如何在两个不同的类中使用getter和setter

Question

how to use getter setter in two different class

Class A{

    int a = 10;
    GetterAndSetter gs = new GetterAndSetter();
    gs.setValue(a);
}


Class GetterAndSetter {
    int a ;
    public void setValue(int a){
    this.a = a;
}

public int getValue(){

return a;

   }

}


class B {

    int c;
    GetterAndSetter gs = new GetterAndSetter();
    c = gs.getValue();
}

While printing c it gives null. And tell me if it is valid or not.

Answer 1

Whenever you write this

GetterAndSetter gs = new GetterAndSetter();

what you're doing is to create a new instance of GetterAndSetter . Two instances that you create won't have any connection between them.

Inside class A , you create a new instance, and set its value. Inside class C , you create a new instance, and read its value. But because you've got two different instances, the value you're reading isn't connected with the value you're setting.

This is roughly like:

1. I buy an envelope, and put some money inside it.
2. Later on, I want to get the money back, so I buy a new envelope, and look for the money inside it.

You have to be looking in the same envelope that you put the money in, if you want to find it!

Answer 2

In class A , your code creates a new instance of GetterAndSetter and sets a value to the property. In class B , however, your code creates again another new instance of GetterAndSetter , then gets the value.

The instances your code works with in classes A and B are not the same - hence you don't obtain the values set in A when trying to get it in B . The instance of GetterAndSetter created in B is not used anymore after the code in B exits.

To fix this, you need to pass a reference to the GetterAndSetter instance from class A to B . You can do this eg by passing it as a parameter to a method of B , or by creating a new instance of A in B and calling a method that provides an instance of GetterAndSetter .

An example of the first option (pass as parameter):

Class A{
...
 GetterAndSetter createAndSet();
    int a = 10;
    GetterAndSetter gs = new GetterAndSetter();
    gs.setValue(a);
    return gs;
  }
...
}

class B {
...
 void getValueFromGetterAndSetter(GetterAndSetter gs) {
    int c;
    c = gs.getValue();
    ...
 }
...
}

To connect the instances, we of course also need to have another piece of code (assuming instances of A and B exist already):

...
        b.getValueFromGetterAndSetter(a.createAndSet());
...

Answer 3

You have used different reference. You should use same reference so that only you can access the value.

Answer 4

你需要了解oops的基础知识，你已经在A类中创建了一个实例，并且你试图在B类中访问，这只有在你将该对象的引用从A类传递给B时才有可能。在这种情况下你必须有你在类B中的A类中创建的GetterAndSetter实例，而是创建了另一个新实例，它将在内存中创建新引用，类变量a将为null。

Answer 5

In your code, both class A and B create new objects for GetterAndSetter. Hence they are not shared between these classes. Thats why you are getting null.

I wounder how your code print null for C. I think it would be "0" instead.

Answer 6

It is valid, here is what happens:

You create object gs in class A

Then you set the value of a of that object to 10

You then create another object gs in class B

Last but not least, you ask the object gs from class B what its value for a is.

Guess what, its NULL as you did not set its value anywhere so it wont return one.