org.apache.commons.beanutils.converters

## Class AbstractConverter

- java.lang.Object
- - org.apache.commons.beanutils.converters.AbstractConverter

- All Implemented Interfaces:
    Converter
  Direct Known Subclasses:
    ArrayConverter, BooleanConverter, CharacterConverter, ClassConverter, DateTimeConverter, FileConverter, NumberConverter, StringConverter, URLConverter

      public abstract class AbstractConverter
      extends Object
      implements Converter

  Base `Converter` implementation that provides the structure for handling conversion to and from a specified type.

  This implementation provides the basic structure for converting to/from a specified type optionally using a default value or throwing a `ConversionException` if a conversion error occurs.

  Implementations should provide conversion to the specified type and from the specified type to a `String` value by implementing the following methods:

  - `convertToString(value)` - convert to a String (default implementation uses the objects `toString()` method).
  - `convertToType(Class, value)` - convert to the specified type

  The default value has to be compliant to the default type of this converter - which is enforced by the generic type parameter. If a conversion is not possible and a default value is set, the converter tries to transform the default value to the requested target type. If this fails, a `ConversionException` if thrown.

  Since:
    1.8.0
  Version:
    $Id$

- - ### Constructor Summary

    Constructors
    Constructor and Description
    `AbstractConverter()` Construct a Converter that throws a `ConversionException` if an error occurs.
    `AbstractConverter(Object defaultValue)` Construct a Converter that returns a default value if an error occurs.

  - ### Method Summary

    All Methods Instance Methods Abstract Methods Concrete Methods
    Modifier and Type | Method and Description
    --- | ---
    `protected ConversionException` | `conversionException(Class<?> type, Object value)` Generates a standard conversion exception with a message indicating that the passed in value cannot be converted to the desired target type.
    `<T> T` | `convert(Class<T> type, Object value)` Convert the input object into an output object of the specified type.
    `protected Object` | `convertArray(Object value)` Return the first element from an Array (or Collection) or the value unchanged if not an Array (or Collection).
    `protected String` | `convertToString(Object value)` Convert the input object into a String.
    `protected abstract <T> T` | `convertToType(Class<T> type, Object value)` Convert the input object into an output object of the specified type.
    `protected Object` | `getDefault(Class<?> type)` Return the default value for conversions to the specified type.
    `protected abstract Class<?>` | `getDefaultType()` Return the default type this `Converter` handles.
    `protected <T> T` | `handleError(Class<T> type, Object value, Throwable cause)` Handle Conversion Errors.
    `protected <T> T` | `handleMissing(Class<T> type)` Handle missing values.
    `boolean` | `isUseDefault()` Indicates whether a default value will be returned or exception thrown in the event of a conversion error.
    `protected void` | `setDefaultValue(Object defaultValue)` Set the default value, converting as required.
    `String` | `toString()` Provide a String representation of this converter.

    - ### Methods inherited from class java.lang.Object

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

- - ### Constructor Detail

    - #### AbstractConverter

          public AbstractConverter()

      Construct a Converter that throws a `ConversionException` if an error occurs.

    - #### AbstractConverter

          public AbstractConverter(Object defaultValue)

      Construct a Converter that returns a default value if an error occurs.
      Parameters:
        `defaultValue` - The default value to be returned if the value to be converted is missing or an error occurs converting the value.

  - ### Method Detail

    - #### isUseDefault

          public boolean isUseDefault()

      Indicates whether a default value will be returned or exception thrown in the event of a conversion error.
      Returns:
        `true` if a default value will be returned for conversion errors or `false` if a `ConversionException` will be thrown.

    - #### convert

          public <T> T convert(Class<T> type,
                               Object value)

      Convert the input object into an output object of the specified type.
      Specified by:
        `convert` in interface `Converter`
      Type Parameters:
        `T` - the target type of the conversion
      Parameters:
        `type` - Data type to which this value should be converted
        `value` - The input value to be converted
      Returns:
        The converted value.
      Throws:
        `ConversionException` - if conversion cannot be performed successfully and no default is specified.

    - #### convertToString

          protected String convertToString(Object value)
                                    throws Throwable

      Convert the input object into a String.

      N.B.This implementation simply uses the value's `toString()` method and should be overriden if a more sophisticated mechanism for conversion to a String is required.

      Parameters:
        `value` - The input value to be converted.
      Returns:
        the converted String value.
      Throws:
        `Throwable` - if an error occurs converting to a String

    - #### convertToType

          protected abstract <T> T convertToType(Class<T> type,
                                                 Object value)
                                          throws Throwable

      Convert the input object into an output object of the specified type.

      Typical implementations will provide a minimum of `String --> type` conversion.

      Type Parameters:
        `T` - Target type of the conversion.
      Parameters:
        `type` - Data type to which this value should be converted.
        `value` - The input value to be converted.
      Returns:
        The converted value.
      Throws:
        `Throwable` - if an error occurs converting to the specified type

    - #### convertArray

          protected Object convertArray(Object value)

      Return the first element from an Array (or Collection) or the value unchanged if not an Array (or Collection). N.B. This needs to be overriden for array/Collection converters.
      Parameters:
        `value` - The value to convert
      Returns:
        The first element in an Array (or Collection) or the value unchanged if not an Array (or Collection)

    - #### handleError

          protected <T> T handleError(Class<T> type,
                                      Object value,
                                      Throwable cause)

      Handle Conversion Errors.

      If a default value has been specified then it is returned otherwise a ConversionException is thrown.

      Type Parameters:
        `T` - Target type of the conversion.
      Parameters:
        `type` - Data type to which this value should be converted.
        `value` - The input value to be converted
        `cause` - The exception thrown by the `convert` method
      Returns:
        The default value.
      Throws:
        `ConversionException` - if no default value has been specified for this `Converter`.

    - #### handleMissing

          protected <T> T handleMissing(Class<T> type)

      Handle missing values.

      If a default value has been specified, then it is returned (after a cast to the desired target class); otherwise a ConversionException is thrown.

      Type Parameters:
        `T` - the desired target type
      Parameters:
        `type` - Data type to which this value should be converted.
      Returns:
        The default value.
      Throws:
        `ConversionException` - if no default value has been specified for this `Converter`.

    - #### setDefaultValue

          protected void setDefaultValue(Object defaultValue)

      Set the default value, converting as required.

      If the default value is different from the type the `Converter` handles, it will be converted to the handled type.

      Parameters:
        `defaultValue` - The default value to be returned if the value to be converted is missing or an error occurs converting the value.
      Throws:
        `ConversionException` - if an error occurs converting the default value

    - #### getDefaultType

          protected abstract Class<?> getDefaultType()

      Return the default type this `Converter` handles.
      Returns:
        The default type this `Converter` handles.

    - #### getDefault

          protected Object getDefault(Class<?> type)

      Return the default value for conversions to the specified type.
      Parameters:
        `type` - Data type to which this value should be converted.
      Returns:
        The default value for the specified type.

    - #### toString

          public String toString()

      Provide a String representation of this converter.
      Overrides:
        `toString` in class `Object`
      Returns:
        A String representation of this converter

    - #### conversionException

          protected ConversionException conversionException(Class<?> type,
                                                            Object value)

      Generates a standard conversion exception with a message indicating that the passed in value cannot be converted to the desired target type.
      Parameters:
        `type` - the target type
        `value` - the value to be converted
      Returns:
        a `ConversionException` with a standard message
      Since:
        1.9

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