Map, Set, and List traversal in Java

1. Five traversal methods of Map

package com.grandage.test;

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

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

        Map<String, String> map = new HashMap<>();
        map.put("key1"."value1");
        map.put("key2"."value2");
        map.put("key3"."value3");

        //1. This parameter is widely used and can be used twice
        System.out.println("Traversal of keys and values by map. keySet");
        for (String key : map.keySet()) {
            System.out.println("key= " + key + " and value= " + map.get(key));
        }

        / / 2.
        System.out.println(Iterator iterator through key and value via map. entrySet:);
        Iterator<Map.Entry<String, String>> it = map.entrySet().iterator();
        while (it.hasNext()) {
            Map.Entry<String, String> entry = it.next();
            System.out.println("key= " + entry.getKey() + " and value= " + entry.getValue());
        }

        //3. Recommended, especially for large capacity
        System.out.println("Traversal key and value via map. entrySet");
        for (Map.Entry<String, String> entry : map.entrySet()) {
            System.out.println("key= " + entry.getKey() + " and value= " + entry.getValue());
        }

        Map.values() traverses all values but not keys
        for (String v : map.values()) {
            System.out.println("value= " + v);
        }
        Lambda traverses the map
        map.forEach((k,v)-> System.out.println(k+"-<key=====value->"+v)); }}Copy the code

2.Set traversal 4 methods

package com.utils;

import java.util.HashSet;
import java.util.Iterator;
import java.util.LinkedHashSet;
import java.util.Set;

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

        //Hashset does not guarantee the iteration order of the collection; In particular, it does not guarantee the constancy of the order
        Set<String> set = new HashSet<>();
        set.add("value1");
        set.add("value2");
        set.add("value3");

        //LinkedHashSet defines the iteration order in which elements are inserted into the collection (insertion order)
        Set<String> setOrder = new LinkedHashSet<>();
        setOrder.add("value1");
        setOrder.add("value2");
        setOrder.add("value3");
        //1. Iterate through
        Iterator<String> it = set.iterator();
        while (it.hasNext()) {
            String str = it.next();
            System.out.println(str);
        }

        //2
        for (String str : setOrder) {
            System.out.println(str);
        }

        //2-1. The enhanced for loop also has the advantage of being generic if :set holds Object
        Set<Object> setObject = new HashSet<>();
        setObject.add(111);
        setObject.add("222");
        for (Object obj : setObject) {
            if (obj instanceof Integer) {
                int resultValue = (Integer) obj;
                System.out.println("Integer:" + resultValue);
            } else if (obj instanceof String) {
                String resultValue = (String) obj;
                System.out.println("String:"+ resultValue); }}//3. Common traversal of collection classes, from very early versions, using iterators
        for (Iterator it2 = set.iterator(); it2.hasNext(); ) {
            System.out.println(it2.next());
        }
        / / 4. Jdk1.8 laterset.forEach(System.out::println); }}Copy the code

3. Five ways to traverse a List collection

package com.utils;

import java.util.ArrayList;
import java.util.Iterator;
import java.util.List;
import java.util.ListIterator;

public class ListTraversing {
    public static void main(String[] args) {
        // Ordered elements can be iterated repeatedly
        List<String> arrayList=new ArrayList<>();
        arrayList.add("value1");
        arrayList.add("value2");

        //1. Foreach traversal (without index recommendation)
        for(String str:arrayList){
            System.out.println(str);
        }

        //2.
        for (int i=0,n=arrayList.size(); i < n ; i++) {
            System.out.println(arrayList.get(i));
        }

        //3. Iterator traversal
        Iterator<String> it = arrayList.iterator();
        while(it.hasNext())
        {
            System.out.println(it.next());
        }

        / / 4. Jdk1.8 later
        arrayList.forEach(System.out::println);

        / / 5. Use a ListIterator
        ListIterator<String> listIt = arrayList.listIterator();
        while(listIt.hasNext()) { System.out.println(listIt.next()); }}}Copy the code

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