CODEWITHSUNDEEP
javalistarraylist
user2282497
user2282497

Reputation: 191

Extending an ArrayList of Lists

Why is it not possible to do something like this?

RowList<List<Value>> extends ArrayList<List<Value>>

Also, how is it possible to do this?

RowList<Value> extends ArrayList<List<Value>>

I'm trying to implement a List to be used in a database, and want to extend ArrayList so I'm able to check that the .add(),.set() etc. methods do not violate the requirements of the Database (not adding two rows with the same keys etc). I realise that it's probably not the best way to implement a database, but it's an assignment for college that required us to do it this way.

EDIT: Using the 2nd option (that compiles), how is it possible to access the Values within the Lists that the RowList class holds?

Upvotes: 4

Views: 495

Answers (1)

Craig
Craig

Reputation: 1390

The following two lines are identical (to the compiler), this is true even if you have a class called Value as the named parameter will mask the class type

class RowList<Value> extends ArrayList<List<Value>>

class RowList<T> extends ArrayList<List<T>>

The reason for this is because value inside the first <> is a named type parameter. So when you attempt to do something like

class RowList<List<Value>> extends ArrayList<List<Value>>

You have attempted to create a named type paramter List<Value> which (in Java) is an invalid identifier, but instead you will get an error something like Syntax error on token(s), misplaced construct(s)

I think instead you are really trying to write

public class RowList extends ArrayList<Value> {

    @Override
    public boolean add(Value e) {
        // TODO Custom code to check and what not
        return super.add(e);
    }
}

Where Value is a custom object in your codebase. Then elsewhere in your code you can do:

RowList rl = new RowList();
rl.add(new Value(...));
Value v = rl.get(i);

EDIT:

The previous example assumes that the Value class is an entry row of data. If instead it is a single item of data, and the row is represented by a List then it would be more like the following:

public class RowList extends ArrayList<List<Value>> {

    @Override
    public boolean add(List<Value> e) {
        // TODO Custom code to check and what not
        return super.add(e);
    }
}

RowList rl = new RowList();
List<Value> row = new ArrayList<Value>();
row.add(new Value(...));
rl.add(row);
List<Value> rowGet = rl.get(i);

Upvotes: 6

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