Java是面向对象的编程语言。继承是最重要的特性之一。我们使用封装来隐藏对象状态。在下面的程序中，我应该对年龄和计数器使用私有访问修饰符，但为了测试这种继承，我使用了公共。
- 为什么在直接对象状态访问（原始类型或对象引用）的情况下继承不起作用。SOP 3 输出与预期不同。
- 由于编译器在 SOP 13 和 SOP 14 行中没有给出任何错误。为什么打印父类详细信息而不是子类。
.
public class Father {
public int age = 50;
/*
* Counter keeps track of total no of instances created so far.
*/
public static int counter = 0;
public Father(){
super();
synchronized (Father.class) {
++Father.counter;
}
}
public int getAge(){
return this.age;
}
public static int getStaticCount(){
return Father.counter;
}
}
public class Child extends Father {
public int age = 25;
public static int counter = 0;
public Child(){
super();
synchronized (Child.class) {
++Child.counter;
}
}
public int getAge(){
return this.age;
}
public static int getStaticCount(){
return Child.counter;
}
public static void main(String args[]){
Father father = new Father();
Father child = new Child();
Child realChild = new Child();
System.out.println("Expecting Father Class details to be printed");
System.out.println("SOP 1 : Father Age : "+father.age); //prints 50 as expected.
System.out.println("SOP 2 : Father Age : "+father.getAge());//prints 50 as expected.
System.out.println("Expecting Child Class details to be printed");
/*
* Why inheritance does not work in case of direct integer access.
*/
System.out.println("SOP 3 : Child Age : "+child.age); //prints 50 ?? , Father Age . Why ?
System.out.println("SOP 4 : Child Age : "+child.getAge());//prints 25 as expected.
System.out.println("Expecting Child Class details to be printed");
System.out.println("SOP 5 : Child Age : "+realChild.age); //prints 25 as expected.
System.out.println("SOP 6 : Child Age : "+realChild.getAge());//prints 25 as expected.
/*
*Total No of static Count : proper way of accessing static field using Class Name.
*/
System.out.println("SOP 7 : Father Instance Count : Using Class Reference :"+Father.counter);
System.out.println("SOP 8 : Father Instance Count : Using Class Reference :"+Father.getStaticCount());
/*
* Incorrect Way to use static. Since Compiler allows this lets see output.
*/
System.out.println("SOP 9 : Father Instance Count : Using Object Reference :"+father.counter); //prints 3 as expected.
System.out.println("SOP 10 : Father Instance Count : Using Object Reference :"+father.getStaticCount());//prints 3 as expected.
/*
*Total No of static Count : proper way of accessing static field using Class Name.
*/
System.out.println("SOP 11 : Child Instance Count : Using Class Reference :"+Child.counter); // output is 2 as expected
System.out.println("SOP 12 : Child Instance Count : Using Class Reference :"+Child.getStaticCount()); // output is 2 as expected
/*
* Incorrect Way to use static.Since Compiler allows this lets see output.
* This invokes function of parent class. Why ? Inheritance does not work for static fields.
*/
System.out.println("SOP 13 : child Instance Count : Using Object Reference :"+child.counter); // output is 3 but expected is 2 . why ?
System.out.println("SOP 14 : child Instance Count : Using Object Reference :"+child.getStaticCount()); // output is 3 but expected is 2 . why ?
/*
* Incorrect Way to use static.Since Compiler allows this lets see output.
* This invokes function of parent class. Why ?
*/
System.out.println("SOP 15 : child Instance Count : Using Object Reference :"+realChild.counter); // output is 2 as expected
System.out.println("SOP 16 : child Instance Count : Using Object Reference :"+realChild.getStaticCount()); // output is 2 as expected
}
}
我的问题是为什么继承只适用于实例方法。为什么 SOP 3、SOP 13 和 SOP 14 的输出不同。