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

运行结果为：