Java遍历List、Map和Set
2021/6/9 22:23:51
本文主要是介绍Java遍历List、Map和Set,对大家解决编程问题具有一定的参考价值,需要的程序猿们随着小编来一起学习吧!
遍历List
首先来创建一个实体类,以供下面使用。
public class News{ private int id; private String title; private String author; public News(int id, String title, String author) { super(); this.id = id; this.title = title; this.author = author; } public int getId() { return id; } public void setId(int id) { this.id = id; } public String getTitle() { return title; } public void setTitle(String title) { this.title = title; } public String getAuthor() { return author; } public void setAuthor(String author) { this.author = author; } }
第一种、最基础的遍历方式:for循环,指定下标长度,使用List集合的size()方法,进行for循环遍历
import java.util.ArrayList; public class Demo01 { public static void main(String[] args) { ArrayList<News> list = new ArrayList<News>(); list.add(new News(1,"list1","a")); list.add(new News(2,"list2","b")); list.add(new News(3,"list3","c")); list.add(new News(4,"list4","d")); for (int i = 0; i < list.size(); i++) { News s = (News)list.get(i); System.out.println(s.getId()+" "+s.getTitle()+" "+s.getAuthor()); } } }
第二种、较为简洁的遍历方式:使用foreach遍历List,但不能对某一个元素进行操作(这种方法在遍历数组和Map集合的时候同样适用)
import java.util.ArrayList; public class Demo02 { public static void main(String[] args) { ArrayList<News> list = new ArrayList<News>(); list.add(new News(1,"list1","a")); list.add(new News(2,"list2","b")); list.add(new News(3,"list3","c")); list.add(new News(4,"list4","d")); for (News s : list) { System.out.println(s.getId()+" "+s.getTitle()+" "+s.getAuthor()); } } }
第三种、适用迭代器Iterator遍历:直接根据List集合的自动遍历
import java.util.ArrayList; public class Demo03 { public static void main(String[] args) { ArrayList<News> list = new ArrayList<News>(); list.add(new News(1,"list1","a")); list.add(new News(2,"list2","b")); list.add(new News(3,"list3","c")); list.add(new News(4,"list4","d")); Iterator<News> iter = list.iterator(); while (iter.hasNext()) { News s = (News) iter.next(); System.out.println(s.getId()+" "+s.getTitle()+" "+s.getAuthor()); } } }
遍历Map
public static void main(String[] args) { Map<String, String> map = new HashMap<String, String>(); map.put("1", "value1"); map.put("2", "value2"); map.put("3", "value3"); //第一种:普遍使用,二次取值 System.out.println("通过Map.keySet遍历key和value:"); for (String key : map.keySet()) { System.out.println("key= "+ key + " and value= " + map.get(key)); } //第二种 System.out.println("通过Map.entrySet使用iterator遍历key和value:"); 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()); } //第三种:推荐,尤其是容量大时 System.out.println("通过Map.entrySet遍历key和value"); for (Map.Entry<String, String> entry : map.entrySet()) { System.out.println("key= " + entry.getKey() + " and value= " + entry.getValue()); } //第四种 System.out.println("通过Map.values()遍历所有的value,但不能遍历key"); for (String v : map.values()) { System.out.println("value= " + v); } }
遍历Set
对 set 的遍历 1.迭代遍历: Set<String> set = new HashSet<String>(); Iterator<String> it = set.iterator(); while (it.hasNext()) { String str = it.next(); System.out.println(str); } 2.for循环遍历: for (String str : set) { System.out.println(str); }
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