Java之期末考试不挂科

考试题型：30道单选题（30'）+ 6道函数题（70'）+ 1道编程题（20'）

函数题：类的定义、接口的实现、继承和多态、数组和排序、自定义异常、多线程

题源：PTA

目录

1. 类的定义

2. 继承和多态

3. 接口的实现

4. 数组和排序

5. 自定义异常类 （最简单的）

6. 多线程(基本的使用)

7. 易错单选题：

【附录】

---

1. 类的定义

练习1：6-6 设计一个矩形类Rectangle

import java.util.Scanner;
/* 你的代码将被嵌入到这里 */
class Rectangle{double width=1;double height=1;public Rectangle() {}public Rectangle(double width, double height) {this.width = width;this.height = height;}public double getArea(){return width*height;}public double getPerimeter(){return 2*(width+height);}
}public class Test1 {public static void main(String[] args) {Scanner input = new Scanner(System.in);double w = input.nextDouble();double h = input.nextDouble();Rectangle myRectangle = new Rectangle(w, h);System.out.println(myRectangle.getArea());System.out.println(myRectangle.getPerimeter());input.close();}
}

2. 继承和多态

练习1：6-7 Shape类

import java.util.Scanner;public class Test2{public static void main(String[] args) {Scanner scan = new Scanner(System.in);double length = scan.nextDouble();Square s = new Square(length);System.out.printf("%.2f ",s.getPerimeter());System.out.printf("%.2f\n",s.getArea());length = scan.nextDouble();double wide = scan.nextDouble();rectangle r = new rectangle(length,wide);System.out.printf("%.2f ",r.getPerimeter());System.out.printf("%.2f\n",r.getArea());double radius = scan.nextDouble();Circle c = new Circle(radius);System.out.printf("%.2f ",c.getPerimeter());System.out.printf("%.2f\n",c.getArea());scan.close();}
}/* 你的代码被嵌在这里 */
abstract class Shape{public abstract double getPerimeter();public abstract double getArea();
}class Square extends Shape{double len;public Square(double len) {this.len = len;}@Overridepublic double getPerimeter() {return len*4;}@Overridepublic double getArea() {return len*len;}
}class rectangle extends Shape{double len;double wid;public rectangle(double len, double wid) {this.len = len;this.wid = wid;}@Overridepublic double getPerimeter() {return 2*(len+wid);}@Overridepublic double getArea() {return len*wid;}
}class Circle extends Shape{double r;public Circle(double r) {this.r = r;}@Overridepublic double getPerimeter() {return 2*Math.PI*r;}@Overridepublic double getArea() {return Math.PI*r*r;}
}

例2：人喂动物

Person类中：

    public void keepPet(Animal a,String something){if(a instanceof Dog){Dog d = (Dog) a;System.out.println(age + "岁的" + name + "在喂狗");} else if (a instanceof Cat) {Cat c = (Cat) a;System.out.println(age + "岁的" + name + "在喂猫");}else {System.out.println("没有这种动物");}

重点是：

 public void keepPet(Animal a,String something){}   #参数是类,可以传入它的所有子类。

Main中：

    public static void main(String[] args) {Person wang = new Person("老王",40);Animal dog = new Dog(2,"黑色");//第一个参数是Animal，当方法的参数是一个类时，可以传入它的所有子类//这里就可以传入Animal的子类：Dog和Cat ，注意:传入的是实例dog而非类型Dogwang.keepPet(dog,"骨头");Person li = new Person("老李",45);Animal cat = new Cat(3,"黄色");li.keepPet(cat,"鱼刺");}

重点是：

Animal dog = new Dog(2,"黑色");   //父类的引用

wang.keepPet(dog,"骨头");   //多态

li.keepPet(cat,"鱼刺");   //多态

3. 接口的实现

小测2：7-1 定义接口（Biology、Animal）、类（Person）、子类（Pupil）

代码：

public interface Biology {public abstract void breathe();
}public interface Animal {public abstract void eat();public abstract void sleep();
}public class Person implements Animal,Biology{@Overridepublic void eat() {System.out.println("我会按时吃饭");}@Overridepublic void sleep() {System.out.println("早睡早起身体好");}@Overridepublic void breathe() {System.out.println("我喜欢呼吸新鲜空气");}public void think(){System.out.println("我喜欢思考");}
}public class Pupil extends Person{private  String school;public Pupil() {}public Pupil(String school) {this.school = school;}public String getSchool() {return school;}public void setSchool(String school) {this.school = school;}}public class Main {public static void main(String[] args) {Pupil zhangsan = new Pupil("福建理工大学");zhangsan.breathe();zhangsan.eat();zhangsan.sleep();zhangsan.think();System.out.println("我的学校是" + zhangsan.getSchool());}
}

运行结果：

 发现：若父类实现了接口，那么子类也会实现该接口。

4. 数组和排序

方法1：实现Comparable接口并重写compareTo方法，然后再用Collections去调用sort方法，在sort方法里面添加list对象进行排序

（类实现parable，然后重写pareTo，最后Main中Collections.sort来排序）

Main函数：public static void main(String[] args) {ArrayList<Worker> list=new ArrayList<>();//排序Collections.sort(list);  //③ Collections调用sort方法
}

