Why is Final used in method parameters
When I was coding android I came across the following:
@Override
public View getView(final int position, View convertView, ViewGroup parent) {
convertView = _inflater.inflate(R.layout.abstract_list_row_item, null);
move.setOnClickListener(new OnClickListener() {
@Override
public void onClick(View v) {
// if btn is clcked then data is changed an we need to refresh framents
MainAppDataController.getInstance().setIsDataChanged(true);
callWhiteListDB = new CallWhiteListDB(_context);
callWhiteListDB.openToWrite();
callWhiteListDB.insert(allContacts.get(position).name, allContacts.get(position).number);
callWhiteListDB.close();
callBlackListDB = new CallBlackListDB(_context);
callBlackListDB.openToWrite();
callBlackListDB.deleteSingleItem(allContacts.get(position).dbId);
callBlackListDB.close();
populateList(position);
notifyListView(view);
}
});
return convertView;
In the above example, the getView()
method has parameters like int position, View convertView, parent of ViewGroup. My observation was, as soon as I started using the position variable internally onclick()
, Eclipse throws a compilation error and asks for the position as final. Why should I make this final? Final AFAIK is used for constants.
final
used in method parameters to make references immutable after they have been passed to the method. This is a specialized way to protect passed parameters. so the reception of the method won't be able to re-initialize it with a new object or value
You are using position
in your anonymous inner class. Therefore, the position is required to be final.
An anonymous class cannot access local variables in its enclosing scope that are not declared as final
or are actually final.
The last modifier indicates that the value of this field cannot be changed.
http://docs.oracle.com/javase/tutorial/java/javaOO/anonymousclasses.html
We use a keyword final
with method parameters to prevent code within the method from changing the value of that parameter.
final makes your variable constant.
How your onclick method will be called after this function finishes and your variable will no longer exist, so when you use the final variable, the constant value is passed inside the onclick.
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A java variable can be declared using the final keyword. Then the final variable can only be assigned once.
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A variable that is declared final and not initialized is called an empty final variable. The net final variable forces the constructors to initialize it.
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Java classes declared final cannot be extended. Limiting inheritance!
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Final methods cannot be overridden. In particular, it is equal to finite, but in variables it is not.
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final parameters - parameter values cannot be changed after initialization. Do a little java exercise to figure out the implications of final parameters in method overriding.
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Local Java classes can only refer to local variables and parameters that are declared final.
- The visible advantage of declaring a java variable as static final, the compiled java class provides better performance.
For more information read this
also see Using Ending Keyword in Java Method Performance?
Him because getView
called when your listView
fans the layout ... and causes onClick
, when depressed view (move)
... At the time when it starts onClick variable position
in getView
already destroyed ... Making his final, you ensure that the value position
is stored for later use