# ConfigMergeable
Date: 2015-03-06

com.typesafe.config

## Interface ConfigMergeable

All Known Subinterfaces:
  Config, ConfigList, ConfigObject, ConfigValue

    public interface ConfigMergeable

Marker for types whose instances can be merged, that is `Config` and `ConfigValue`. Instances of `Config` and `ConfigValue` can be combined into a single new instance using the `withFallback()` method.

Do not implement this interface; it should only be implemented by the config library. Arbitrary implementations will not work because the library internals assume a specific concrete implementation. Also, this interface is likely to grow new methods over time, so third-party implementations will break.

### Method Summary

All Methods Instance Methods Abstract Methods
`ConfigMergeable` `withFallback(ConfigMergeable other)` Returns a new value computed by merging this value with another, with keys in this value "winning" over the other one.

### Method Detail

#### withFallback

    withFallback(ConfigMergeable other)

Returns a new value computed by merging this value with another, with keys in this value "winning" over the other one.

This associative operation may be used to combine configurations from multiple sources (such as multiple configuration files).

The semantics of merging are described in the spec for HOCON. Merging typically occurs when either the same object is created twice in the same file, or two config files are both loaded. For example:

      foo = { a: 42 }
      foo = { b: 43 }

Here, the two objects are merged as if you had written:

      foo = { a: 42, b: 43 }

Only `ConfigObject` and `Config` instances do anything in this method (they need to merge the fallback keys into themselves). All other values just return the original value, since they automatically override any fallback. This means that objects do not merge "across" non-objects; if you write `object.withFallback(nonObject).withFallback(otherObject)`, then `otherObject` will simply be ignored. This is an intentional part of how merging works, because non-objects such as strings and integers replace (rather than merging with) any prior value:

     foo = { a: 42 }
     foo = 10

Here, the number 10 "wins" and the value of `foo` would be simply 10. Again, for details see the spec.

Parameters:
  `other` - an object whose keys should be used as fallbacks, if the keys are not present in this one
Returns:
  a new object (or the original one, if the fallback doesn't get used)