我们可以在Java中使用公共静态变量，在其他类中使用一个类的变量吗？

Question

Can we use public static variable, to use variable of one class in other class in Java? Some people said it should not be used beacuse when class unloaded the variables will assigned to null .

Is this correct if it is correct we should not use public static variable, to use variable of one class in other class in Java. So what is the easy method.

1. creating object and use the variable
2. create interface class.

Which is the standard method is being used for Java to use the variable of one class in other class?

Update:

public class BluetoothLePlugin extends CordovaPlugin
{
public static CallbackContext callBack;
public boolean execute(final String action, JSONArray args, CallbackContext callbackContext) throws JSONException {

callBack=callbackContext;


}


}

Here I am getting Callbackcontext i want to use this callback context in other classes like

public class mainClass{

  public void onCreate() {


        BluetoothUtils mBluetoothUtils = new BluetoothUtils(mActivity);
        BluetoothAdapter.LeScanCallback  mLeScanCallback =null;
        mLeScanCallback = new BluetoothAdapter.LeScanCallback() {
            @Override
            public void onLeScan(final BluetoothDevice device, final int rssi, final byte[] scanRecord) {


                Log.d("BluetoothLeDevice", " " + device + " " + device.getName() + "      " + rssi + "    " + scanRecord + "  " + device.getAddress());
                mDeviceAdress = "80:98:00:98:00:5A";

                jsonobdevice = new JSONObject();
                if (device.getName() != null && device.getName().trim().length() > 0) {

                    try {

                        if (!jsonobdevice.has("uuid)") ){
                            jsonobdevice.put("name", device.getName());
                            jsonobdevice.put("uuid", device.getAddress());
                            jsonobdevice.put("state", "");
                            jsonarraydevice.put(jsonobdevice);
                            Log.d("", " " + jsonarraydevice);
                            PluginResult result = new PluginResult(PluginResult.Status.OK, jsonarraydevice);
               result.setKeepCallback(true);

                            BluetoothLePlugin.callBack.sendPluginResult(result);//Here I am using callback context like this
                        }

                    } catch (JSONException e) {
                        e.printStackTrace();
                    }
                }
              //  Callbackpublic.webView.loadUrl("javascript:");


            }
        };

        BluetoothLeScanner mScanner = new BluetoothLeScanner(mLeScanCallback, mBluetoothUtils);
        boolean mIsBluetoothOn = mBluetoothUtils.isBluetoothOn();
        boolean mIsBluetoothLePresent = mBluetoothUtils.isBluetoothLeSupported();

        mBluetoothUtils.askUserToEnableBluetoothIfNeeded();
        if (mIsBluetoothOn && mIsBluetoothLePresent) {
            mScanner.scanLeDevice(-1, true);

        }

    }
}

I am using like this in the above class

BluetoothLePlugin.callBack.sendPluginResult(result);

Is what I am doing. Is correct my seniors told dont use public static. So I am Asking question.If it is wrong, what is the solution.

Answer 1

Yes, you can use a public static method to access variables of another class in Java. Something like:

public static int variable;

And then from another class:

ExternalClass.variable;

Please keep in mind that making a variable static means that it is not tied to any specific instance of your class. If you have a class Example , and you do:

Example e = new Example();

a static variable of class Example belongs to the class ( Example.variable ) not the instance ( e.variable ).

Answer 2

Yes you can use static variables to use values in other class. because static variables resides in memory throughout the application life cycle.

Answer 3

You can access public static variables of another class in Java. See some basic tutorial Understanding Class Members first.

Sometimes, you want to have variables that are common to all objects. This is accomplished with the static modifier. Fields that have the static modifier in their declaration are called static fields or class variables. They are associated with the class, rather than with any object. Every instance of the class shares a class variable, which is in one fixed location in memory. Any object can change the value of a class variable, but class variables can also be manipulated without creating an instance of the class.

Answer 4

IMHO, the quick answer is no. In general terms, you should be careful about exposing VARIABLES; even more if they are static fields. This approach changes with regards to CONSTANTS. I would use static modifier to expose constants because there is no risk of accidentally modifying its value.

Remember, changes in value of a static field affects to ALL instances of that class; because it is a property of the class itself and not the individual instances.

Answer 5

You could use setters and getters to create a new variable in another class of that value. This is called instance variables.

Heres and example.

public class Car
{

 private int milage = 100;

  //........


 public int getMilage()
 {
    return this.milage;
 }

Then in the other class create an instance of that class.

 public class ExampleClass
{
  public static void main(String [] args)
  { 
    Car c = new Car();

    int newMilageVariable = c.getMilage();

    System.out.println(newMilageVariable);
  }
}

---

Additionally, you can use a static variable that can be accessed trough out other classes. The following code example should suffice.

 public static int milage;


 //********************
public class ExampleClass
{

  //..............

  Car c = new Car();   // new car object
  int milage = c.milage;  //access the static variable
}