Java SE 048 Map in-depth detailed explanation and two implementation methods of traversing Map

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Java SE 048 Map in-depth detailed explanation and two implementation methods of traversing Map

1.Map (mapping)

Map a key to its value. A Map cannot contain duplicate keys. Each key can be mapped to one value, but not to multiple values.

As long as there is a map, we must think of a pair.
put(K key,V value);Associate the specified value to the associated key

## 2. Why
are the types returned by values () and keySet() different? Because there can be duplicate values ​​in the value set, but there can be no duplicate values ​​in the key set.
Set conforming key set cannot be repeated, and Collection conforming value set allows duplicate values.

package com.javase.map;

import java.util.HashMap;

public class MapTest2 {
    
    
	@SuppressWarnings("unchecked")
	public static void main(String[] args) {
    
    
		HashMap map = new HashMap();
		
		String str = "zhangsan";
		map.put("a", str);
		map.put("b", str);
		
		System.out.println(map);
	}
}

The keySet() method of Map will return a collection of keys, because the keys of Map cannot be repeated, so the return type of the keySet() method is Set; the value of Map can be repeated, so the return type of the values() method is Collection , Can accommodate repeated elements.

3. Traverse the map collection

package com.javase.map;

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

public class MapTest3 {
    
    
	@SuppressWarnings("unchecked")
	public static void main(String[] args) {
    
    
		HashMap map = new HashMap();
		map.put("a", "aa");
		map.put("b", "bb");
		map.put("c", "cc");
		map.put("d", "dd");
		map.put("e", "ee");
		map.put("f", "ff");
		
		Set set = map.keySet();
		for(Iterator iter = set.iterator(); iter.hasNext();){
    
    
			String key = (String)iter.next();
			String value =(String)map.get(key);
			System.out.println(key+"="+value);
		}
	}
}

4. Pass in parameters through the command line, and count the number of times the same parameter appears in the command line parameter

package com.javase.map;

import java.util.HashMap;
import java.util.Iterator;

public class MapTest4 {
    
    
	@SuppressWarnings("unchecked")
	public static void main(String[] args) {
    
    
		HashMap map = new HashMap();
		
		for(int i = 0; i < args.length; i++){
    
    
			//如果不存在这个元素就返回null，则此时可将这个值放进map进行记录,不为null表明以前它已经存在过了
			//存在则可以将其取出来，然后再放回去即可
			if(map.get(args[i])==null){
    
    
				map.put(args[i], new Integer(1));
			}else{
    
    
				Integer in= (Integer)map.get(args[i]);
				in = new Integer(in.intValue()+1);
				map.put(args[i], in);
			}
		}
		
		for(Iterator iter = map.keySet().iterator();iter.hasNext();){
    
    
			String key = (String)iter.next();
			Integer value = (Integer)map.get(key);
			System.out.println(key+"出现"+value+"次");
		}
	}
}