Package org.apache.commons.configuration2.interpol

## Interface Lookup

- All Known Implementing Classes:
    `ConfigurationLookup`, `ConstantLookup`, `DummyLookup`, `EnvironmentLookup`, `ExprLookup`, `SystemPropertiesLookup`

      public interface Lookup

  Definition of an interface for looking up variables during interpolation.

  Objects implementing this interface can be assigned a variable prefix and added to a `ConfigurationInterpolator` object. Whenever the `ConfigurationInterpolator` encounters a property value referencing a variable, e.g. `${prefix:variableName}`, it extracts the prefix and finds the matching `Lookup` object. Then this object is asked to resolve the variable name and provide the corresponding value.

  This interface defines a single method for performing variable lookup. It is passed the name of a variable and has to return the corresponding value. It is of course up to a specific implementation how this is done. If the variable name cannot be resolved, an implementation has to return null.

  Note: Implementations must be aware that they can be accessed concurrently. This is for instance the case if a configuration object is read by multiple threads or if a `Lookup` object is shared by multiple configurations.

  Since:
    2.0

- - ### Method Summary

    All Methods Instance Methods Abstract Methods
    Modifier and Type | Method | Description
    --- | --- | ---
    `Object` | `lookup​(String variable)` | Looks up the value of the specified variable.

- - ### Method Detail

    - #### lookup

          Object lookup​(String variable)

      Looks up the value of the specified variable. This method is called by `ConfigurationInterpolator` with the variable name extracted from the expression to interpolate (i.e. the prefix name has already been removed). A concrete implementation has to return the value of this variable or null if the variable name is unknown.
      Parameters:
        `variable` - the name of the variable to be resolved
      Returns:
        the value of this variable or null