Collections in Java

1.ArrayList

ArrayList collection class

Construction method	illustrate
public ArrayList()	Constructs an empty list with an initial capacity of 10

1.1. Adding elements

ArrayList collection class

member method	illustrate
public boolean add(E e)	adds the specified element to the end of this list
public void add(int index, E element)	Adds an element at the specified index, moving the previous element to the right

 　　Define an ArrayList class and add elements:

1 ArrayList<String> al = new ArrayList<String>();
2 al.add("hello");                               
3 al.add(0, "world");                            
4                                                
5 System.out.println(al); // [world, hello]      

　　When specifying an index, the index out of bounds exception: (the specified index does not exist in the collection)

al.add(7, "USA");                        
System.out.println(al);

 

1.2. Delete, modify, check

Types of	member method	illustrate
check	public E get(int index)	Returns the element at the specified position in the list (risk of index out of bounds)
check	public int size()	Returns the number of elements in the collection
check	public int indexOf(Object o)	Returns the index of the first occurrence of the specified element in the list collection (does not return -1)
check	public boolean isEmpty()	no element, returns true
delete	public E remove(int index)	Deletes the element at the specified position; the return value is the deleted element
delete	public boolean remove(Object o)	removes the first occurrence of the element; returns true for the containing element
change	public E set(int index, E element)	Replaces the contents of the specified index with the element; returns the replaced old element

 

1.3. Traverse the collection ArrayList

　　Traversal through the size and get methods of the ArrlyList collection

1 //Sony                                                  
2 //said                                                  
3 //hello                                                 
4 //JavaEE                                                
5 for (int i = 0; i < al.size(); i++) {                   
6     System.out.println(al.get(i));                      
7 }                                                       
8 System.out.println(al); //[Sony, said, hello, JavaEE, .]