can someone help me with my global variables?

Question

Ok, I am creating a program that uses a normal global class but it doesn't seam to work. I can get the value of variables from the global but I cant set a new value.

I created a really short test program just to get this to work so I can change it to my other much bigger program.

In this small program I have a textview and a edit text with a button in the middle

all it is designed to do is when the button is clicked take what was typed in the editText and place it in a variable and send that variable to globals

then at the same time get the variable from globals and set the textView text to that variable...EASY...

but it will not save to globals...

can someone help me please?

here is the global code that I am using.

 package com.example.arnoldray007.globalstest;

    /**
     * Created by arnoldray007 on 5/3/2016.
     */
    public class Globals
    {
        private static Globals instance;

        //this is the global variable
        private String received;

        //this restricts the constructor from being instantiated
        private Globals(){}

        public void setReceived(String r)
        {
            this.received = r;
        }

        public String getReceived()
        {
            return this.received;
        }


        //this is the global variable
        private String send = "Sorry im busy right now.";

        public void setSend(String s)
        {
            this.send = s;
        }

        public String getSend()
        {
            return this.send;
        }
      public static synchronized Globals getInstance()
        {
            if(instance == null)
            {
                instance = new Globals();
            }
            return instance;
        }

and here is the MainActivity code that I am using

public class MainActivity extends AppCompatActivity
{
    Globals g = Globals.getInstance();
    String received = g.getReceived();  //this retrieves the variable


    Button button;
    EditText editText;
    TextView textView;
    String temp;

    public void myClickThree(View v)//this is my onclickThree for my refresh button
    {
        temp = editText.getText().toString();
        g.setReceived(temp); //this sends the new  message to the global variable adding it to it
        received = g.getReceived();
        editText.setText(received);
    }

    @Override
    protected void onCreate(Bundle savedInstanceState)
    {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        button = (Button) findViewById(R.id.button);
        editText = (EditText) findViewById(R.id.editText);
        textView = (TextView) findViewById(R.id.textView);
    }
}

Thanks for any help that I can get......

I am getting an error now can anyone tell me what this means?

05-05 09:41:47.476 9601-9601/com.example.arnoldray007.arnoldfinalproject E/AndroidRuntime: FATAL EXCEPTION: main
  java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.arnoldray007.arnoldfinalproject/com.example.arnoldray007.arnoldfinalproject.Received_messages}: java.lang.NullPointerException
      at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2059)
      at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2084)
      at android.app.ActivityThread.access$600(ActivityThread.java:130)
      at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1195)
      at android.os.Handler.dispatchMessage(Handler.java:99)
      at android.os.Looper.loop(Looper.java:137)
      at android.app.ActivityThread.main(ActivityThread.java:4745)
      at java.lang.reflect.Method.invokeNative(Native Method)
      at java.lang.reflect.Method.invoke(Method.java:511)
      at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:786)
      at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:553)
      at dalvik.system.NativeStart.main(Native Method)
   Caused by: java.lang.NullPointerException
      at android.widget.TextView.append(TextView.java:3118)
      at com.example.arnoldray007.arnoldfinalproject.Received_messages.onCreate(Received_messages.java:91)
      at android.app.Activity.performCreate(Activity.java:5008)
      at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1079)
      at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2023)
      at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2084) 
      at android.app.ActivityThread.access$600(ActivityThread.java:130) 
      at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1195) 
      at android.os.Handler.dispatchMessage(Handler.java:99) 
      at android.os.Looper.loop(Looper.java:137) 
      at android.app.ActivityThread.main(ActivityThread.java:4745) 
      at java.lang.reflect.Method.invokeNative(Native Method) 
      at java.lang.reflect.Method.invoke(Method.java:511) 
      at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:786) 
      at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:553) 
      at dalvik.system.NativeStart.main(Native Method)

