Getting "Null Object Reference" While Trying Encapsulation

Question

It's probably stupidly simple, but I can't seem to figure out what's wrong. I'm trying to create a demo for showcasing Encapsulation.

Encapsulation.class:

public class Encapsulation {
// 'numberOfEggs' is an instance variable
// it was made 'private', because we do NOT want it to be directly accessed by other classes
// being 'private' also means that only code within the class can read or write its value
private int numberOfEggs = 10;

// getter
public int getNumberOfEggs() {
    return numberOfEggs;
}

// setter
public void setNumberOfEggs(int numberOfEggs) {
    if (numberOfEggs > 0) {
        // we used the 'this' keyword to differentiate between the method parameter 'currentNumberOfEggs' and
        //  the instance variable 'currentNumberOfEggs'
        this.numberOfEggs = numberOfEggs;
    }
}
}

MainActivity.class:

public class MainActivity extends AppCompatActivity {

TextView defaultNumberOfEggs;
Encapsulation encapsulation;

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

    defaultNumberOfEggs = (TextView) findViewById(R.id.default_number_of_eggs);
    defaultNumberOfEggs.setText(encapsulation.getNumberOfEggs());
}
}

activity_main.xml:

<RelativeLayout android:id="@+id/activity_main"
xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:paddingBottom="@dimen/activity_vertical_margin"
android:paddingLeft="@dimen/activity_horizontal_margin"
android:paddingRight="@dimen/activity_horizontal_margin"
android:paddingTop="@dimen/activity_vertical_margin"
tools:context="com.example.vladp.studyandroid1.MainActivity">

<TextView
    android:id="@+id/default_number_of_eggs"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content" />
</RelativeLayout>

And I get a "null object reference" error, when starting the app and setting the text via defaultNumberOfEggs.setText(encapsulation.getNumberOfEggs());, which should display "10" (without the quotation marks).

What's the exact error? – Carcigenicate Apr 29 '17 at 15:44 — Carcigenicate, Apr 29 '17 at 15:44
You never instantiated the class `Encapsulation`. – t0mm13b Apr 29 '17 at 15:45 — t0mm13b, Apr 29 '17 at 15:45
You need to initiate `encapsulation`... – Yoav Sternberg Apr 29 '17 at 15:46 — Yoav Sternberg, Apr 29 '17 at 15:46

score 2 · Answer 1 · answered Apr 29 '17 at 15:55

You forgot to initialize your encapsulation object. By default, it points to null.

So you should either define it on class level:

Encapsulation encapsulation = new Encapsulation();

Or in constructor:

MainActivity() {
    encapsulation = new Encapsulation();
}

Or inside onCreate method after calling super:

protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    encapsulation = new Encapsulation();
    setContentView(R.layout.activity_main);

    ....
}

Also correct. I checked the above answer as correct, just because I saw it first, but yours is on-point, as well. Thank you! — Vlad Pintea, Apr 29 '17 at 16:01

score 1 · Accepted Answer · answered Apr 29 '17 at 15:45

1

In the onCreate method, add a first line as

encapsulation = new Encapsulation();

answered Apr 29 '17 at 15:45

Deepesh Choudhary

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Of course it was stupidly simple :P Thank you! – Vlad Pintea Apr 29 '17 at 16:01

Getting "Null Object Reference" While Trying Encapsulation

2 Answers2