ArrayList() 输出

import java.util.ArrayList;
import java.util.HashMap;
import java.util.Iterator;
import java.util.List;
import java.util.Map;
import java.util.Set;


public class zengzhiwei {
    public static void main(String agrs[]) {
        System.out.println("hello world!");
        Map<String, String> m = new HashMap<String, String>();
        m.put("zengzhiwei", "zzw");
        m.put("zengzhijie", "zzj");
    
        Set<String> s = m.keySet();
        Iterator<String> itr = s.iterator();
        while(itr.hasNext()) {
            Object o = itr.next();
            System.out.println(m.get(o));
        }
        
        List<Integer> l = new ArrayList<Integer>();
        l.add(1);
        l.add(2);
        l.add(3);
        System.out.println(l.toString());
        for (int i = 0; i < l.size(); i++) {
            System.out.println(l.get(i));
            
        }
        
    }

}

hello world!
zzw
zzj
[1, 2, 3]
1
2
3