Question

我在线学习初学者java课程，我需要更好地理解作业如何与继承一起工作。例如，如果我有这些类......

public class Cell{...}
public class BloodCell extends Cell {...}
public class RedBloodCell extends BloodCell {...}

这些有效吗？

Cell c = new BloodCell();
Cell c = new RedBloodCell();
BloodCell c = new RedBloodCell();
RedBloodCell c = new BloodCell();

Answer 1

extends

用于继承，在外行术语中它类似于“is-a”属性

所以A类扩展了其他一些B类，意味着“A类是-B类”。换句话说，B类是A类的父母。

父类引用可以包含子对象，反之亦然。

Cell c = new BloodCell();   // OK, as BloodCell is a Cell,
Cell c = new RedBloodCell();   // OK, RedBloodCell is a Cell
BloodCell c = new RedBloodCell();   // OK, RedBloodCell is a BloodCell
RedBloodCell c = new BloodCell();  // ERROR, BloodCell is not a RedBloodCell

Answer 2

Cell c = new BloodCell(); //OK: all blood cells are cells
Cell c = new RedBloodCell();//OK: all red blood cells are cells
BloodCell c = new RedBloodCell();//OK: all red blood cells are blood cells
RedBloodCell c = new BloodCell();//ERROR: not all blood cells are red blood cells

理解Java中的赋值和“扩展”

2 个答案: