Java ArrayList array syntax

Catching basis, record what common syntax array of ArrayList

1. Import

import java.util.ArrayList;

2. Define the array list 

ArrayList <class name> list = new ArrayList <class name> (); can not be a basic type, must be a class

3. Get set size

size()

4. Data stored

add (Object object); start from index 0 was added

add (int idx, Object object); inserting the object position index idx, idx <= list.size ();

Different deposited into a single object to new, not by changing the value after addition of the array. (Involved in memory)

5. Delete

remove (int idx); delete the index idx of the element, the element returns, to receive may be variable, or not received

6. Empty

Clear (); empty array

7. Replace

set (int idx, Object object); the original object element and the element index idx is replaced

8. Get the specified location element

Object get(int idx);

9. sentenced empty

bool isEmpty (); generally it does not, size () may be used to empty judgment

10. determines whether an element

bool contains (Object object); the basic need, can be solved by finding the index of the element

11. Find the index element

int indexOf (Object object); if the element is present, the index is returned, otherwise -1, -1 is not determined by the elements in the array is not

12. The sorting array list

Introducing Collections class;

import java.util.Collections;

(1) default natural order, from small to large

Collections.sort (list); // not a new Collections object directly

(2) custom ordering

Comparator introducing the like;

java.util.Comparator import;

When creating an object needs to implement an abstract method compare (), implement custom sorting

Package my_acm; 

Import of java.util.ArrayList;
 Import java.util.Collections;
 Import java.util.Comparator; // custom sorting
 // Import java.lang.Integer; // lang classes in the package can not be directly turned using 

public  class MyTest4 {
     public  static  void main (String [] args) { 
        
        the ArrayList <Point> = List1 new new the ArrayList <Point> (); 
        the ArrayList <Integer> = List2 new new the ArrayList <Integer> ();
         // the ArrayList <int> the ArrayList = new new list3 <int> (); //Error, the basic data types can not 
        Comparator <Point> Comparator = new new Comparator <Point> () {
             public  int Compare (Point P1, P2 Point) {
                 IF (p1.id! = P2.id)
                     return p1.id- p2.id ;
                 the else  
                { 
                    IF (p1.age =! p2.age)
                         return p1.age- p2.age;
                     the else  
                        return 0; // like C ++, where the need to match if-else 
                } 
            } 
        }; 
        
        Point P1 = new newPoint (); 
        p1.id =. 11; = 21 is p1.age ; 
        list1.add (P1); 
        Point P2 = new new Point (); 
        p2.id =. 9; = 44 is p2.age ; 
        list1.add (P2); 
        P3 Point = new new Point (); 
        p3.id = 2; 68 = p3.age ; 
        list1.add (P3); 
        
        / ** values applied to change the variable in the array is not feasible, ( 
        p3.id = 14 ; = 23 is p3.age; 
        list1.add (P3); 
        * / 
        for ( int I = 0; I <list1.size (); I ++ ) { 
            System.out.println ( "I =" + I + "ID =" + list1.get (i) .id + " age =" +list1.get(i).age);
        }
        Collections.sort(list1,comparator);
        System.out.println("按id排序后");
        
        for(int i=0;i<list1.size();i++) {
            Point x = new Point();
            x=list1.get(i);
            System.out.println( "i="+i+" id="+x.id+" age="+x.age );
        }
    }
}

class Point{
    int id;
    int age;
}

Output:

i=0 id=11 age=21
i=1 id=9 age=44
i=2 id=2 age=68
按id排序后
i=0 id=2 age=68
i=1 id=9 age=44
i=2 id=11 age=21