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).
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);
....
}
In the onCreate method, add a first line as
encapsulation = new Encapsulation();