Java教程

Java遍历List、Map和Set

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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