抽象类不能实例化（创建对象）：

抽象类中不一定有抽象方法：

有抽象方法的类一定是抽象类：

可以有构造方法：（作用：在创建子类对象时，给属性进行赋值的）

Person类：

public abstract  class Person {

    private String name;
    private int age;

    public Person() {
    }

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

    public String getName() {
        return name;
    }

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

    public int getAge() {
        return age;
    }

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

    public abstract void work();
    public void sleep(){
        System.out.println("睡觉");
    }
}

Student类：

public class Student extends Person{
    public Student() {
    }

    public Student(String name, int age) {
        super(name, age);
    }

    @Override
    public void work() {
        System.out.println("学生的工作是学习");
    }
}

Test类：

public class Test {
    public static void main(String[] args) {
        Student s = new Student ("张三",23);
        System.out.println(s.getName()+","+s.getAge());
    }
}

代码运行结果：

抽象类的子类：

有两种选择：1、重写所有抽象方法；2、子类也是抽象类（推荐第一种）

否则会报错：

案例：

test类代码：

public class test {
    public static void main(String[] args) {
        Dog d = new Dog("小狗",2);
        frog f = new frog("小蛙",4);
        Sheep s = new Sheep("小羊",3);

        System.out.println(d.getName()+","+d.getAge());
        d.eat();
        d.drink();
        System.out.println(f.getName()+","+f.getAge());
        f.eat();
        f.drink();
        System.out.println(s.getName()+","+s.getAge());
        s.eat();
        s.drink();
    }


}

Aniaml类代码：

public abstract class Animal {
    private String name;
    private int age;

    public Animal() {
    }

    public Animal(String name, int age) {
        this.name = name;
        this.age = age;
    }

    public String getName() {
        return name;
    }

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

    public int getAge() {
        return age;
    }

    public void setAge(int age) {
        this.age = age;
    }
    public void drink(){
        System.out.println("喝水");
    }
    public abstract void eat();
}

frog类代码：

public class frog extends Animal{
    public frog() {
    }

    public frog(String name, int age) {
        super(name, age);
    }

    @Override
    public void eat() {
        System.out.println("吃虫子");
    }
}

Dog类代码：

public class Dog extends Animal{
    public Dog() {
    }

    public Dog(String name, int age) {
        super(name, age);
    }

    @Override
    public void eat() {
        System.out.println("吃骨头");
    }
}

Sheep类代码：

public class Sheep extends Animal{
    public Sheep() {
    }

    public Sheep(String name, int age) {
        super(name, age);
    }

    @Override
    public void eat() {
        System.out.println("吃草");
    }
}

运行结果为：