Working with maps

This page explains how op4j can deal with map objects (objects implementing java.util.Map<K,V>).

Map input objects are considered immutable, so they will not be changed when used in an op4j expression and a new map will always be returned when executing get().

This, however, does not apply to the input map key and value objects, which could be changed if you executed on them any functions which might change their state instead of substituting them for new elements.

1. Creating map expressions

Operation expressions

There are two equivalent ways of creating an operation expression on a map. Just use the method you like most:

Map<String,String> map = ...;
Map<String,String> map = ...;
Maps from their elements

Map expressions can be created by specifying the map elements (entries), as key/value pairs, like:

Op.onMapFor(key1, value1).and(key2, value2).and(key3, value3)...

Once any action is called, the and(...) method will not be available anymore, as it is only meant to build the map.

Maps from arrays, lists or sets.

Maps can be built from arrays, lists or sets in a variety of ways (see the arrays, lists or sets documentation for details).

For example, zipping can be used:

  Map<String,Integer> map = Op.onListFor("a","b","c").zipValues(1,2,3).get();

Function expressions

Function expressions are created as usual with other structures:

// Create a function which receives a Map<Integer,String> variable as input
function = Fn.onMapOf(Types.INTEGER, Types.STRING)...get();

2. Extracting keys and values

All keys of a map can be extracted as a Set:

Map<Integer,String> map = 
Set<Integer> keys = Op.on(map).extractKeys().get();

Also values can be extracted, as a List:

Map<Integer,String> map = 
List<String> values = Op.on(map).extractValues().get();

3. Iterating

Maps can be iterated, resulting in an operator acting on its map entries (Map.Entry<K,V>).


And once iterating each entry, actions can be executed on the entry itself (having thus access to both key and value):

Op.on(map).forEachEntry().[ACTIONS ON MAP ENTRIES]...

But also the keys can be selected, so that subsequent actions will only apply on the map's keys:

Op.on(map).forEachEntry().onKey().[ACTIONS ON MAP KEYS]...

...and the same for the values:

Op.on(map).forEachEntry().onValue().[ACTIONS ON MAP VALUES]...

The result obtained when executing get() after iterating will be a map with the results of applying the subsequently chained actions on the entries/keys/values of the map.

Iteration on entries can be ended with the endFor() action:

Op.on(map).forEachEntry().[ENTRY ACTIONS].endFor().[MAP ACTIONS].get();

And selection of keys or values can be ended with the endOn() action:

Op.on(map).forEachEntry().onKey().[KEY ACTIONS].endOn().endFor().[MAP ACTIONS].get();
Op.on(map).forEachEntry().onValue().[VALUE ACTIONS].endOn().endFor().[MAP ACTIONS].get();

4. Modifying

A map can be modified by adding or removing elements from it.

Maps in op4j are ordered (LinkedHashMap), and several options for adding new elements at the end of a map:

Map<String,String> map = ...;
Map<String,String> newElements = ...;
Op.on(map).put("new Key", "new Value")...

New elements can also be inserted into a specific position. Positions start with 0.

Map<String,String> map = ...;
Map<String,String> newElements = ...;
Op.on(map).insert(0, "new Key", "new Value")...
Op.on(map).insertAll(0, newElements)...

Removal of elements can be done in several ways. Elements can be removed attending to their keys:

Op.on(map).removeAllKeys("one Key", "another Key")...
Op.on(map).removeAllKeysNot("one Key", "another Key")...

A function returning Boolean can be used as evaluator to determine whether an element should be removed or not:

Map<String,String> map = ...;
IFunction<Map.Entry<String,String>,Boolean> eval = ...;

5. Executing functions

Executing functions on the map keys or values

Functions can be executed on each of the map keys or values after a forEachEntry() followed by and an onKey() or onValue() action:

Map<String,String> map = ...;
Map<String,String> newMap = 

A condition can be added for conditional execution, if needed:

Map<String,String> map = ...;
IFunction<String,String> myFunction = ...;
Map<String,String> newMap = 
Map<String,String> map = ...;
IFunction<String,Boolean> condition = ...;
IFunction<String,String> myFunction = ...;
Op.on(map).forEachEntry().onValue().execIfTrue(condition, myFunction)... else side can it also be added, in which case the expression can change the type of the operator:

Map<String,String> map = ...;
IFunction<String,Boolean> condition = ...;
IFunction<String,Integer> myThenFunction = ...;
IFunction<String,Integer> myElseFunction = ...;
Map<String,Integer> newMap = ...;
    Op.on(map).forEachEntry().onValue().execIfTrue(condition, myThenFunction, myElseFunction).get();

Executing functions on map entries

Functions can be executed on entire map entries.

