Question

java初学者，下面程序为什么运行结果有问题，一个类访问不到另一个类的变量？？

public class Calculation {
public double weight;
public double height;
public double BMI;
public String state;

public void calculate(double weight, double height){
this.weight=weight;
this.height=height;
BMI=(weight*1.0*10000/(height)/(height));
BMI=(int)(BMI*10+0.5)/10.0;// round the number
if (BMI<18.5){
state="Underweight";
}
if(BMI>=18.5&&BMI<=24.9){
state="Normal";
}
if (BMI>=25&&BMI<=29.9){
state="Overweight";
}
if(BMI>=29.9){
state="Obese";
}
}
public double getWeight(){
return weight;
}
}

public class Display {

private Calculation cal= new Calculation();
public void display(){
System.out.println("Your weight: "+cal.getWeight()+"kg");
System.out.println("Your height: "+new Calculation().height+"cm");
System.out.println("Your BMI: "+new Calculation().BMI+"kg");
System.out.println("Your are in the "+new Calculation().state+" range");
}
}

public class BMI_tests {

public static void main(String[] args) {
double para1=Double.parseDouble(args[0]);
System.out.println(para1);
double para2=Double.parseDouble(args[1]);
System.out.println(para2);

Calculation ob=new Calculation();
Display ob2=new Display();
ob.calculate(para1,para2);
ob2.display();
}
}
输入72kg和170cm时，结果为：
Your weight: 0.0kg
Your height: 0.0cm
Your BMI: 0.0kg
Your are in the null range。
为什么Display类中，weight等变量没有得到Calculation中这些变量的数值？

Answer 1

Display类中定义了一个自己的对象，如下
private Calculation cal= new Calculation();
还有后面三句打印语句的new Calculation()，都是重新定义了变量。
运行是打印的是这些对象变量的值，在Display类中没有给他们赋值，所以打印时，是没有结果的。

修改如下：
public class Display {

private Calculation cal ;
public void display(Calculation calPara ){
cal = calPara ;
System.out.println("Your weight: "+cal.getWeight()+"kg");
System.out.println("Your height: "+cal..height+"cm");
System.out.println("Your BMI: "+cal.BMI+"kg");
System.out.println("Your are in the "+cal..state+" range");
}
}

public class BMI_tests {

public static void main(String[] args) {
double para1=Double.parseDouble(args[0]);
System.out.println(para1);
double para2=Double.parseDouble(args[1]);
System.out.println(para2);

Calculation ob=new Calculation();
Display ob2=new Display();
ob.calculate(para1,para2);
ob2.display(ob);
}
}