Advanced JAVA - Traversal methods and common APIs for Map series collections

Table of contents

1.0 Description of Map collection

1.1 Common methods of Map collection

1.2 Characteristics of Map series collections

 2.0 Traversal methods for Map series collections (three methods)

 2.1 Use keySet() method to traverse

 2.2 Use the entrySet() method to traverse

 2.3 Traversing using the forEach() method (Java 8+)

1.0 Description of Map collection

Map is a collection type used to storekey-value pairs( key-value pairs). Each key is unique, while values can be repeated. Map can quickly find the corresponding value based on the key. In a Map keys and values can be objects of any type. Simply put, the Map class is an interface and a container that stores elements of key-value pair type.

1.1 Common methods of Map collection

Introduce in code Map Common methods of collection:

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

public class MapAPI {
    public static void main(String[] args) {
        //由于 Map 是一个接口，不能直接 new 一个 Map 类型的对象，
        //所以需要用到 Map 的实现类 HashMap 来创建对象。

        Map<String,Integer> map = new HashMap<>();//一行经典代码

        //1. put(k,v) :实例方法，添加元素
        map.put("二哈",250);
        map.put("金毛",750);
        map.put("拉布拉多",1250);
        map.put("中华田园犬",1359);
        System.out.println(map);
    //输出结果为:{二哈=250, 金毛=750, 中华田园犬=1359, 拉布拉多=1250}

        //2. size() :实例方法，获取元素大小
        int s = map.size();
        System.out.println(s);
        //输出结果为:4

        //3. isEmpty() :实例方法，判断集合是否为空，为空返回true，反之
        boolean b = map.isEmpty();
        System.out.println(b);
        //输出结果为:false

        //4. get(k) :实例方法，根据键获取对应的值
        int i = map.get("二哈");
        System.out.println(i);
        //输出结果为:250

        //5. remove(k) :实例方法，根据键删除整个元素
        map.remove("二哈");
        System.out.println(map);
        //输出结果为:{金毛=750, 中华田园犬=1359, 拉布拉多=1250}

        //6. containsKey(k) :实例方法，判断是否包含某个键
        boolean bk = map.containsKey("二哈");
        System.out.println(bk);
        //输出结果为:false

        //7. containsValue(k) :实例方法，判断是否包含某个值
        boolean bv = map.containsValue(750);
        System.out.println(bv);
        //输出结果为:true

        //8. keySet() :实例方法，获取全部键的集合，
        // 需要用 Set 系列集合来接受。
        Set<String> stringSet = map.keySet();
        System.out.println(stringSet);
        //输出结果为:[金毛, 中华田园犬, 拉布拉多]

        //9. values() :实例方法，获取全部值
        //需要用 Collection 系列集合来接受
        Collection<Integer> collection = map.values();
        System.out.println(collection);
        //输出结果为:[750, 1359, 1250]

        //10. putAll(Map m) :将 m 中的内容全部倒入到调用该方法的集合中。
        Map<String,Integer> map1 = new HashMap<>();
        map1.put("小黑",19999);
        map1.put("小白",29999);
        map.putAll(map1);
        System.out.println(map);
  //输出结果为:{小白=29999, 金毛=750, 小黑=19999, 中华田园犬=1359, 拉布拉多=1250}

        //11. clear() :实例方法，删除整个集合中的元素
        map.clear();
        System.out.println(map);
        //输出结果为：{}

    }
}

The running results are as follows:

1.2 Characteristics of Map series collections

Unordered: means that it will not be output according to the added elements, but will be output randomly, as follows:

Non-repeatable: The "key" specified is not allowed to appear repeatedly. Repeated elements added later will overwrite the elements with the same "key", but the "value" is not allowed. Make a request and it can appear repeatedly, such as the following:

No index:Because the Map series collection does not support a fixed order, the existence of an index is naturally meaningless.

2.0 Traversal methods for Map series collections (three methods)

2.1 Use keySet() method to traverse

Get all through the keySet() method of Map keys, then use for-each to loop through the keys and pass get() Method to obtain the corresponding value.

This is achieved through specific code:

public class MapIteration {
    public static void main(String[] args) {

        Map<String,Integer> map = new HashMap<>();
        map.put("二哈",250);
        map.put("金毛",1250);
        map.put("拉布拉多",2250);
        map.put("中华田园犬",3250);

        //先取出该集合的全部”键“
        Set<String> stringSet = map.keySet();
        //然后用增强 for 方法根据”键“通过 get(k) 来查询”值“
        for (String k:stringSet) {
            int v = map.get(k);
            System.out.println(k+"=="+v);
        }
    }
}

operation result:

Step: First remove all the "keys" of the collection, and then useto enhance the for methodAccording to the "key" pass get(k) method to query "value".

2.2 Use the entrySet() method to traverse

Get all through Map ’s entrySet() method Entry object of key-value pairs, and then use for-each loop TraverseEntry objects,. To put it simply, consider a key-value pair in the collection as a whole, and then use the enhancedfor loop to traverse the collection, and then passfor The a> methods get the key and value. getValue() and getKey()

This is achieved through specific code:

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

public class MapIteration {

    public static void main(String[] args) {
        Map<String,Integer> map = new HashMap<>();
        map.put("二哈",250);
        map.put("金毛",1250);
        map.put("拉布拉多",2250);
        map.put("中华田园犬",3250);

        //通过调用 map 的实例方法 entrySet()，键值对包装成一个整体，
        //Map.Entry<String,Integer>这代码可以理解为是一个类型，用 Set 系列集合来存储
        Set<Map.Entry<String,Integer>> entrySet = map.entrySet();
        for (Map.Entry<String,Integer> entry:entrySet) {
            String k = entry.getKey();
            int v = entry.getValue();
            System.out.println(k+"=="+v);
        }
    }
}

The running result is:

2.3 Traversing using the forEach() method (Java 8+)

Pass Map ’s forEach() method, pass Enter a BiConsumer function interface to traverse key-value pairs. BiConsumer interface’s accept() method receives two parameters, namely keys and values.

It should be noted that here forEach() theMap instance method.

This is achieved through specific code:

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

public class MapIteration {
    public static void main(String[] args) {
        Map<String, Integer> map = new HashMap<>();
        map.put("二哈", 250);
        map.put("金毛", 1250);
        map.put("拉布拉多", 2250);
        map.put("中华田园犬", 3250);
/*        map.forEach(new BiConsumer<String, Integer>() {
            @Override
            public void accept(String k, Integer v) {
                System.out.println(k+"=="+v);
            }
        });*/
        
        //进一步简化为：
        map.forEach((k, v) -> System.out.println(k + "==" + v));
    }
}

The running results are as follows:

Additionally, in fact, this method is essentially a packaging of the second method. The essence is to use the entrySet() method to traverse. The source code is as follows:

It can be seen that the third method is very easy to implement the traversal of the Map series collection, and it only requires one line of code. Easy to remember:

map.forEach((k, v) -> System.out.println(k + "==" + v));

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