第4讲Java面向对象程序设计(一).pptVIP

4.2.2 null、this、super 每个类均定义3个对象值：null、this、super * null null：指代“空”对象; 即该类不存在相应的对象； 如：Point p1=null;? //没有构造相应的对象，只是声明，该对象名不能再被定义 * this This：指代“当前”对象； 系统自动将this与当前对象的变量相关联。 str = 名字: + name + \nSalary: + salary; 可以换成: str = 名字: + this.name + \nSalary: + this.salary; * this package ch4; public class TriPoint extends Point_1 { private int z; public TriPoint(int x, int y, int z) { super(x, y); this.z = z; } public int getZ() { return z; } TriPoint tp = new TriPoint(1,2,3); this.z = z; ? tp.z = 3 int a = tp.getZ(); * super super：指代直接“父”对象。 super作为方法名代表父类的构造方法。 * super package ch4; public class TriPoint extends Point_1 { private int z; public TriPoint(int x, int y, int z) { super(x, y); this.z = z; } public int getZ() { return z; } TriPoint tp = new TriPoint(1,2,3); //this.z = z; ? tp.z = 3 int a = tp.getZ(); * 继承 Person:name,IdNo,sex,add Student:stNo,corseId super(name,IdNo,sex,add)=Person(name,IdNo,sex,add) C_Student:os,compile super(name,IdNo,sex,add,stNo,corseId)=Student(name,IdNo,sex,add,stNo,corseId) * package ch4; class Employee1 { public String name;// 名字 public int salary;// 薪水 // 方法 public String getSalary() { String str; str = 名字: + name + \nSalary: + salary; return str; } } * class Manager extends Employee1 { public String department;// 部门 // 方法 public String getSalary() { // 使用super变量调用超类Employee1的方法getSalary() return super.getSalary() + \nDepartment: + department; } } * public class UseSuper { public static void main(String[] args) { Manager m = new Manager(); m.name = 王飞; m.salary = 10000; m.department = 业务部; System.out.println(m.getSalary()); //调用Manager类的方法getSalary() } } * 4.2.3 多态性 例4-14 ?多态示例。 package ch4; public class Point { protected int x; protected int y; protected Point(int x, int y) { this.x=x; this.y=y; } public void draw()? { ????? ????? //父类的draw()方法 System.out.println(Point:a point); }????????????? } * 多态性 多态性（Polymorphism）:指在超类中定义的属性或行为，被子类继承