentPrint() {
System.out.println("parent print called");
}
public static void staticMethod() {
System.out.println("parent static method called");
}
}
public class SubclassReference extends Parent {
public void parentPrint() {
System.out.println("child print called");
}
public static void staticMethod() {
System.out.println("child static method called");
}
public static void main(String[] args) {
//SubclassReference invalid = new Parent();//Type mismatch: cannot convert from Parent to SubclassReference
Parent obj = new SubclassReference();
obj.parentPrint(); //method of subclass would execute as subclass object at runtime.
obj.staticMethod(); //method of superclass would execute as reference of superclass.
Parent obj1 = new Parent();
obj1.parentPrint(); //method of superclass would execute as superclass object at runtime.
obj1.staticMethod(); //method of superclass would execute as reference of superclass.
SubclassReference obj3 = new SubclassReference();
obj3.parentPrint(); //method of subclass would execute as subclass object at runtime.
obj3.staticMethod(); //method of subclass would execute as reference of subclass.
}
}输出结果
child print called parent static method called parent print called parent static method called child print called child static method called
因此，使用超类引用进行引用和使用子类引用进行引用之间的区别在于，使用超类引用可以保留子类的对象，并且只能访问由子类定义/覆盖的方法，而使用子类引用不能保留超类的对象，并且可以访问超类和子类。