Java常见数据类型用法

HashMap

package org.example;
import java.util.HashMap;
import java.util.Map;


public class Main {
    public static void main(String[] args) {
     Map<Integer, String> m = new HashMap<>();
     m.put(1, "Mike");
     m.put(2, "John");
     for (Map.Entry<Integer, String> item : m.entrySet()) {
      System.out.println(item.getValue());
     }
    }
}

ArrayList

package org.example;
import java.util.ArrayList;

public class Main {
    public static void main(String[] args) {
     ArrayList<String> ls = new ArrayList<>();
     ls.add("Mike");
     ls.add("John");
     ls.add("Chen");
     System.out.println(ls.isEmpty());
     System.out.println(ls.size());

     for (String ele : ls){
	      System.out.println(ele);
	  }
    }
}

LinkedList

package org.example;
import java.util.Arrays;
import java.util.LinkedList;


public class Main {
    public static void main(String[] args) {
     LinkedList<String> languages = new LinkedList<>(Arrays.asList("Java","go","python"));
     languages.add("rust");
     languages.addFirst("C#");
     for(String ele : languages){
      System.out.println(ele);
     }
     System.out.println(languages.size());
     System.out.println(languages.isEmpty());
    }

}