描述：父类拥有income字段

子类也拥有income字段

使用多态访问income字段，默认访问的是父类的income，而不是子类的

public class Father{
    double income;    
    public Father(double income) {
        this.income = income*2;    
 }
}
class Salary extends Father{
    protected double income;   
    protected double some= 1.0;    
    public Salary(double income) {
        super(income);        
        this.income = income;    
 }
}
class Main{
    public static void main(String[] args) {
        Father d = new Salary(7500);       
        System.out.println(d.getClass().getName());        //输出 Salary
        System.out.println(d.income);                      //输出 15000.0
 }
}