I suppose I could help you by telling you global variables are, generally, a terrible idea. — Louis Wasserman, May 04 '16 at 21:41
this is a test program lol will not be used, only to figure out how to change the value of my global variable — Danny Arnold, May 04 '16 at 21:41
BUt, I have a receiver and inorder to take a variable from that and use it on another class I need a global — Danny Arnold, May 04 '16 at 21:42
the receiver gets all messages that are sent to me by text and I want to store those messages so I can display them on a different screen and in order to do that I need to send that variable to a different class. and the only way I know how to do that is by Intent or global variables — Danny Arnold, May 04 '16 at 21:43
I can get the variables from the globals but it will not save new values to globals — Danny Arnold, May 04 '16 at 21:46
added a answer to better explain: a wrong getinstance() implementation could cause the problem — Lorenzo Barbagli, May 04 '16 at 21:48
I have spent days on this and a few other problems, I haven't taken a Java class, and im teaching myself everything. its slow going and android studio always has problems.... — Danny Arnold, May 04 '16 at 21:48
Suggestion, move the declaration of `String temp` to the body of `myClickThree()`. The way it's coded now, `temp` is acting like a global variable of `MainActivity ` (a Bad Thing). — Frecklefoot, May 04 '16 at 21:55
if someone has a better idea on how to take a variable and send it to another class without a intent im up for that idea as well — Danny Arnold, May 04 '16 at 21:56
yea , I had it their and it didn't work so I movend it to see if it changed anything — Danny Arnold, May 04 '16 at 21:57
I think that this part of the code may be wrong(g.setReceived(temp);)???? — Danny Arnold, May 04 '16 at 22:03
because when the button returns the variable it returns a null value(empty) that means that the global value was never changed... — Danny Arnold, May 04 '16 at 22:04
What exactly is this supposed to do? You take some string out of an EditText, but you put it back in the same EditText.... Anyways, options: 1) Use an Intent because why not? 2) Use SharedPreferences if you plan on saving that data in the long-term. — OneCricketeer, May 04 '16 at 22:21
I take data out of the editText and place it in a global variable and then get that variable back and place that text in a textView — Danny Arnold, May 04 '16 at 22:27
the receiver only runs when I get a text, it has no intent in it — Danny Arnold, May 04 '16 at 22:28

score 1 · Answer 1 · edited May 04 '16 at 22:14

1

in singleton pattern getInstance() should be:

private static Globals instance;

public static Globals getInstance() {
    if (instance == null)
        instance = new Globals();
    return instance;
}

Are you sure to have this exact code inside Globals class body?

EDIT:

I think for this kind of problem the best solution is using a superclass Application, where you mantain all the global stuffs. You will find thousands of tutorial on this android programming pattern, just google "extending application android".

edited May 04 '16 at 22:14

OneCricketeer

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answered May 04 '16 at 21:47

Lorenzo Barbagli

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That MAY be the problem. If you're creating the instance every time you call getInstance(), it will look like the object is not saving the new values. – Frecklefoot May 04 '16 at 21:52
1

did you look at what im using – Danny Arnold May 04 '16 at 21:54
its the same but inbetween Globals and Static I have Synchronized – Danny Arnold May 04 '16 at 21:55
public static synchronized Globals getInstance() { if(instance == null) { instance = new Globals(); } return instance; } – Danny Arnold May 04 '16 at 21:55
I saw it after you added it. It looks like you added it after my comment. – Frecklefoot May 04 '16 at 21:56
but, I still cant change the value of the variable in the global class using the button – Danny Arnold May 04 '16 at 21:59
that's what I was talking about: http://stackoverflow.com/questions/11165852/java-singleton-and-synchronization. I'll probably use synchronized in getReceived and setReceived methods, but it depends on what you want to do – Lorenzo Barbagli May 04 '16 at 22:00
if your talking about a class that Extends application, then change the name of the program to the name of your class, I tried that as well – Danny Arnold May 04 '16 at 22:08

score 0 · Accepted Answer · answered May 04 '16 at 22:46

0

There isn't anything wrong with the code you've shown except your comment seems misleading.

this sends the new message to the global variable adding it to it

Nothing is being added to anything. Because your method there only does an assignment.

public void setReceived(String r)
{
    this.received = r;
}

If you want to see that your class is actually working, maybe you should do something more than get text from an EditText and assign the string back into the EditText. That will only appear like nothing is happening.

So, try this instead in onCreate()

g = Globals.getInstance();

button = (Button) findViewById(R.id.button);
editText = (EditText) findViewById(R.id.editText);
textView = (TextView) findViewById(R.id.textView);

button.setOnClickListener(new View.OnClickListener() {
    @Override
    public void onClick(View v) {
        String temp = editText.getText().toString();
        g.setReceived(temp);
        received = g.getReceived();
        textView.append("\n"+received);
    }
});

answered May 04 '16 at 22:46

OneCricketeer

179,855
19
132
245

Thanks a lot, this works great on my test application, but when I try to implement it on my real application it pops up with an error – Danny Arnold May 05 '16 at 14:39
The error `java.lang.NullPointerException at android.widget.TextView.append` means you didn't initialize your TextView or findViewById returned null – OneCricketeer May 05 '16 at 15:05
I don't understand , because I did. here is what I have for the display( EditText editText9, changeEditText;)---(changeEditText = (EditText) findViewById(R.id.changeEditText);)---rember that I said that the error was coming from my real program not the test program..... the test program works fine with the answer below... but when I ad the fix to my program it gives me this error... – Danny Arnold May 06 '16 at 13:12
That's an EditText, not a Textview. Anyways. I'm not sure why you'd get an error in one device, but not another. I'm just telling you what the error means – OneCricketeer May 06 '16 at 13:30

can someone help me with my global variables?

2 Answers2