尝试在空对象引用上调用虚拟方法-全局变量类

Question

I'm learning about MVC. My project has tons of variables. So I made a new class for them called MainVariables.

public class MainVariables {

private String mPictureDirectory;
private String mNameOfThePictureFile;
private String mFullPathPicture;
private double mLongitude;
private double mLatitude;
private String mAddress;
private String mCity;
private String mState;
private String mCountry;
private String mPostalCode;
private String mKnownName;
private String mDescription;
private String mSolicitationType;
...
...

The rest is composed by automatic getters and setters for each variable.

I'm having a problem accessing and casting those variables across my application.

I tried accessing it by casting the following in other files:

private MainVariables mMainVariables;

The above code throws the error Attempt to invoke virtual method on a null object reference

Then I tried the following:

private MainVariables mMainVariables = new MainVariables();

Now, this does work. Only in the file it's using though. For Example, I set variables from within the "SolicitationFragment" and when I try to access it on "PostFragment", I get an empty result.

That's because I'm having to initialize MainVariables on each file.

How can I get around this and be able to access my variables globally?

Answer 1

Make the variables static, or final if you're not going to change them. This way you don't have to create a new instance and can call MainVariables.mPictureDirectory immediately

public class MainVariables {
    public static String mPictureDirectory;
}

Another option is a singleton pattern, this way you create only one instance of an object and still can use getters and setters

public class MainVariables {
    private static MainVariables mInstance = null;

    private String mString;

    private MainVariables(){
        mString = "Hello";
    }

    public static MainVariables getInstance(){
        if(mInstance == null)
        {
            synchronized (MainVariables.class) {
            if (mInstance== null) {
                mInstance= new MainVariables();
            }            
        }
        return mInstance;
    }

    public String getString(){
        return this.mString;
    }

    public void setString(String value){
        mString = value;
    }
}

In your MainActivity you can declare a field
MainVariables mainVariables = MainVariables.getInstance()
and call
mainVariables.[METHOD] from basically anywhere in your MainActivity

Answer 2

Create a class extending your Application class and create a method to get instance of MainVariables :

AppController.java

public class AppController extends Application {

    private MainVariables mMainVariables;
    private static AppController mInstance;

    @Override
    public void onCreate() {
        super.onCreate();
        mInstance = this;
    }

    public static synchronized AppController getInstance() {
        return mInstance;
    }

    public MainVariables getMainVariables() {

        if (mMainVariables == null) {
            mMainVariables = new MainVariables();
        }
        return mMainVariables;
    }
}

MainVariables.java

public class MainVariables {

    private String string;

    public String getString(){
        return this.string;
    }

    public void setString(String string){
        this.string = string;
    }
}

USE:

// SET VALUE
AppController.getInstance().getMainVariables().setString("Hello Android");

// GET VALUE
String str = AppController.getInstance().getMainVariables().getString();

FYI, You have to add AppController class under application name in your AndroidManifest.xml file.

<application
    android:name=".AppController">

</application>

Hope this will help~