集合类的学习

import java.util.ArrayList;
import java.util.Collection;
import java.util.Iterator;
//collection接口
public class coll {
    public static void main(String[] args) {
        Collection<String> list = new ArrayList<>();
        list.add("a");
        list.add("b");
        list.add("c");
        Iterator<String> it = list.iterator();
        while (it.hasNext()){
            String str = (String)it.next();
            System.out.println(str);
        }
    }
}

import java.util.ArrayList;
import java.util.List;

public class list {
    //    list集合
    public static void main(String[] args) {
        List<String> list = new ArrayList<>();
        list.add("a");
        list.add("b");
        list.add("c");
        int i = (int)(Math.random()*list.size());
        System.out.println("随机获取list中的元素"+list.get(i));
        list.remove(i);
        for (int j = 0; j < list.size(); j++) {
            System.out.println(list.get(j));
        }
    }
}

import java.util.HashSet;
import java.util.Iterator;

public class set {
    public static void main(String[] args) {
        HashSet<String> set = new HashSet<String>();
        set.add("a");
        set.add("a");
        set.add("b");
        Iterator<String> it = set.iterator();
        while (it.hasNext()){
            System.out.println(it.next());
        }
    }
}

import java.util.HashMap;

public class map {
    public static void main(String[] args) {
        HashMap<Integer,String> maps = new HashMap<Integer, String>();
        maps.put(1,"a");
        maps.put(2,"aa");
        maps.put(3,"aaa");
        System.out.println(maps.get(1));
    }
}