1. Información general
Este artículo ilustrará cómo aplicar la ordenación a Array , List , Set y Map en Java 7 y Java 8.
2. Ordenar con matriz
Comencemos ordenando matrices de enteros primero usando el método Arrays.sort () .
Definiremos las siguientes matrices int en un método @Before jUnit:
@Before public void initVariables () { toSort = new int[] { 5, 1, 89, 255, 7, 88, 200, 123, 66 }; sortedInts = new int[] {1, 5, 7, 66, 88, 89, 123, 200, 255}; sortedRangeInts = new int[] {5, 1, 89, 7, 88, 200, 255, 123, 66}; ... }2.1. Clasificación de matriz completa
Usemos ahora la API simple Array.sort () :
@Test public void givenIntArray_whenUsingSort_thenSortedArray() { Arrays.sort(toSort); assertTrue(Arrays.equals(toSort, sortedInts)); }La matriz sin clasificar ahora está completamente ordenada:
[1, 5, 7, 66, 88, 89, 123, 200, 255]Como se menciona en el JavaDoc oficial, Arrays.sort usa Quicksort de doble pivote en primitivas . Ofrece un rendimiento de O (n log (n)) y suele ser más rápido que las implementaciones de Quicksort tradicionales (un pivote). Sin embargo, utiliza una implementación iterativa, adaptativa y estable del algoritmo mergesort para Array of Objects.
2.2. Ordenar parte de una matriz
Arrays.sort tiene un tipo más de API, que discutiremos aquí:
Arrays.sort(int[] a, int fromIndex, int toIndex)Esto solo ordenará una parte de la matriz, entre los dos índices.
Echemos un vistazo a un ejemplo rápido:
@Test public void givenIntArray_whenUsingRangeSort_thenRangeSortedArray() { Arrays.sort(toSort, 3, 7); assertTrue(Arrays.equals(toSort, sortedRangeInts)); }La clasificación se realizará solo en los siguientes elementos de la submatriz ( toIndex sería exclusivo):
[255, 7, 88, 200]La submatriz ordenada resultante, incluida la matriz principal, sería:
[5, 1, 89, 7, 88, 200, 255, 123, 66]2.3. Java 8 Arrays.sort frente a Arrays.parallelSort
Java 8 viene con una nueva API, paraleloSort , con una firma similar a la API Arrays.sort () :
@Test public void givenIntArray_whenUsingParallelSort_thenArraySorted() { Arrays.parallelSort(toSort); assertTrue(Arrays.equals(toSort, sortedInts)); }Detrás de escena de ParalleSort (), divide la matriz en diferentes submatrices (según la granularidad en el algoritmo de ParalleSort ). Cada submatriz se ordena con Arrays.sort () en diferentes subprocesos para que la ordenación se pueda ejecutar de forma paralela y se fusionen finalmente como una matriz ordenada.
Tenga en cuenta que el fondo común de ForJoin se utiliza para ejecutar estas tareas paralelas y luego fusionar los resultados.
El resultado de Arrays.parallelSort será el mismo que Array.sort, por supuesto, es solo una cuestión de aprovechar los subprocesos múltiples.
Finalmente, también hay variantes similares de API Arrays.sort en Arrays.parallelSort :
Arrays.parallelSort (int [] a, int fromIndex, int toIndex);3. Ordenar una lista
Usemos ahora la API Collections.sort () en java.utils.Collections - para ordenar una lista de enteros:
@Test public void givenList_whenUsingSort_thenSortedList() { List toSortList = Ints.asList(toSort); Collections.sort(toSortList); assertTrue(Arrays.equals(toSortList.toArray(), ArrayUtils.toObject(sortedInts))); }La lista antes de ordenar contendrá los siguientes elementos:
[5, 1, 89, 255, 7, 88, 200, 123, 66]Y, naturalmente, después de clasificar:
[1, 5, 7, 66, 88, 89, 123, 200, 255]Como se menciona en Oracle JavaDoc for Collections.Sort , utiliza un mergesort modificado y ofrece un rendimiento n log (n) garantizado .
4. Clasificación de un conjunto
A continuación, usemos Collections.sort () para ordenar un LinkedHashSet .
Estamos usando LinkedHashSet porque mantiene el orden de inserción.
Observe cómo, para usar la API de clasificación en Colecciones , primero estamos envolviendo el conjunto en una lista :
@Test public void givenSet_whenUsingSort_thenSortedSet() { Set integersSet = new LinkedHashSet(Ints.asList(toSort)); Set descSortedIntegersSet = new LinkedHashSet( Arrays.asList(new Integer[] {255, 200, 123, 89, 88, 66, 7, 5, 1})); List list = new ArrayList(integersSet); Collections.sort(Comparator.reverseOrder()); integersSet = new LinkedHashSet(list); assertTrue(Arrays.equals( integersSet.toArray(), descSortedIntegersSet.toArray())); }El método Comparator.reverseOrder () invierte el orden impuesto por el orden natural.
