java练习题之多态练习

1：关于多态描述错误的是(D)

A. 父类型的引用指向不同的子类对象

B. 用引用调用方法，只能调用引用中声明的方法

C. 如果子类覆盖了父类中方法，则调用子类覆盖后的方法

D. 子类对象类型会随着引用类型的改变而改变

2：class Super{

public void m1(){}

public void m2(){}

???}

???class Sub extends Super{

public void m2(){}

public void m3(){}

public void m4(){}

???}

???创建对象Super s=new Sub();用s引用可以调用的方法(A B)

A.m1() ?????B. m2() ????C. m3() ????D. m4()

3：class Super{}

class Sub extends Super{}

class ClassA extends Super{}

public class TestSuper{

public static void main(String[] args){

//1

}

}

对于1处代码及对其描述错误的是(CD)

A. Super s=new Sub();

B. Sub s=new Sub();

???Super sup=s; ?????

???

C. Super s=new Sub();

???Sub s2=s; ???强转

D. Super s=new Sub();

???ClassA c=(ClassA)s; ?

4:class Super{

public int getLength(){

return 4;

}

????}

class Sub extends Super{

public long getLength(){

return 5;

}

}

public class Test{

public static void main(String[] args){

Super s1=new Super();

Super s2=new Sub();

System.out.println(s1.getLength()+"\t"+s2.getLenght());

}

}

以上程序输出的结果是(D)

1. 4 ?4 ????B. 4 ?5 ?????C.5 ?5 ???D.编译报错??父类不能调用子类独有方法

5:完成(1、2、3、4、5)处代码（要求将完整题目抄写）

??

1：public Animal(){}

public Anamal(String name){

this.name=name;

}

2:public String getName(){

return name;

}

public void setName(String name){

this.name=name;

}

???????3:public Dog(){}

?public Dog(String name){

this.name=name;}

??????

???????4:Animal[] animal={new Dog,new Cat,new Dog,new Cat}

???????int count=0;

for(int i=0;i<animal.length;i++){

if(animal[i]instanceof new Dog){

count++;

}

}

System.out.println(count);

}

6:(多态)仔细阅读以下代码，编译是否通过，如果通过，写出输出结果；如果不能通过，则如何修改？

不能通过

Sub sub=(Sub)s;

sub.method(“Hello”);

7:(多态)仔细阅读以下代码，写出程序运行之后输出的结果。

method() in sub

method() in super

8:(多态)仔细阅读以下代码，下列几个选项中，有哪几个放在//1 位置能够编译通过()

1. return null;
2. return new Animal();
3. return new Dog();
4. return new Cat();

9:(多态)在继承题目的基础上，定义一个 Person 类型的数组，存储多个不同类型的子类 型对象，

(1) 统计并打印输出数组中所有学生干部的个数

(2) 打印输出所有学生的信息

package com.by.homework4.person;

public class Person {
????private String name;
????private String sex;
????private int age;
????private String nationality;

????public Person() {
????}

????public Person(String name, String sex, int age, String nationality) {
????????this.name = name;
????????this.sex = sex;
????????this.age = age;
????????this.nationality = nationality;
????}

????public String getName() {
????????return name;
????}

????public void setName(String name) {
????????this.name = name;
????}

????public String getSex() {
????????return sex;
????}

????public void setSex(String sex) {
????????this.sex = sex;
????}

????public int getAge() {
????????return age;
????}

????public void setAge(int age) {
????????this.age = age;
????}

????public String getNationality() {
????????return nationality;
????}

????public void setNationality(String nationality) {
????????this.nationality = nationality;
????}

????public void eat(){
????????System.out.println("人的吃饭方法");
????}
????public void sleep(){
????????System.out.println("人的睡觉方法");
????}
????public void work(){
????????System.out.println("人的工作方法");
????}
}





package com.by.homework4.person;

public class Student extends Person {
????private String school;
????private String schoolNum;
????public Student(){}

????public Student(String school, String schoolNum) {
????????this.school = school;
????????this.schoolNum = schoolNum;
????}

????public Student(String name, String sex, int age, String nationality, String school, String schoolNum) {
????????super(name, sex, age, nationality);
????????this.school = school;
????????this.schoolNum = schoolNum;
????}

????public String getSchool() {
????????return school;
????}

????public void setSchool(String school) {
????????this.school = school;
????}

????public String getSchoolNum() {
????????return schoolNum;
????}

????public void setSchoolNum(String schoolNum) {
????????this.schoolNum = schoolNum;
????}

????public void work(){
????????System.out.println("学生的学习方法");
????}
}



package com.by.homework4.person;

public class StudentLead extends Student {
????private String zhiwu;

????public StudentLead(){}

????public StudentLead(String zhiwu) {
????????this.zhiwu = zhiwu;
????}

????public StudentLead(String school, String schoolNum, String zhiwu) {
????????super(school, schoolNum);
????????this.zhiwu = zhiwu;
????}

????public StudentLead(String name, String sex, int age, String nationality, String school, String schoolNum, String zhiwu) {
????????super(name, sex, age, nationality, school, schoolNum);
????????this.zhiwu = zhiwu;
????}

????public String getZhiwu() {
????????return zhiwu;
????}

????public void setZhiwu(String zhiwu) {
????????this.zhiwu = zhiwu;
????}

????public void kaihui(){
????????System.out.println("学生的开会方法");
????}
}





package com.by.homework4.person;

public class Test {
????public static void main(String[]args){
????????Person []person={new Student("张yi","男",18,"中国","一中","123"),
????????????????new Student("张er","男",18,"中国","一中","123"),
????????????????new Student("张三","男",18,"中国","一中","123"),
????????????????new Student("张si","男",18,"中国","一中","123"),
????????????????new Student("张wu","男",18,"中国","一中","123"),
????????????????new Student("张liu","男",18,"中国","一中","123")};
????????for(int i=0;i< person.length;i++){
????????????Student students=(Student) person[i];
????????????System.out.println(students.getName()+students.getSex()+students.getAge()+students.getNationality()
????????????????????+students.getSchool()+students.getSchoolNum());
????????}
????????Student []student={new StudentLead("张yi","男",18,"中国","一中","123","团长"),
????????????????new StudentLead("张二","男",18,"中国","一中","123","团长"),
????????????????new StudentLead("张三","男",18,"中国","一中","123","团长"),
????????????????new StudentLead("张四","男",18,"中国","一中","123","团长"),
????????????????new StudentLead("张五","男",18,"中国","一中","123","团长"),
????????????????new StudentLead("张六","男",18,"中国","一中","123","团长")};
????????for(int i=0;i<student.length;i++){
????????????StudentLead studentLead=(StudentLead)student[i];
????????????System.out.println(studentLead.getName()+studentLead.getSex()+studentLead.getAge()+studentLead.getNationality()
????????????????????+studentLead.getSchool()+studentLead.getSchoolNum()+studentLead.getZhiwu());
????????}
????}
}

10:编程：在继承题目的基础上，创建一个长度为 3 的数组，里面有三个不同类型的对象， 分别打印这三个圆形、矩形、正方形的周长和面积。

11:编程：阅读以下代码，根据要求完成程序功能

(1) 在程序的 1、2、3 处填上适当的 构造方法或 get/set 方法

(2) 完成 4 处的填空：getAllDog 方法从一个 Animal 数组中挑选出所有的 Dog 对象，并把这

些对象放在一个 Dog 数组中返回

12:编程：在在继承题目的基础上，创建一个 Employee 数组，分别创建若干不同的 Employee 对象，并打印某个月的工资。