These functions can return Map.Entry<K,V> objects as well, in which case the target object will be still considered a map:

Map<String,String> map = ...;
IFunction<Map.Entry<String,String>,Map.Entry<String,String>> myFunction = ...;
Map<String,String> newMap = 

...or they can return something different from a map entry, in which case the target object will be considered a List from then on:

Map<String,String> map = ...;
IFunction<Map.Entry<String,String>,Integer> myFunction = ...;
List<Integer> newList = 

Executing functions on the whole map

If a map has not been iterated (forEach()) (or it has, but endFor() has been called), functions can be executed on the whole map.

There are three ways of executing functions on a map as a whole:

  • Executions which return a map (example: Map<String,String> -> Map<String,Integer>)
  • Executions which do not return a map (example: Map<String,String> -> Calendar)
Returning a map

Functions will be executed using the execAsMap(...) action:

Map<String,String> map = ...;
IFunction<Map<String,String>,Map<String,Integer>> myFunction = ...;

A conditional check can be added (null, not null, condition true, condition false):

Map<String,String> map = ...;
IFunction<Map<String,String>,Map<String,Integer>> myFunction = ...;
Not returning a map
Map<String,String> map = ...;
IFunction<Map<String,String>,Calendar> myFunction = ...;
Calendar calendar = Op.on(map).exec(myFunction).get();

6. Selecting (conditional code)

op4j allows the conditional execution of actions. Once the condition (an action starting with "if") is executed, all subsequent actions will apply only on the selected parts of the target object.

For example, lets convert into upper case only the first String key of the map:

Map<String,String> map = ...;
Map<String> newMap = Op.on(map).forEachEntry().ifIndex(0).onKey().exec(FnString.toUpperCase()).get();

Selections can be ended with endIf(...):

Map<String> newMap =  
    Op.on(map).forEachEntry().ifIndex(0,1,3).[ACTIONS ON SELECTED ENTRIES].endIf()...

Selecting specific map keys/values

If a selection is executed after forEachEntry().onKey() or forEachEntry().onValue(), it will be applied on the map keys or values, respectively.

Selection can be based on the nullity of the element:


And also on the value returned by the evaluation of a function returning Boolean:

Map<String,String> map = ...;
IFunction<String,Boolean> eval = ...;

Selecting specific map entries

If a selection is executed after a forEachEntry() action without onKey() or onValue() being executed, it will be applied on the map entries.

Selection can be done on the entry's position in the map:


...on the value of its key:

Op.on(map).forEachEntry().ifKeyEquals("a key", "another key")....
Op.on(map).forEachEntry().ifKeyNotEquals("a key", "another key")....

...and also on the value returned by the evaluation of a function returning Boolean:

Map<String,String> map = ...;
IFunction<Map.Entry<String,String>,Boolean> eval = ...;

Selecting the map as a whole

Selection can also be performed on the map itself as a whole, effectively deciding whether subsequent actions will be executed at all or not.

Map<String,String> map = ...;
IFunction<Map<String,String>,Boolean> eval = ...;

Restricions on execution actions after selection

After executing a selection action, function executed by means of an exec action cannot change the map key and/or value types (i.e., a Map<String,String> must remain Map<String,String>.

So this would not be valid:

// Will not compile!
Map<String,String> newMap = 

...because converting only some of the map keys to integer would render the map type inconsistent (some elements would be String, some other Integer).

This is perfectly valid, though:

// FnString.toUpperCase is IFunction<String,String>
Map<String,String> newMap = 

7. Replacing

Map keys, values, entries or even the map itself can be replaced by other objects:

Map<String,String> newMap = 
    Op.on(map).forEachEntry().onValue().ifNull().replaceWith("[no value]").get();

...which is equivalent to:

Map<String,String> newMap = 
    Op.on(map).forEachEntry().onValue().replaceIfNullWith("[no value]").get();

8. Reversing

Maps can be reversed, so that the order of its entries gets inverted:

  // map = {{"a"=1},{"b"=2},{"c"=3}}
  // newMap = {{"c"=3},{"b"=2},{"a"=1}}
  Map<String,Integer> newMap = Op.on().reverse().get();