# Class TableColumn<V>

Object
TableColumn<V>

Type Parameters:
  `V` - base type of all values in the column identified by this instance.

All Implemented Interfaces:
  `CheckedContainer<V>`

public class TableColumn<V> extends Object implements CheckedContainer<V>
Identifies a column in `TreeTable​.Node` instances. Each `TableColumn` instance contains the column header and the type of values for a particular column. `TableColumn`s are used for fetching values from nodes as in the following example:

    public class CityLocation {
        public static final ColumnTable<String> CITY_NAME = new ColumnTable<>(String.class, "City name");
        public static final ColumnTable<Float>  LATITUDE  = new ColumnTable<>(Float.class,  "Latitude");
        public static final ColumnTable<Float>  LONGITUDE = new ColumnTable<>(Float.class,  "Longitude");

        private String name;
        private float  latitude;
        private float  longitude;

        CityLocation(TreeTable.Node myNode) {
            name      = myNode.getValue(CITY_NAME);
            latitude  = myNode.getValue(LATITUDE);
            longitude = myNode.getValue(LONGITUDE);
        }
    }

## Identity comparisons and serialization

This base class relies on identity comparisons instead of defining the `equals(Object)` method, because the element type is not a sufficient criterion for differentiating the columns (many columns have values of the same type) and the header is arbitrary. Consequently developers who create their own instances are encouraged to declare them as static final constants as in the above example, and use those constants consistently.

This base class is not serializable because the default deserialization mechanism does not resolve automatically the deserialized instances to the above-cited singleton instances. Developers who need serialization support for their own instances have to resolve them in their own subclass. The following example is one possible way to achieve that goal:

    public class CityLocation {
        public static final ColumnTable<String> CITY_NAME = new Column<>("CITY_NAME", String.class, "City name");
        public static final ColumnTable<Float>  LATITUDE  = new Column<>("LATITUDE",  Float.class,  "Latitude");
        public static final ColumnTable<Float>  LONGITUDE = new Column<>("LONGITUDE", Float.class,  "Longitude");

        private static final class Column<V> extends TableColumn<V> implements Serializable {
            private final String field;

            private Column(String field, Class<V> type, CharSequence header) {
                super(type, header);
                this.field = field;
            }

            private Object readResolve() throws ObjectStreamException {
                try {
                    return CityLocation.class.getField(field).get(null);
                } catch (Exception cause) { // Many exceptions, including unchecked ones.
                    throw new InvalidObjectException(cause.toString());
                }
            }
        }
    }

The constants defined in this class use a similar approach for providing serialization support.

Since:
  0.3

- ## Field Summary

  Fields
  Modifier and Type
  Field
  Description
  `static final TableColumn<String>`
  `IDENTIFIER`
  Predefined constant for a column of object identifiers.
  `static final TableColumn<Integer>`
  `INDEX`
  Predefined constant for a column of index values.
  `static final TableColumn<CharSequence>`
  `NAME`
  Predefined constant for a column of object names.
  `static final TableColumn<Obligation>`
  `OBLIGATION`
  Predefined constant for a column of obligation (mandatory, optional, conditional).
  `static final TableColumn<CharSequence>`
  `REMARKS`
  Predefined constant for a column of remarks.
  `static final TableColumn<Class<?>>`
  `TYPE`
  Predefined constant for a column of object types.
  `static final TableColumn<Object>`
  `VALUE`
  Predefined constant for a column of object values.
  `static final TableColumn<Number>`
  `VALUE_AS_NUMBER`
  Predefined constant for a column of object numerical values.
  `static final TableColumn<CharSequence>`
  `VALUE_AS_TEXT`
  Predefined constant for a column of object textual values.

- ## Constructor Summary

  Constructors
  Modifier
  Constructor
  Description
  `protected`
  `TableColumn()`
  Invoked on deserialization for creating an initially empty instance.
  ``
  `TableColumn(Class<V> type, CharSequence header)`
  Creates a new instance for the given type of values.

- ## Method Summary

  Modifier and Type
  Method
  Description
  `final Class<V>`
  `getElementType()`
  Returns the base type of all values in any column identified by this `TableColumn` instance.
  `InternationalString`
  `getHeader()`
  Returns the text to display as column header.
  `String`
  `toString()`
  Returns a string representation of this table column.

  ### Methods inherited from class Object

  `clone, equals, finalize, getClass, hashCode, notify, notifyAll, wait, wait, wait`

- ## Field Details

  - ### NAME

    public static final TableColumn<CharSequence> NAME
    Predefined constant for a column of object names. The column header is Name (eventually localized) and the column elements are typically instances of `String` or `InternationalString`, depending on whether the data provide localization support or not.

  - ### IDENTIFIER

    public static final TableColumn<String> IDENTIFIER
    Predefined constant for a column of object identifiers. The column header is Identifier (eventually localized) and the column elements are instances of `String`.

  - ### INDEX

    public static final TableColumn<Integer> INDEX
    Predefined constant for a column of index values. The column header is Index (eventually localized) and the column elements are instances of `Integer`.

  - ### TYPE

    public static final TableColumn<Class<?>> TYPE
    Predefined constant for a column of object types. The column header is Type (eventually localized).

  - ### OBLIGATION

    public static final TableColumn<Obligation> OBLIGATION
    Predefined constant for a column of obligation (mandatory, optional, conditional). The column header is Obligation (eventually localized) and the column elements are instances of `Obligation`.
    Since:
      1.5

  - ### VALUE

    public static final TableColumn<Object> VALUE
    Predefined constant for a column of object values. The column header is Value (eventually localized) and the column elements can be instance of any kind of objects.
    See Also:
      - `VALUE_AS_TEXT`
      - `VALUE_AS_NUMBER`

  - ### VALUE_AS_TEXT

    public static final TableColumn<CharSequence> VALUE_AS_TEXT
    Predefined constant for a column of object textual values. The column header is Value (eventually localized) and the column elements are typically instances of `String` or `InternationalString`, depending on whether the data provide localization support or not.

  - ### VALUE_AS_NUMBER

    public static final TableColumn<Number> VALUE_AS_NUMBER
    Predefined constant for a column of object numerical values. The column header is Value (eventually localized).

  - ### REMARKS

    public static final TableColumn<CharSequence> REMARKS
    Predefined constant for a column of remarks. The column header is Remarks (eventually localized) and the column elements are typically instances of `String` or `InternationalString`, depending on whether the data provide localization support or not.
    Since:
      1.0

- ## Constructor Details

  - ### TableColumn

    protected TableColumn()
    Invoked on deserialization for creating an initially empty instance. This constructor has `protected` visibility only because the Java deserialization mechanism requires so; this constructor shall not be invoked in any other context. See the Identity comparisons and serialization section in the class javadoc for more information.

  - ### TableColumn

    public TableColumn(Class<V> type, CharSequence header)
    Creates a new instance for the given type of values.
    Parameters:
      `type` - base type of all values in the column identified by this instance.
      `header` - the text to display as column header.

- ## Method Details

  - ### getHeader

    public InternationalString getHeader()
    Returns the text to display as column header.
    Returns:
      the text to display as column header.

  - ### getElementType

    public final Class<V> getElementType()
    Returns the base type of all values in any column identified by this `TableColumn` instance.
    Specified by:
      `getElementType` in interface `CheckedContainer<V>`
    Returns:
      the element type.

  - ### toString

    public String toString()
    Returns a string representation of this table column. The default implementation returns the header in its default locale.
    Overrides:
      `toString` in class `Object`