60.《Java集合框架-List-Set-Map》

此篇所写 不知道你们是否在网页开发的时候 当看到要写Map集合什么HashMap之类的突然蒙了 虽然之前学过 突然让你调用方法 懵了 所以在此总结一下 以备后需

对比数组 可存储任意类型对象 且存储长度是可以变的 集合类乃内存层面对数据进行存储 数据库是持久层 断电后仍长期存在

都来自这个包：java.util.*

List:

package List;

import pojo.Student;

import java.util.ArrayList;

public class ArrayListTest {
    public static void main(String[] args) {
    /*
     *  有序不唯一
     *  ArrayList适合元素的查找
     * */
        ArrayList<Object> one = new ArrayList<>();
        one.add("高远");
        one.add("哈哈");
        one.add("文雅");
        one.add("文雅");
        Student student = new Student(1,"草地");
        one.add(student);
        System.out.println(one);
        System.out.println(one.get(0));
    }
}

package List;

import pojo.Student;

import java.util.Arrays;
import java.util.LinkedList;

public class LinkedListTest {
    public static void main(String[] args) {
        /*
         * 有序不唯一
         * LinkedList适合大量的增删操作
         * foreach循环不可以对元素进行修改
         */
        LinkedList<Object> one = new LinkedList<>();
        Student student = new Student(1, "高远");
        one.add(student);
        one.add("student");
        one.add("哈哈");
        one.add("哈哈");
        System.out.println(one);
        for (Object o : one) {
            System.out.println(o);
        }
    }
}

Set:

package Set;

import pojo.Student;

import java.util.HashSet;

public class HashSetTest {
    public static void main(String[] args) {
        /*
         *  无序唯一
         *  HashSet add()->调用hashCode()方法->根据元素的哈希值计算元素存储位置->判断是否有元素/是否相等
         *  若使用Student对象 必须重写hashCode()和equals()方法
         */

        HashSet<Object> one = new HashSet<>();
        one.add("ok");
        one.add("ok");
        one.add("高远");
        System.out.println(one);

        HashSet<Object> two = new HashSet<>();
        Student student1 = new Student(1, "高远");
        Student student2 = new Student(2, "哈哈");
        Student student3 = new Student(2, "高远");
        two.add(student1);
        two.add(student2);
        two.add(student3);
        System.out.println(two);
    }
}

package Set;

import java.util.LinkedHashSet;

public class LinkedHashSetTest {
    public static void main(String[] args) {
        /*
         * 唯一 LinkedHashSet让存取顺序一致(有序)
         */
        LinkedHashSet<Object> one = new LinkedHashSet<>();
        one.add("高远");
        one.add("哈哈");
        one.add("ok");
        one.add("高远");
        System.out.println(one);
    }
}

package Set;

import java.util.TreeSet;

public class TreeSetTest {
    public static void main(String[] args) {
        /*
         * 无序 唯一
         * TreeSet利用二叉树存储元素 来去掉重复元素
         */
        TreeSet<Object> one = new TreeSet<>();
        one.add("高远");
        one.add("哈哈");
        one.add("高远");
        System.out.println(one);
    }
}

Map:

package Map;

import pojo.Student;

import java.util.HashMap;
import java.util.Iterator;
import java.util.Set;

public class HashMapTest {
    public static void main(String[] args) {
        /*
         * 键唯一 值可以重复
         * 若键相同 值覆盖 HashMap 元素无序
         */
        HashMap<String, Object> one = new HashMap<>();
        one.put("1", new Student(1, "高远"));
        one.put("2", new Student(2, "哈哈"));
        one.put("3", new Student(3, "白台"));
        one.put("4", new Student(1, "高远"));
        one.put("1", new Student(4, "高远2"));

        Set<String> keySet = one.keySet();
        Iterator<String> it = keySet.iterator();
        while (it.hasNext()) {
            String key = it.next();
            Object value = one.get(key);
            System.out.println(key + " " + value);
        }
    }
}

package Map;

import java.util.HashMap;
import java.util.LinkedHashMap;
import java.util.Map;

public class LinkedHashMapTest {
    public static void main(String[] args) {
        /*
         * 键唯一 值可以重复
         * 若键相同 值覆盖
         * LinkedHashMap有序(存入顺序)
         */
        Map one = new HashMap<>();
        one.put("2", "高远");
        one.put("4", "哈哈");
        one.put("3", "高远");
        System.out.println(one);
        Map two = new LinkedHashMap();
        two.put("2", "高远");
        two.put("4", "哈哈");
        two.put("3", "高远");
        two.put("2", "高远2");
        System.out.println(two);
    }
}

package Map;


import java.util.Map;
import java.util.TreeMap;

public class TreeMapTest {
    public static void main(String[] args) {
        /*
         * 键唯一 值可以重复
         * 若键相同 值覆盖
         * TreeMap 键有序
         */
        Map one = new TreeMap<>();
        one.put(2, "高远");
        one.put(5, "哈哈");
        one.put(3, "ok");
        one.put(2, "高远2");
        System.out.println(one);
    }
}

还有一个比较常用的Arrays工具类：
参考 https://www.cnblogs.com/soberw/p/15876454.html#asList_28