Map collection framework

Introduction to Map collection framework

Method classification

A set of mapping relationships is stored in the map set, with key = value

Supplement: pseudocode described by Map set mapping relation May.Entry

Add & remove

Increase:
put(K key, V value) 

putAll(Map<? extends K,? extends V> m) 

//Delete:
clear() 

remove(Object key) 

Judgment & Acquisition

Judgement:
containsKey(Object key) 

containsValue(Object value) 

isEmpty() 

//Obtain:
get(Object key)

size()

values()  

entrySet() 

keySet()

Compare the differences between Hashtable, HashMap and TreeMap

Be careful:

	When adding an element, if the key already exists in the collection, the value added later will overwrite the original value, and the put method will return the original value

Map collection framework application

Application 1

package com.Map;

import java.util.Comparator;
import java.util.HashMap;
import java.util.Map;
import java.util.Map.Entry;
import java.util.Set;
import java.util.TreeMap;


/**
 * Application 1:

	1,The student is used as the key, the address is used as the value for storage, and the name with the same age is recognized as a person, and finally output

	2,Last sorted by age

	3,Demand change, sort by name
 *
 *Steps:
 *1,Create a student class, instantiate it and add it to the map set as key
 *2,Judge students by weight
 *	
 * @author 86182
 *
 */
public class TreeMapDemo {

	public static void main(String[] args) {

		//Map<Student, String> map = new HashMap<>();
		//Map<Student, String> map = new TreeMap<>();
		Map<Student, String> map = new TreeMap<>(new StuComp());
		map.put(new Student("aa", 25), "The Republic of Korea");
		map.put(new Student("ee", 21), "Macao");
		map.put(new Student("bb", 23), "Beijing");
		map.put(new Student("dd", 26), "The Republic of Korea");
		map.put(new Student("cc", 21), "Thailand");
		map.put(new Student("dd", 25), "The Republic of Korea");
		
		//Duplicate elements cannot be added because the underlying calls the hashCode and equals methods. If the elements are at the same address, the equals method will be called.
		
		//Not the same address
		System.out.println(map.size());//6 5
		
		Set<Entry<Student, String>> entrySet = map.entrySet();
		for (Entry<Student, String> entry : entrySet) {
			System.out.println(entry.getKey()+"=="+entry.getValue());
		}
	}

}

class Student implements Comparable<Student>{
	private String name;
	private int age;
	public String getName() {
		return name;
	}
	public void setName(String name) {
		this.name = name;
	}
	public int getAge() {
		return age;
	}
	public void setAge(int age) {
		this.age = age;
	}
	public Student(String name, int age) {
		super();
		this.name = name;
		this.age = age;
	}
	public Student() {
		super();
	}
	@Override
	public String toString() {
		return "Student [name=" + name + ", age=" + age + "]";
	}
	@Override
	public int hashCode() {
		return this.getName().hashCode()+this.getAge()*39;//Widen the gap between hash values
	}
	@Override
	public boolean equals(Object obj) {
		Student stu = (Student)obj;
		return this.getAge() ==stu.getAge() && this.getName().equals(stu.getName());
	}
	@Override
	public int compareTo(Student o) {
		int num = this.getAge() - o.getAge();
		if(num == 0) {
			return this.getName().compareTo(o.getName());
		}
		//Sort by age
		return num;
	}
	
}


class StuComp implements Comparator<Student>{

	@Override
	public int compare(Student o1, Student o2) {
		int num = o1.getName().compareTo(o2.getName());
		if(num == 0) {
			return o1.getAge()-o2.getAge();
		}
		//Sort by name
		return num;
	}
	
}

Application two

package com.Map;

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

/**
 * 
 * map Generics in a collection use wrapper classes or application data types
 * 
 * Application two:

	Count the number of characters in the string
	adsfdsfsfaadfsfdsfas
	Sort by times
 *	
 *Function: statistical sorting
 *
 *		Statistics:
 *			1,Convert string to character array
 *			2,Take the characters in this string as the key in the map set, and the number of occurrences as the value value
 *			3,When the character first appears, change the null returned to 1
 *			4,If the character does not appear for the first time, the value corresponding to the character will be + 1
 *
 *
 * @author 86182
 *
 */
public class HashMapDemo {

	public static void main(String[] args) {
		String str = "basdfsfasaasadsfadsfa";
		
		str = count(str);
		System.out.println(str);
	}

	/**
	 * Character occurrence statistics
	 * count-frequency
	 * 
	 * @param str
	 * @return
	 */
	private static String count(String str) {
		char[] charArray = str.toCharArray();
		Map<Character, Integer> map = new HashMap<>();
		for (char c : charArray) {
			Integer value = map.get(c);
			if(value == null) {
				map.put(c, 1);
			}else {
				map.put(c, value+1);
			}
		}
		StringBuffer sb = new StringBuffer();
		for(Map.Entry<Character, Integer> entry:map.entrySet()) {
			sb.append(entry.getKey()+"("+entry.getValue()+")");
		}
		return sb.toString();
	}

}