5. Mapa de clasificación
En esta sección, comenzaremos a ver cómo ordenar un mapa, tanto por claves como por valores.
Primero definamos el mapa que estaremos ordenando:
@Before public void initVariables () { .... HashMap map = new HashMap(); map.put(55, "John"); map.put(22, "Apple"); map.put(66, "Earl"); map.put(77, "Pearl"); map.put(12, "George"); map.put(6, "Rocky"); .... }5.1. Ordenar mapa por claves
We'll now extract keys and values entries from the HashMap and sort it based on the values of the keys in this example:
@Test public void givenMap_whenSortingByKeys_thenSortedMap() { Integer[] sortedKeys = new Integer[] { 6, 12, 22, 55, 66, 77 }; List
entries = new ArrayList(map.entrySet()); Collections.sort(entries, new Comparator
() { @Override public int compare( Entry o1, Entry o2) { return o1.getKey().compareTo(o2.getKey()); } }); Map sortedMap = new LinkedHashMap(); for (Map.Entry entry : entries) { sortedMap.put(entry.getKey(), entry.getValue()); } assertTrue(Arrays.equals(sortedMap.keySet().toArray(), sortedKeys)); }
Note how we used the LinkedHashMap while copying the sorted Entries based on keys (because HashSet doesn't guarantee the order of keys).
The Map before sorting :
[Key: 66 , Value: Earl] [Key: 22 , Value: Apple] [Key: 6 , Value: Rocky] [Key: 55 , Value: John] [Key: 12 , Value: George] [Key: 77 , Value: Pearl]The Map after sorting by keys:
[Key: 6 , Value: Rocky] [Key: 12 , Value: George] [Key: 22 , Value: Apple] [Key: 55 , Value: John] [Key: 66 , Value: Earl] [Key: 77 , Value: Pearl] 5.2. Sorting Map by Values
Here we will be comparing values of HashMap entries for sorting based on values of HashMap:
@Test public void givenMap_whenSortingByValues_thenSortedMap() { String[] sortedValues = new String[] { "Apple", "Earl", "George", "John", "Pearl", "Rocky" }; List
entries = new ArrayList(map.entrySet()); Collections.sort(entries, new Comparator
() { @Override public int compare( Entry o1, Entry o2) { return o1.getValue().compareTo(o2.getValue()); } }); Map sortedMap = new LinkedHashMap(); for (Map.Entry entry : entries) { sortedMap.put(entry.getKey(), entry.getValue()); } assertTrue(Arrays.equals(sortedMap.values().toArray(), sortedValues)); }
The Map before sorting:
[Key: 66 , Value: Earl] [Key: 22 , Value: Apple] [Key: 6 , Value: Rocky] [Key: 55 , Value: John] [Key: 12 , Value: George] [Key: 77 , Value: Pearl]The Map after sorting by values:
[Key: 22 , Value: Apple] [Key: 66 , Value: Earl] [Key: 12 , Value: George] [Key: 55 , Value: John] [Key: 77 , Value: Pearl] [Key: 6 , Value: Rocky]6. Sorting Custom Objects
Let's now work with a custom object:
public class Employee implements Comparable { private String name; private int age; private double salary; public Employee(String name, int age, double salary) { ... } // standard getters, setters and toString }We'll be using the following Employee Array for sorting example in the following sections:
@Before public void initVariables () { .... employees = new Employee[] { new Employee("John", 23, 5000), new Employee("Steve", 26, 6000), new Employee("Frank", 33, 7000), new Employee("Earl", 43, 10000), new Employee("Jessica", 23, 4000), new Employee("Pearl", 33, 6000)}; employeesSorted = new Employee[] { new Employee("Earl", 43, 10000), new Employee("Frank", 33, 70000), new Employee("Jessica", 23, 4000), new Employee("John", 23, 5000), new Employee("Pearl", 33, 4000), new Employee("Steve", 26, 6000)}; employeesSortedByAge = new Employee[] { new Employee("John", 23, 5000), new Employee("Jessica", 23, 4000), new Employee("Steve", 26, 6000), new Employee("Frank", 33, 70000), new Employee("Pearl", 33, 4000), new Employee("Earl", 43, 10000)}; }We can sort arrays or collections of custom objects either:
- in the natural order (Using the Comparable Interface) or
- in the order provided by a ComparatorInterface
6.1. Using Comparable
The natural order in java means an order in which primitive or Object should be orderly sorted in a given array or collection.
Both java.util.Arrays and java.util.Collections have a sort() method, and It's highly recommended that natural orders should be consistent with the semantics of equals.
