Map

Summary
1. Map: Translation is "Map"
Map: The correspondence between a point on paper and an actual location in real life
2. Map in java:
Correspondence between recording one data and another
3. Corresponding relationship:

4. The second way to express correspondence in java. Maps also express correspondence in an exhaustive way. Dictionaries (by exhaustive way, the correspondence between words and explanations)

5. The characteristics of Map:

Simple Method in Map
1. Description:
Map is an interface that defines what functions should be available in a two-column collection
2. Common functions:

	put(K key, V value)
 	 	If the key does not exist in the collection, add a pair of key-value pairs
 	remove(Object key)
 	 	Delete key-value pairs based on specified keys
 	put(K key, V value)
 	 	If the key already exists in the collection, then the corresponding value is modified.
 	get(Object obj)
 	 	Get the corresponding value according to the given key
 	size()
 	 	Get the logarithm of key-value pairs
 	clear()
 	 	Empty Set
 	containsKey(Object obj)
 	 	Determine whether obj keys exist in a set
 	containsValue(Object obj)
 	 	Determine whether there is a value in the set
 	isEmpty()
 	 	Judging whether a set is empty

Code examples

import java.util.HashMap;
import java.util.Map;

public class Demo04_Map {
	public static void main(String[] args) {
		Map<String, Integer> map = new HashMap<>();
		map.put("abc", 3);
		map.put("qq", 2);
		map.put("zxcv", 4);
		map.put("", 666);
		System.out.println(map);
		System.out.println(map.get("abc"));
		map.remove("abc");
		System.out.println(map);
		
		map.put("", 0);
		System.out.println(map);
		
		System.out.println(map.isEmpty());
		System.out.println(map.size());
		System.out.println(map.containsKey(""));
		System.out.println(map.containsValue(666));
		
		map.clear();
		System.out.println(map);
	}
}

Traversal of Map

1. Overview:

2. Two ideas, each corresponding to two traversal modes

Map's first traversal idea: getting values based on keys
1. Steps:

2. The methods involved are as follows:

1,Set<K> keySet()  	
2,V get(K key)

Map's second traversal idea: Getting keys and values from keys to objects

1. Steps:

2. Classes and methods involved

Code examples

import java.util.HashMap;
import java.util.Iterator;
import java.util.Map;
import java.util.Set;

public class Demo05_Map The traversal of ___________ {
	public static void main(String[] args) {
		Map<String, Integer> map = new HashMap<>();
		map.put("abc", 3);
		map.put("qq", 2);
		map.put("zxcv", 4);
		map.put("", 666);
		
		test22_Getting keys and values from key-value pairs of objects_Enhance for(map);
	}

	private static void test22_Getting keys and values from key-value pairs of objects_Enhance for(Map<String, Integer> map) {
		for (Map.Entry<String, Integer> en : map.entrySet()) {
			System.out.println(en.getKey() + "..." + en.getValue());
		}
	}

	private static void test21_Getting keys and values from key-value pairs of objects_iterator(Map<String, Integer> map) {
		//Get the Set set of all key-value pair objects
		Set<Map.Entry<String, Integer>> entrys = map.entrySet();
		//iterator
		Iterator<Map.Entry<String, Integer>> it = entrys.iterator();
		while (it.hasNext()) {
			//Get the next key-value pair object
			Map.Entry<String, Integer> en = it.next();
			//Gets the key of the key to the object
			String key = en.getKey();
			//Gets the value of the key to the object
			int value = en.getValue();
			System.out.println(key + "..." + value);
		}
	}

	private static void test12_Get values based on keys_Enhance for(Map<String, Integer> map) {
		//Get values based on keys
		for (String key : map.keySet()) {
			System.out.println(key + "..." + map.get(key));
		}
	}

	private static void test11_Get values based on keys_iterator(Map<String, Integer> map) {
		//Get the Set of Sets for all keys
		Set<String> keys = map.keySet();
		//Traversing Set Sets
		Iterator<String> it = keys.iterator();
		while (it.hasNext()) {
			String key = it.next();
			int value = map.get(key);
			System.out.println(key + "..." + value);
		}
	}
}

Exercise: Keyboard input a string, count the number of occurrences of each character
Code examples

import java.util.HashMap;
import java.util.Map;
import java.util.Scanner;

public class Demo06_Statistical exercises {
	public static void main(String[] args) {
//		Keyboard enters a string and counts the number of occurrences of each character
		Scanner sc = new Scanner(System.in);
		String line = sc.nextLine();
		char[] chs = line.toCharArray();
		//Prepare a container for statistics, the correspondence between characters and numbers
		Map<Character, Integer> map = new HashMap<>();
		for (char c : chs) {
			/*if (map.containsKey(c)) {
				int count = map.get(c);
				map.put(c, count + 1);
			} else {
				map.put(c, 1);
			}*/
			map.put(c, map.containsKey(c) ? map.get(c) + 1 : 1);
		}
		
		System.out.println(map);
		
		//The required printing results are as follows: a has 4, b has 4,.... There are two.
		System.out.println(getString(map));
	}
	
	public static String getString(Map<Character, Integer> map) {
		StringBuilder sb = new StringBuilder();
		for (Map.Entry<Character, Integer> en : map.entrySet()) {
			char c = en.getKey();
			int v = en.getValue();
			sb.append(c).append("Yes").append(v).append("One,");
		}
		
		sb.replace(sb.length() - 1, sb.length(), ". ");
		return sb.toString();
	}
}

HashMap

Summary
1. It is an implementation class of Map, which is implemented by hash storage.
2. View the source code of HashSet:

3. Source screenshots:

LinkedHashMap

1. Keys can be stored and removed in the same order.
2. It is a subclass of HashMap and an implementation class of Map.
Code examples

import java.util.LinkedHashMap;

public class Demo08_LinkedHashMap {
	public static void main(String[] args) {
		LinkedHashMap<String, Integer> lhm = new LinkedHashMap<>();
		lhm.put("asdf", 4);
		lhm.put("qq", 2);
		lhm.put("xyz", 3);
		System.out.println(lhm);
	}
}

Comparison of HashMap and Hashtable
1. Similarities:

2. Differences:

Code examples

import java.util.HashMap;
import java.util.Hashtable;

public class Demo09_HashMap and Hashtable Comparison {

	public static void main(String[] args) {
		HashMap<String, String> hm = new HashMap<>();
		hm.put(null, "");
		hm.put("kong", null);
		hm.put(null, null);
		System.out.println(hm);
	}

	private static void test1_Hashtable Unable to store null key null value() {
		Hashtable<String, String> ht = new Hashtable<>();
//		ht.put(null, "");
//		ht.put("", null);
//		ht.put(null, null);
		System.out.println(ht);
	}
}

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