2016/5/21学习记录

1.重大教训！先上代码

package Pra;

public class Read {
    
    public static void main(String[] args) {
    T a = new T();
    Read.swap(a);
    System.out.println("e1 = "+a.e1+"e2 = "+a.e2);
    }
    public static void swap(T t){
    int temp = t.e1;
    t.e1 = t.e2;
    t.e2 = temp;
    
}
}
 class T {
     T(){}
     int e1 = 1;
     int e2 = 2;
}

这段代码之前一直出错“No enclosing instance of type  is accessible. ”百度了一下，如下，跟我的情况一样，把下面的类class改成static确实解决了问题。

但总觉得哪里不对，因为理论上静态方法不能调用实例方法或者访问实例数据域，因为静态方法和静态数据域不属于某个特定的对象，但是实例化之后就可以了啊。

后来我发现我的括号打错了！！！！结果就是Class T写在了Class Read里面，实际上在mian函数里面实例化了T，也可以进行引用的。

所以不能轻信楼上这种做法！吸取点教训。

2.P305 exercise9.1

package Exercise;

public class Rectangle {
    private double width = 1;
    private double height = 1;
    Rectangle(){
    }
    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 void print(){
        System.out.println("The width of the rectangle is "+width);
        System.out.println("The height of the rectangle is "+height);
        System.out.println("The Area of the rectangle is "+getArea());
        System.out.println("The Perimeter of the rectangle is "+getPerimeter());
    }
    public static void main(String[] args) {
     Rectangle t1 = new Rectangle(4,40);
    t1.print();
    }

}

3.Exercise 9.2

package Exercise;


public class Stock {
    private String symbol ;
    private String name;
    private double PCR;
    private double CP;
    Stock(String symbol,String name){
        this.symbol = symbol;
        this.name = name;
    }
    double getChangePercent(){
        return ((CP-PCR)/PCR*100);
    }
    void print(){
        System.out.println("the symbol of the stock is "+symbol);
        System.out.println("the name of the stock is "+name);
        System.out.printf("%4.2f",Math.abs(getChangePercent()));
    }
    public static void main(String[] args) {
    Stock s = new Stock("Orcl","Oracle Corp");
    s.PCR = 34.5;
    s.CP = 34.35;
    s.print();
    }

}