从一个类获取价值以在另一个类中使用它(java) [英] Get value from one class to use it in another class (java)

问题描述

我有两节课

ABC类

double a = 0.5 
public double lala()
{
     return a;
}

我想在另一个类中使用它，让我们说DEF类

I want use it in another class, let us say class DEF

ABC abc;
double baba = abc.lala();

但是它说java.lang.NullPointerException，你知道吗?谢谢

But it says that java.lang.NullPointerException, any idea? Thx

public class AdapterDB 
{ 
        double cal;
    double rcarbohydrate;
    double rfat;
    double rprotein;

    public AdapterDB(double cal, double rcarbohydrate, double rfat, double rprotein, Context ctx)
    {
        this.cal = cal;
        this.rcarbohydrate = rcarbohydrate;
        this.rfat = rfat;
        this.rprotein = rprotein;
        this.context = ctx;
        DBHelper = new DatabaseHelper (context);        
    }

        public double Calorie()
    {
        return cal;
    }

    public double Carbohydrate()
    {
        return rcarbohydrate;
    }

    public double Protein()
    {
        return rprotein;
    }

    public double Fat()
    {
        return rfat;
    }

}

这是我的ABC类的代码，我想在另一类> _<

Here is my code for class ABC, and I want to use some of them at another class >_<

推荐答案

您需要在内存中分配ABC，否则它指向 null ，因此出现NullPointerException:

You need to allocate ABC in memory, otherwise it points to null, hence NullPointerException:

ABC abc = new ABC();
double baba = abc.lala();

编辑:好的，以防ABC没有零参数的构造函数，如:

Edit: Ok, in case ABC does not have a zero-argument constructor like:

public ABC() { }

您有两种选择来解决您的问题.要么添加一个，然后在内部将其他字段的默认值赋予默认值.喜欢:

you have two options to solve your problem. Either add one and inside give default values to your other fields. Like:

public AdapterDB()
{
    DBHelper = new DatabaseHelper (context);        
}

或者，修改对象的创建，例如:

Or, modify the creation of the object like:

double p1 = <some value>;
double p2 = <some value>;
double p3 = <some value>;
double p4 = <some value>;
Context ctx = new Context(); // don't know what this does.
ABC abc = new ABC(p1, p2, p3, p4, context);
double baba = abc.lala();

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