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)

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()

return null;
return new Animal();
return new Dog();
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

复制代码

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());
        }
    }
}