java：什么是抽象类，以及抽象类的作用和用法（abstract）

package com.jredu.oopch03;
/**
* 抽象类
* @author Administrator
*
*/
public abstract class Shape2 {

protected int bottom;//底
protected int height;//高

public Shape2(int bottom, int height) {
super();
this.bottom = bottom;
this.height = height;
}
public abstract void perimeter();//抽象方法
public abstract void area();

}

@Override
public void perimeter() {
// TODO Auto-generated method stub
int perimeter = (bottom*height)*2;
System.out.println("平行四边形的周长是："+perimeter);
}

@Override
public void area() {
// TODO Auto-generated method stub
int area = bottom*height;
System.out.println("平行四边形的面积是："+area);
}