①实现Comparable接口，class Worker implements Comparable<Worker>{}

class Worker implements Comparable<Worker>{private int age;private String name;private double salary;public int getAge() {return age;}public void setAge(int age) {this.age = age;}public String getName() {return name;}public void setName(String name) {this.name = name;}public double getSalary() {return salary;}public void setSalary(double salary) {this.salary = salary;}public Worker(int age, String name, double salary) {this.age = age;this.name = name;this.salary = salary;}@Overridepublic String toString() {return "Worker{" +"age=" + age +", name='" + name + '\'' +", salary=" + salary +'}';}@Overridepublic int compareTo(Worker o) {/*按照年龄age进行排序*/return this.age-o.age;
//        /*按照姓名name进行排序*/
//        return this.name.compareTo(o.name);}
}

② 重写compareTo函数：

    public int compareTo(Worker o) {
        /*按照年龄age进行排序*/
        return this.age-o.age;   //用自己减别人是升序的

//        /*按照姓名name进行排序*/
//        return this.name.compareTo(o.name); //字符串比较要用compareTo()
    }

 方法2：直接用Collections去调用sort方法，但这个sort方法含有两个参数，一个参数是list,另一个参数是new一个Comparator<Worker>，然后再去重写compare方法。

（类不变，全都在Main里了：两参一列一parator,最后重写pare方法）

    public static void main(String[] args) {ArrayList<Worker> list=new ArrayList<>();//在list中添加几个对象list.add(new Worker(15,"张3",3522.0));list.add(new Worker(17,"张1",3512.0));list.add(new Worker(16,"张2",3532.0));Collections.sort(list, new Comparator<Worker>() {@Overridepublic int compare(Worker o1, Worker o2) {/*按照age排序*/return o1.getAge()-o2.getAge();
//                /*按照name排序*/
//                return o1.getName().compareTo(o2.getName());}});}

重点是：

        Collections.sort(list, new Comparator<Worker>() {  @Overridepublic int compare(Worker o1, Worker o2) {  /*按照age排序*/return o1.getAge()-o2.getAge();   
//                /*按照name排序*/
//                return o1.getName().compareTo(o2.getName()); }});

Tips：比较数字用减法，比较字符用pareTo()

5. 自定义异常类 （最简单的）

【方法】所有异常继承RuntimeException或Exception，异常类中写3个东西：空参构造、带参构造和toString()函数。

可能就像这样：名字不合法异常、分数不合法异常

Main函数中：

try {
    String name = sc.next();
    int score = sc.nextInt();
    Stu stu = new Stu(name,score);
    System.out.println(stu);
    }catch (illegalNameException | illegalScoreException e){
          System.out.println(e);
    }catch (Exception e){
          System.out.println("java.util.NoSuchElementException");
    }
}

重点是：

//异常类
//名字不合法异常
class illegalNameException extends RuntimeException{String name;public illegalNameException() {}public illegalNameException(String message) {this.name = message;}@Overridepublic String toString() {return "IllegalNameException: the first char of name must not be digit,name="+name;}}//分数不合法异常
class illegalScoreException extends RuntimeException{int score;public illegalScoreException(){}public illegalScoreException(String message){this.age=message;}public String toString(){return "return "IllegalScoreException: score out of range, score="+score;"}
}

 public Stu(String name, int score) {
        if(name.charAt(0)>='0'&&name.charAt(0)<='9'){
            throw new illegalNameException(name);
        }
        if(score<0||score>100){
            throw new illegalScoreException(score+"");
        }
        this.name = name;
        this.score = score;
    }

6. 多线程(基本的使用)

练习3：6-1（大概率是这种题）

【方法】线程类继承自Thread，里面包括3个东西：无参构造、带参构造和run()函数。

public class Test1 {public static void main(String[] args) {Scanner sc = new Scanner(System.in);Thread t1 = new MyThread(Integer.parseInt(sc.next()));t1.start();}
} 

class MyThread extends Thread{int n;public MyThread() {}public MyThread(int n) {this.n = n;}public void run(){for (int i=0;i<n;i++){System.out.println(i);}System.out.println(Thread.currentThread().getName()+" "+isAlive());}
//    int sum=0;
//    public void run(){
//        for (int i=0;i<n;i++){
//            sum+=i;
//        }
//        System.out.println("sum:" + sum);
//    }
}

7. 易错单选题：

练习1

 

练习2：

 

 

 

 

练习3：

练习4：

 

小测1：

小测2：

 【附录】

1. 在当前类中调用成员变量直接使用即可。如:

public class Person {String name;int age;System.out.println(age +"岁的" + name +"在喂狗");
}

但若是子类调用从父类继承下来的成员变量，就不直接写变量名了，要写成 get成员方法

public class Animal {int age;String color;
}public class Dog extends Animal{public void eat(String something){System.out.println("我家的小狗" + getAge()+"岁,颜色是"+getColor());}
}

2. 输入

Scanner sc = new Scanner(System.in);
zhangsan.setSchool(sc.next());

3. Ctrl + P 显示提示

4. Alt + Enter 报错修正

5. Fn+ Alt +Insert ，插入

好啦，此次的Java复习就先到这，还有1小时55分钟就要奔赴考场啦~祝我们都能考个好成绩！(ᕑᗢᓫ∗)˒(ᕑᗢᓫ∗)˒(ᕑᗢᓫ∗)˒(ᕑᗢᓫ∗)˒(ᕑᗢᓫ∗)˒(ᕑᗢᓫ∗)˒(ᕑᗢᓫ∗)˒(ᕑᗢᓫ∗)˒(ᕑᗢᓫ∗)˒(ᕑᗢᓫ∗)˒(ᕑᗢᓫ∗)˒(ᕑᗢᓫ∗)˒

                                                                                                              ——2023.6.19 下午 🌤