Java Collection Framework

Key elements in Collection Framework

1. Interfaces
2. Implementations
3. Algorithms

Interfaces

• Collection
  • Set
    • SortedSet
  • List
  • Queue
  • Deque
• Map
  • SortedMap

The Collection Interface

Collection Basic Operations

• size()
• isEmpty()
• contains()
• add(Object o)
• remove(Object o)
• iterator()

Collection Bulk Operations

• containsAll()
• addAll()
• removeAll()
• retainAll()
• clear()

Traverse Collections

Aggregate Operations

In JDK 8 and later, the preferred method of iterating over a collection is to obtain a stream and perform aggregate operations on it.

• stream()

myShapesCollection.stream()
  .filter(e -> e.getColor() == Color.RED)
  .forEach(e -> System.out.println(e.getName()));

• parallelStream()

myShapesCollection.parallelStream()
  .filter(e -> e.getColor() == Color.RED)
  .forEach(e -> System.out.println(e.getName()));

• map() & collect()

String joined = elements.stream()
    .map(Object::toString)
    .collect(Collectors.joining(", "));

• collect()

int total = employees.stream()
  .collect(Collectors.summingInt(Employee::getSalary)));

For-each Construct

for (Object o : collection)
    System.out.println(o);

Iterator

An Iterator is an object that enables you to traverse through a collection and to remove elements from the collection selectively, if desired.

public interface Iterator<E> {
    boolean hasNext();
    E next();
    void remove(); //optional
}

Use Iterator instead of the for-each construct when you need to:

• Remove the current element.
• Iterate over multiple collections in parallel.

  static void filter(Collection<?> c) {
    for (Iterator<?> it = c.iterator(); it.hasNext(); )
        if (!cond(it.next()))
            it.remove();
  }
  

Collection Array Operations

• toArray()
  • return a new Object Array containing elements in old one
• toArray(T[] a)
  • return a new T Array containing elements in old one