Detailed explanation of common syntax of Java array ArrayList

Fill in the foundation and record the common syntax of array ArrayList

1. Import

import java.util.ArrayList;

2. Define array list

ArrayList < class name > List = new ArrayList < class name > (); Cannot be a basic type, must be a class

3. Get collection size

size()

4. Store data

add(Object object); Join from subscript 0

add(int idx,Object object); Insert the object into the position with index idx, IDX < = list size();

When saving different objects, you need a single new. You can't add an array by changing the value. (memory involved)

5. Delete

remove(int idx); Delete the element with index idx and return the element. It can be received with variables or not

6. Empty

clear(); Empty array

7. Replacement

set(int idx,Object object); Replace the object element with the element originally indexed as IDX

8. Get the specified location element

Object get(int idx);

9. Air judgment

bool isEmpty(); Generally not, size () can be used to judge null

10. Determine whether there is an element

bool contains(Object object); Basically not. It can be solved by looking up the index of the element

11. Find the index of the element

int indexOf(Object object); If the element exists, the index is returned. Otherwise, it returns - 1. Judge whether the element is in the array by whether it is - 1 or not

12.. Sort array list

Import collections class;

import java.util.Collections;

(1) Default natural sorting, from small to large

Collections. sort(list); // You can't new out the object of collections. Use it directly

(2) Custom sorting

Import comparator class;

import java.util.Comparator;

When creating objects, you need to implement the abstract method compare() to implement custom sorting

package my_acm;
import java.util.ArrayList;
import java.util.Collections;
import java.util.Comparator;//自定义排序
//import java.lang.Integer;//lang包中的类不用导也能直接用
public class MyTest4 {
  public static void main(String [] args) {
    ArrayList<Point> list1 = new ArrayList<Point>();
    ArrayList<Integer> list2 = new ArrayList<Integer>();
    //ArrayList<int> list3 = new ArrayList<int>();//报错，基本数据类型不能
    Comparator<Point> comparator = new Comparator<Point>() {
      public int compare(Point p1,Point p2) {
        if(p1.id!=p2.id)
          return p1.id-p2.id;
        else
        {
          if(p1.age!=p2.age)
            return p1.age-p2.age;
          else
            return 0;//不像C++，这里的if-else需要匹配
        }
      }
    };
    Point p1 = new Point();
    p1.id=11;p1.age=21;
    list1.add(p1);
    Point p2 = new Point();
    p2.id=9;p2.age=44;
    list1.add(p2);
    Point p3 = new Point();
    p3.id=2;p3.age=68;
    list1.add(p3);

    /** 修改变量的值在加到数组里是不可行的，（
    p3.id=14;p3.age=23;
    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 results:

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

After sorting by ID

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

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