[Android study notes] JAVA basics - class set framework

Types
of collections The basic structure of

the collection framework What is the collection

framework Three major categories: sets, lists, and maps Objects in a




set
are not sorted in a specific way, and there are no duplicate

objects
. Objects in a list set are corrupted by index position, and there can be duplicate objects in

a
map set. Each element of contains a key object and a value object, the key can not be repeated, the value can be repeated
Interator <-- Collection <-- List

                         <-- Set  

//list routine						 
import java.util.List;//Import List interface
import java.util.ArrayList;//Import the ArrayList class


public class Test{
	public static void main(String args []){
		//Generate an ArrayList object
		//ArrayList<xxx> xxx represents the type of object
		ArrayList<String> arrayList = new ArrayList<String>();
		
		arrayList.add("a");//Add elements
		arrayList.add("b");
		arrayList.add("c");
		arrayList.add("d");
		
		arrayList.remove(1);//Remove an element according to the subscript (the following elements will not come up automatically after removal)
		int lenght = arrayList.size();//Get the length of the list
		String s = arrayList.get(1);//Take out an element corresponding to the following table
		System.out.println(s);
	}
}

//collection routine
import java.util.Set;//Import the Set interface
import java.util.HashSet;//Import HashSet implementation class
import java.util.Iterator; // import Iterator interface
public class Test{
	public static void main(String args []){
		//
		HashSet<String> hashSet = new HashSet<String>();
		Set<String> = set = hashSet;//Upcast
		
		set.add("a");//Add element
		set.add("b");
		set.add("c");
		set.add("d");
		set.add("a");//The "a" here is repeated, and the length will not change
		
		
		set.remove("a");//Remove the element "a"
		boolean b = set.isEmpty();//Determine whether the element is empty
		int lenght = set.size();//Get the number of elements
		set.clear();//Clear all elements of the collection
	
		// Generate an iterator object, which is used to traverse the entire Set
		Iterator<String> it =set.iterator();
		
		boolean h = it.hasNext();//Return whether there is a next object
		String s = it.Next();//Remove the value of the current cursor and move it back one bit


		System.out.println(s);
	}
}

//mapping routine
import java.util.Map;
import java.util.HashMap;


public class Test{
	public static void main(String args []){
	//Define a hashMap class
	//Hashmap<a,b> where a represents the type of the key and b represents the type of the value
	HashMap<String,String> hashMap = new HashMap<String,String>();
	
	Map<String,String> map = hashMap;//Upcast to map
	
	
	map.put("1","a");//Store key-value pairs
	map.put("2","b");
	map.put("3","c");
	map.put("3","d");//The key value is the same as the existing one, the old value will be overwritten by the new one
	
	String s = map.get("2");//Read the object corresponding to the key
}

By Urien April 14, 2018 14:06:47