Eighth Chapter Collection Class

8 Integrated

Used to store objects with variable lengths to store different types of objects

8.1 Integrated Frame

Collection

List

ArrayList: The underlying data structure uses an array structure. Features: Fast query speed, adding and deleting slow. Thread is out of sync

LinkedList: The underlying data structure uses a linked list data structure. Features: Slow query, adding and deleting fast. Thread is out of sync

Vector: The underlying data structure is used by the array-of-structures. Thread synchronization, replaced by ArrayList.

Set

HashSet: The underlying data structure uses a hash table, and the thread is out of sync.

TreeSet: The underlying data structure uses a two-fork tree to sort the elements in the set set, but the comparable interface needs to be implemented. The basis for guaranteeing the uniqueness of the element: CompareTo method return 0.

1. Common methods:

Add (); adding elements

Remove (); Delete element

Clear (); empty collection

Contains (); Contains element

IsEmpty (); Whether the collection is empty

Size ();

Retainall (); Take the intersection of two sets

Example: A.retainall (b); A only retains the same elements as B

2. Traversing elements

Using iterators: Iterator

Iterator it = A.iterator ();

while (It.hasnext ()) {

It.next ();

}

for (Iterator it = A.iterator (); It.hasnext ();) {

It.next ();

}

8.2 List

1, Characteristics: elements are ordered, can be repeated, the collection has an index

2. Unique methods

Add (index,element); Specify the location of the element

AddAll (index,collection); Add a collection at the specified location

Remove (index); Delete the element at the specified position

Set (index,element); Sets the value of the specified element

Get (index); Gets the element at the specified position

IndexOf (); Gets the position of the element

Sublist (From,to), intercepting the collection, containing the From, does not contain the same

Listiterator (); list iterator, iterator sub-interface

In an iteration, you cannot manipulate the elements in the collection through the methods of the collection object, because the concurrentmodificationexception exception occurs. Therefore, in the iteration, only through the iterator amount method operation elements, free to judge, remove and delete operations.

If you want other operations, such as add, modify, and so on, you need to use its subinterface listiterator.

8.3 LinkedList

1. Unique methods

AddFirst (); Adding elements to the head

AddLast (); Adding elements at the tail

GetFirst (); get the head element

GetLast (); get tail element

Removefirst (); Remove and remove the head element

Removelast (); Remove and remove trailing elements

Where get and remove are used, nosuchelementexception occurs if there are no elements in the collection.

2, there is an alternative method in JDK1.6

adding elements

Offerfirst ();

Offerlast ();

Gets the element that returns NULL if there are no elements in the collection;

Peekfirst ();

Peeklast ();

Gets and deletes an element that returns null if there are no elements in the collection;

Pollfirst ();

Polllast ();

8.4 Set

1, Characteristics: elements are unordered, elements can not be repeated

2, the function of set set and collection is consistent

3. HashSet guarantees the uniqueness of the element: True by Hashcode and equals

• If the hashcode value of the element is the same, the equals is not determined to be true.

• If the Harbin code value of the element is not the same, the Equals method is not called.

For actions that determine whether an element exists, as well as additions and deletions, the dependent method is the hashcode of the element and the Equals method.

8.5 generic type

After the JDK1.5 version, which is used to resolve security issues, is a security mechanism that uses <> representations

Arraylist<string> a = new arraylist<string> ();

1. Generic type

When the reference data type to be manipulated in the class is not deterministic

Class utiles<tt>{

Private TT t;

public void Settt (TT t) {

THIS.T = t;

}

Public TT gettt () {

return t;

}

}

A static method cannot access a generic class and can access a generic method.

2. Generic method

Public <T> void Show (T t) {

...... ；

}

3. Generic Limit

ArrayList <? Extends e>;

？ Represents a wildcard character

You can receive an e-type or a subtype of e, the upper limit.

ArrayList <? Super e>;

You can receive the type E or the parent type of e, the lower bound.

8.6 Map

The collection stores key-value pairs

HashTable: The underlying data structure uses a hash table, which does not deposit null keys and null values, and thread synchronization.

HASHMAP: The underlying data structure uses a hash table that can use NULL keys and null values, and threads are out of sync.

TREEMAP: The underlying data structure uses a two-fork tree, and the thread is out of sync. Can be used to sort the keys in the map collection.

1. Method:

Add to

Put (K key,v value);

Putall (map<? extends K,? extends v> m);

Delete

Clear ();

Judge

ContainsKey (Object key);

Containsvalues (Object value);

IsEmpty ();

Get

Get (Object key);

Size ();

Values ();

EntrySet ();

Set <Map.Entry<k,v>> entryset; the mapping relationships in the map collection are stored in the set collection, and the data type of the relationship is map.entry. Once you get map.entry, you can get the keys and values in the relationship through the Getkey and GetValue methods in them.

KeySet (); Save all the keys to the set set because the set has iterators, so you can remove all the keys in an iterative way, and then get the values for each key according to the map's Get method

8.7 tool classes for the Integrated Framework

Collections.fill (List,object);

You can replace all the elements in the list collection with the specified elements

Collections.repalceall (List,oldvalue,newvalue);

Replaces the old value in the list collection with the new value.

Collections.reverse (List);

Reverses the elements in the list collection

8.8 Advanced for Loop

Format:

For (data type variable name: traversed collection or array) {

}

Only elements can be obtained, but the collection cannot be manipulated.

8.9 variable Parameters

When passing parameters, use ...

public void Show (int ... arr) {}

Where arr is actually an array

8.10 Static Import

Import static java.util.arrays.*;

All static members in the arrays class are imported.

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Eighth Chapter Collection Class