Section 16 Collections and Generics

Collections:

A collection (container) is an object that groups multiple elements into a single unit.

TreeSet: Keeps the elements sorted and prevents duplicates.Use red-black tree.

HashSet: Prevents duplicates.

LinkedList: Better performance for insertion and deletion

HashMap: Store name/value pairs. No duplicate keys. Can have duplicate values.

LinkedHashMap: Remember the order elements inserted.

Map is not a true collection.

Use Set /List/Map as the reference type instead of implementation types.

Set: prevent duplicates.

HashSet: hashtable.

TreeSet: Keep the elements sorted. Use red-black tree.

LinkedHashSet: Keep insertion order.

List:

ArrayList

LinkedList: Better performance for insertion and deletion

Vector

Deque: (deck)

Implement both stack and queue.

Map:

TreeMap:

HashMap:

LinkedHashMap:

HashTable: 

1.hashMap去掉了HashTable 的contains方法，但是加上了containsValue（）和containsKey（）方法。
2.hashTable同步的，而HashMap是非同步的，效率上逼hashTable要高。
3.hashMap允许空键值，而hashTable不允许。

Ordering for sort:

Classes Implementing ComparableClassNatural OrderingByteSigned numericalCharacterUnsigned numericalLongSigned numericalIntegerSigned numericalShortSigned numericalDoubleSigned numericalFloatSigned numericalBigIntegerSigned numericalBigDecimalSigned numericalBooleanBoolean.FALSE < Boolean.TRUEFileSystem-dependent lexicographic on path nameStringLexicographicDateChronologicalCollationKeyLocale-specific lexicographic

1. public static <T extends Comparable<? super T>> void sort(List<T> list)

sort() method only takes comparable objects. T must be comparable. Implement Comparable interface.

2. public static <T> void sort(List<T> list, Comparator<? super T> c)

Use custom Comparator.

Generics: for type-safe collections

Generic Method:

public <T extends Animal> void takeThing(ArrayList<T> list)

HashSet 首先判断hashcode是否一致，利用hashcode产生数组下标。如果hashcode一致（可能是冲突），则调用equals()判断reference是否一样。

a.equals(b) implies a.hashCode()==b.hashCode(). Not verse.

If you override equals(), you must override hashcode().

Polymorphism:

public void takeAnimals(Animals[] animals); // can take subclass of Animal as arguments.public void takeAnimals(ArrayList<Animal> animals); // can take only take Animal class.

To avoid adding wrong type to the ArrayList, use wildcards:

public <T extends Animal> void takeThing(ArrayList<T> list)public void takeThing(ArrayList<? extends Animal> list)