Java - List (sequence)

About

A list (also known as a sequence):

• is ordered
• allow duplicates (multiple null elements if they allow them)
• is zero based. The first element is at the position 0.
• can contain themselves as elements (but caution as the equals and hashCode methods are no longer well defined on such a list)

A special iterator, called a ListIterator, allows element insertion and replacement, and bidirectional access (from begin or from end) in addition to the normal operations.

Interface and Implementation

List is an interface from the Java Collections Framework representing an ordered collection (also known as a sequence).

See List for all implementations. The most known is the ArrayList: a Resizable-array that permits all elements, including null.

Management

Instantiation

From Array

From an array

List<Class> myList = new ArrayList<Class>();

// With String as Class
List<String> messages = new java.util.ArrayList(Arrays.asList("Nico", "Gerard"));

// The java.util.ArrayList transform the list implementation of the return list from Arrays.asList

See asList

• As generic function

public static <T> List<T> asList(T... object) {
    return Arrays.stream(object)
      .collect(Collectors.toList());
}

New Operator

List<Class> myList = new ArrayList<Class>();

The ArrayList(int) constructor gives a List that has size 0. It only ensures that n elements can be added before it needs to reallocate the underlying array.

From scalar

Collections.singletonList(myObject)

Concatenation

List<String> list = new ArrayList<String>();
newList.addAll(secondList);

Join

String join = String.join("joinSeparator", list);

• With a stream: Example with columns, to create the first line of a csv

final String headers = columns.stream()
                .map(s->s.getColumnName())
                .collect(Collectors.joining(","));

Copy

The element are not copied.

List<String> copyList = new ArrayList<String>();
copyList.addAll(list);

Deep Copy

The element are also copied.

for(Object obj:list){
    copyList.add(obj.clone());
    copyList.add(obj.clone());
}

Cast

• cast of generic list with parameters. You can't cast a generic type of one parameter to another directly. However, since you can cast to and from wildcard types, you can cast through an intermediate wildcard type.

List<B> variable = (List<B>)(List<?>) collectionOfListA;

Sort

• Ascending order

foreignKeys = foreignKeys.stream()
                .sorted((s1,s2)->(s1.getForeignPrimaryKey().getTableDef().getName()).compareTo(s2.getForeignPrimaryKey().getTableDef().getName()))
                .collect(Collectors.toList());
// same as
foreignKeys = foreignKeys.stream()
                .sorted(Comparator.comparing(s -> (s.getForeignPrimaryKey().getTableDef().getName())))
                .collect(Collectors.toList());

• Descending order

foreignKeys = foreignKeys.stream()
                .sorted((s1,s2)->(s2.getForeignPrimaryKey().getTableDef().getName()).compareTo(s1.getForeignPrimaryKey().getTableDef().getName()))
                .collect(Collectors.toList());
// same as
foreignKeys = foreignKeys.stream()
                .sorted(Comparator.comparing(s -> (s.getForeignPrimaryKey().getTableDef().getName())).reversed())
                .collect(Collectors.toList());

Reverse

• Stream

List<String> elements  = elements.stream()
   .sorted(Collections.reverseOrder())
   .collect(Collectors.toList());

• Collections methods

Collections.reverse() 

Stream to

Stream

toList

• List of object to a list of attribute

List<Person> persons = new ArrayList<>();
// ... add persons
List<String> names = persons.stream()
                .map(a -> a.getName())
                .collect(Collectors.toCollection(ArrayList::new));
                
// or 
//     .collect(Collectors.toList());

• List of list to list

List<String> groups =
	properties.stream()
	.flatMap(x->x.getListOfStrings().stream())
	.collect(Collectors.toList());

toMap

• from a list of object

List<Person> persons = new ArrayList<>();
// ... add persons
Map<Integer, CliWord> personsMap = persons.stream()
                .filter(p -> p.getNummer()!=null)
                .collect(Collectors.toMap(Person::getNummer, Function.identity()));

• extends a list of string with attributes.

List<String> persons = new ArrayList<>();
Map<String, String> personsMap = persons.stream()
            .collect(Collectors.toMap(
                s -> s, 
                s -> lookupFirstName(s)
            ));

• groupBy

Map<String, List<Person>> groupedByName = persons
        .stream()
        .collect(Collectors.groupingBy(Person::getName));

• from a list of generic

Map<T, Double> buckets = genericList
        .stream()
        .collect(Collectors.toMap(
          g->g,
          g->1.0
        ));

toArray

List to array

• General construction

list.toArray(new Class[0])
// before version I don't know anymore
list.toArray(new Class[arraySize])

• from a list of scalar

// For a list of string
list.toArray(new String[5])
// since opendJdk java 6 better not to pre-size
list.toArray(new String[0])

// Example
List<String> myList = new ArrayList<>();
myList.add("first");
myList.add("second");

String[] myArray = myList.toArray(new String[myList.size()])

• from a list of object Object

// from Stream
final String[] nativeColumns = foreignKeyDef.getNativeColumns().stream()
                        .map(ColumnDef::getColumnName)
                        .toArray(String[]::new);

toGenericArray

// Based on
T[] arrayGeneric = (T[]) Array.newInstance(clazz,size)

// With Stream
T[] objects = valuesList.stream()
        .map(o -> Casts.cast(o, columnDef.getClazz()))
        .collect(Collectors.toList())
        .toArray((T[]) Array.newInstance(clazz,0));

ToSet

mySet = new HashSet<>(list);

Documentation / Reference

• List Api