In this example, we will consider employees with the same name as equal:
@Test public void givenEmpArray_SortEmpArray_thenSortedArrayinNaturalOrder() { Arrays.sort(employees); assertTrue(Arrays.equals(employees, employeesSorted)); }You can define the natural order for elements by implementing a Comparable interface which has compareTo() method for comparing current object and object passed as an argument.
To understand this clearly, let's see an example Employee class which implements Comparable Interface:
public class Employee implements Comparable { ... @Override public boolean equals(Object obj) { return ((Employee) obj).getName().equals(getName()); } @Override public int compareTo(Object o) { Employee e = (Employee) o; return getName().compareTo(e.getName()); } }Generally, the logic for comparison will be written the method compareTo. Here we are comparing the employee order or name of the employee field. Two employees will be equal if they have the same name.
Now when Arrays.sort(employees); is called in the above code, we now know what is the logic and order which goes in sorting the employees as per the age :
[("Earl", 43, 10000),("Frank", 33, 70000), ("Jessica", 23, 4000), ("John", 23, 5000),("Pearl", 33, 4000), ("Steve", 26, 6000)]We can see the array is sorted by name of the employee – which now becomes a natural order for Employee Class.
6.2. Using Comparator
Now, let's sort the elements using a Comparator interface implementation – where we pass the anonymous inner class on-the-fly to the Arrays.sort() API:
@Test public void givenIntegerArray_whenUsingSort_thenSortedArray() { Integer [] integers = ArrayUtils.toObject(toSort); Arrays.sort(integers, new Comparator() { @Override public int compare(Integer a, Integer b) { return Integer.compare(a, b); } }); assertTrue(Arrays.equals(integers, ArrayUtils.toObject(sortedInts))); }Now lets sort employees based on salary – and pass in another comparator implementation:
Arrays.sort(employees, new Comparator() { @Override public int compare(Employee o1, Employee o2) { return Double.compare(o1.getSalary(), o2.getSalary()); } });The sorted Employees arrays based on salary will be:
[(Jessica,23,4000.0), (John,23,5000.0), (Pearl,33,6000.0), (Steve,26,6000.0), (Frank,33,7000.0), (Earl,43,10000.0)] Note that we can use Collections.sort() in a similar fashion to sort List and Set of Objects in Natural or Custom order as described above for Arrays.
7. Sorting With Lambdas
Start with Java 8, we can use Lambdas to implement the Comparator Functional Interface.
You can have a look at the Lambdas in Java 8 writeup to brush up on the syntax.
Let's replace the old comparator:
Comparator c = new Comparator() { @Override public int compare(Integer a, Integer b) { return Integer.compare(a, b); } }With the equivalent implementation, using Lambda expression:
Comparator c = (a, b) -> Integer.compare(a, b);Finally, let's write the test:
@Test public void givenArray_whenUsingSortWithLambdas_thenSortedArray() { Integer [] integersToSort = ArrayUtils.toObject(toSort); Arrays.sort(integersToSort, (a, b) -> { return Integer.compare(a, b); }); assertTrue(Arrays.equals(integersToSort, ArrayUtils.toObject(sortedInts))); }As you can see, a much cleaner and more concise logic here.
8. Using Comparator.comparing and Comparator.thenComparing
Java 8 comes with two new APIs useful for sorting – comparing() and thenComparing() in the Comparator interface.
These are quite handy for the chaining of multiple conditions of the Comparator.
Let's consider a scenario where we may want to compare Employee by age and then by name:
@Test public void givenArrayObjects_whenUsingComparing_thenSortedArrayObjects() { List employeesList = Arrays.asList(employees); employees.sort(Comparator.comparing(Employee::getAge)); assertTrue(Arrays.toString(employees.toArray()) .equals(sortedArrayString)); }In this example, Employee::getAge is the sorting key for Comparator interface implementing a functional interface with compare function.
Here's the array of Employees after sorting:
[(John,23,5000.0), (Jessica,23,4000.0), (Steve,26,6000.0), (Frank,33,7000.0), (Pearl,33,6000.0), (Earl,43,10000.0)]Here the employees are sorted based on age.
We can see John and Jessica are of same age – which means that the order logic should now take their names into account- which we can achieve with thenComparing():
... employees.sort(Comparator.comparing(Employee::getAge) .thenComparing(Employee::getName)); ... After sorting with above code snippet, the elements in employee array would be sorted as:
[(Jessica,23,4000.0), (John,23,5000.0), (Steve,26,6000.0), (Frank,33,7000.0), (Pearl,33,6000.0), (Earl,43,10000.0) ]Thus comparing() and thenComparing() definitely make more complex sorting scenarios a lot cleaner to implement.
9. Conclusion
In this article, we saw how we can apply sorting to Array, List, Set, and Map.
También vimos una breve introducción sobre cómo las características de Java 8 podrían ser útiles para clasificar el uso de Lambdas, comparar () y luego Comparar () y En paraleloSort () .
Todos los ejemplos utilizados en el artículo están disponibles en GitHub.