Java nested list to array conversion

Question

What is the most efficient way to convert data from nested lists to an object array (which can be used i.e. as data for JTable)?

List<List> table = new ArrayList<List>();

for (DATAROW rowData : entries) {
    List<String> row = new ArrayList<String>();

    for (String col : rowData.getDataColumn())
        row.add(col);

    table.add(row);
}

// I'm doing the conversion manually now, but
// I hope that there are better ways to achieve the same
Object[][] finalData = new String[table.size()][max];
for (int i = 0; i < table.size(); i++) {
    List<String> row = table.get(i);

    for (int j = 0; j < row.size(); j++)
        finalData[i][j] = row.get(j);
}

Many thanks!

Patrick · Accepted Answer · 2008-12-16 18:02:41Z

13

//defined somewhere
List<List<String>> lists = ....

String[][] array = new String[lists.size()][];
String[] blankArray = new String[0];
for(int i=0; i < lists.size(); i++) {
    array[i] = lists.get(i).toArray(blankArray);
}

I don't know anything about JTable, but converting a list of lists to array can be done with a few lines.

answered Dec 16, 2008 at 18:02

Patrick

3,9811 gold badge28 silver badges30 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

Michael Myers · Accepted Answer · 2008-12-16 16:42:43Z

8

For JTable in particular, I'd suggest subclassing AbstractTableModel like so:

class MyTableModel extends AbstractTableModel {
    private List<List<String>> data;
    public MyTableModel(List<List<String>> data) {
        this.data = data;
    }
    @Override
    public int getRowCount() {
        return data.size();
    }
    @Override
    public int getColumnCount() {
        return data.get(0).size();
    }
    @Override
    public Object getValueAt(int row, int column) {
        return data.get(row).get(column);
    }
    // optional
    @Override
    public void setValueAt(Object aValue, int row, int column) {
        data.get(row).set(column, aValue);
    }
}

Note: this is the most basic implementation possible; error-checking is omitted for brevity.

Using a model like this, you don't have to worry about pointless conversions to Object[][].

answered Dec 16, 2008 at 16:42

Michael Myers♦

193k47 gold badges301 silver badges297 bronze badges

2 Comments

Paul Brinkley Over a year ago

Good answer. However, double-check that the data passed to this model isn't visible elsewhere, and prone to ninja-editing. It probably isn't, but if it is, it may need to be cloned here.

Michael Myers Over a year ago

Good point. I thought of that, but DefaultTableModel actually doesn't check either (it uses Vectors). Anyway, this is the most basic implementation possible. ;)

djm.im · Accepted Answer · 2020-11-16 08:35:21Z

4

Java 11 answer.

List<List<String>> table = List.of(List.of("A", "B"), List.of("3", "4"));
String[][] finalData = table.stream()
        .map(arr -> arr.toArray(String[]::new))
        .toArray(String[][]::new);
    
System.out.println(Arrays.deepToString(finalData));

[[A, B], [3, 4]]

The Collection.toArray(IntFunction<T[]> generator) method is new in Java 11.

Of course you may also use a stream in Java 8+. Just use this mapping instead:

.map(arr -> arr.toArray(new String[0]))

(The List.of method was introduced in Java 9.)

edited Nov 16, 2020 at 8:35

djm.im

3,3334 gold badges32 silver badges47 bronze badges

answered Dec 19, 2018 at 10:09

Anonymous

87.4k15 gold badges163 silver badges181 bronze badges

Comments

mamen · Accepted Answer · 2021-03-23 14:39:46Z

0

To convert a nested list to a two-dimensional object-array, you can use this code:

public Object[][] ToObjectMatrix(List<List<String>> data) throws IllegalArgumentException {
    if(data == null) {
        throw new IllegalArgumentException("Passed data was null.");
    }

    Object[][] result = new Object[data.size()][];
     
    for(int i = 0; i < data.size(); i++) {
        result[i] = data.get(i).toArray();
    }
     
    return result;
}

answered Mar 23, 2021 at 14:39

mamen

1,86914 silver badges37 bronze badges

Collectives™ on Stack Overflow

Java nested list to array conversion

4 Answers 4

Comments

2 Comments

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

4 Answers 4

Comments

2 Comments

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related