集合

集合的框架体系

Collection

Collection 接口实现类的特点

public interface Collection extends Iterable

collection 实现子类可以存放多个元素，每个元素可以是 Object
有些 Collection 的实现类，可以存放重复的元素，有些不可以
Collection 的实现类，有些是有序的(List)，有些不是有序的(Set)
Collection 接口没有直接实现子类，是通过它的子接口 Set 和 List 来实现的

常用方法

package com.bbedu.collection_;

import java.util.ArrayList;
import java.util.List;
@SuppressWarnings({"all"})
public class CollectionMethod {
    public static void main(String[] args) {

        List list = new ArrayList();

        // add
        list.add("Tim");
        list.add(10);   // 相当于list.add(new Integer(10))
        list.add(true);
        System.out.println("list=" + list);

        // remove
        list.remove(0);     // 删除第一个元素
        list.remove((Integer) 10);
        System.out.println("list=" + list);

        // contains
        System.out.println(list.contains(true));

        // size
        System.out.println(list.size());

        // isEmpty
        System.out.println(list.isEmpty());

        // clear
        list.clear();
        System.out.println("list=" + list);

        // addAll
        ArrayList list2 = new ArrayList();
        list2.add("红楼梦");
        list2.add("三国演义");
        list.addAll(list2);
        System.out.println("list=" + list);

        // containsAll
        System.out.println(list.containsAll(list2));

        // removeAll
        list.add("Macbeth");
        list.removeAll(list2);
        System.out.println("list=" + list);
    }
}

迭代器遍历

Iterable 类中，可以返回一个 Iterator 对象，迭代器，可以用来遍历

Iterator对象称为迭代器，主要用于遍历Collection集合中的元素。
所有实现了Collection接口的集合类都有一个iterator()方法，用以返回一个实现了lterator接口的对象,即可以返回一个迭代器。
lterator的结构：
Iterator仅用于遍历集合,Iterator本身并不存放对象。

增强 for 循环

增强 for 循环，可以替代 iterator 迭代器，特点：增强 for 就是简化版的 iterator，本质相同。但只能用于遍历集合和数组

基本语法：

for(元素类型元素名 : 集合名或数组名){

访问元素;

}

练习

package com.bbedu.collection_;

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

public class CollectionExercise {
    public static void main(String[] args) {
        List<Dog> dogs = new ArrayList<>();
        Dog dog1 = new Dog("小黑", 3);
        Dog dog2 = new Dog("大黄", 5);
        Dog dog3 = new Dog("旺财", 6);

        dogs.add(dog1);
        dogs.add(dog2);
        dogs.add(dog3);

        Iterator<Dog> iterator = dogs.iterator();

        System.out.println("=====迭代器=====");

        while (iterator.hasNext()) {
            Dog next =  iterator.next();
            System.out.println(next);
        }

        System.out.println("=====增强for循环=====");

        for (Dog dog : dogs) {
            System.out.println(dog);
        }
    }
}

class Dog {
    private String name;
    private int age;

    public Dog(String name, int age) {
        this.name = name;
        this.age = age;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public int getAge() {
        return age;
    }

    public void setAge(int age) {
        this.age = age;
    }

    @Override
    public String toString() {
        return "Dog{" +
                "name='" + name + '\'' +
                ", age=" + age +
                '}';
    }
}

List 接口

基本介绍

List 接口是 Collection 接口的子接口

List集合类中元素有序(即添加顺序和取出顺序一致)、且可重复
List集合中的每个元素都有其对应的顺序索引，即支持索引
List容器中的元素都对应一个整数型的序号记载其在容器中的位置,可以根据序号存取容器中的元素

常用方法

package com.bbedu.list_;

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

public class ListMethod {
    public static void main(String[] args) {

        // add
        ArrayList list = new ArrayList();
        list.add("计算机组成原理");
        list.add("操作系统");
        list.add(1, "数据结构");    // 插入指定位置
        System.out.println("list=" + list);

        // addAll
        ArrayList list2= new ArrayList();
        list2.add("Tim");
        list2.add("Mike");
        list.addAll(1, list2);
        System.out.println("list=" + list);

        // indexOf lastIndexOf
        System.out.println(list.indexOf("Tim"));
        list.add("Tim");
        System.out.println(list.lastIndexOf("Tim"));

        // remove
        list.remove(0);
        System.out.println("list=" + list);

        // set 相当于替换
        list.set(0, "Jim");
        System.out.println("list=" + list);

        // subList  范围：[fromIndex, toIndex)
        List retList = list.subList(0, 2);
        System.out.println("returnList=" + retList);
    }
}

List 的三种遍历

iterator
增强 for
普通 for

练习

package com.bbedu.list_;

import java.util.ArrayList;
import java.util.LinkedList;
import java.util.List;
import java.util.Vector;

public class ListExercise02 {
    public static void main(String[] args) {
        List list = new ArrayList();
//        List list = new LinkedList();
//        List list = new Vector();

        list.add(new Book("红楼梦", 100, "曹雪芹"));
        list.add(new Book("西游记", 10, "吴承恩"));
        list.add(new Book("水浒传", 19, "施耐庵"));
        list.add(new Book("三国演义", 20, "罗贯中"));
        list.add(new Book("西游记", 10, "吴承恩"));

        for (Object o : list) {
            System.out.println(o);
        }

        sort(list);
        System.out.println("=====排序后=====");
        for (Object o : list) {
            System.out.println(o);
        }
    }

    public static void sort(List list){
        for (int i = 0; i < list.size() - 1; i++) {
            for (int j = 0; j < list.size() - i - 1; j++) {
                Book book1 = (Book) list.get(j);
                Book book2 = (Book) list.get(j + 1);
                if(book1.getPrice() > book2.getPrice()){
                    list.set(j, book2);
                    list.set(j+1, book1);
                }
            }
        }
    }
}

class Book {
    private String name;
    private int price;
    private String author;

    public Book(String name, int price, String author) {
        this.name = name;
        this.price = price;
        this.author = author;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public int getPrice() {
        return price;
    }

    public void setPrice(int price) {
        this.price = price;
    }

    public String getAuthor() {
        return author;
    }

    public void setAuthor(String author) {
        this.author = author;
    }

    @Override
    public String toString() {
        return "名称：" + name + "\t 价格：" + price + "\t 作者：" + author;
    }
}

ArrayList

注意事项

permits all elements, including null
ArrayList 是由数组来实现数据存储的
ArrayList 基本等同于 Vector，除了 ArrayList 是线程不安全，在多线程情况下，不建议使用 ArrayList

底层结构和源码分析

先结论，再源码

ArrayList中维护了一个Object类型的数组elementData. [debug 看源码]
transient Object[] elementData; //transient表示瞬间,短暂的,表示该属性不会被序列化
当创建ArrayList对象时，如果使用的是无参构造器，则初始elementData容量为0，第1次添加，则扩容elementData为10，如需要再次扩容，则扩容elementData为1.5倍。
如果使用的是指定大小的构造器，则初始elementData容量为指定大小,如果需要扩容，则直接扩容elementData为1.5倍。

package com.bbedu.list_;

import java.util.ArrayList;
import java.util.Vector;

public class ArrayListSource {
    public static void main(String[] args) {
        ArrayList list = new ArrayList(8);
        for (int i = 1; i <= 10; i++) {
            list.add(i);
        }
        for (int i = 11; i <= 15; i++) {
            list.add(i);
        }
        list.add(100);
        list.add(200);
        list.add(300);
    }
}

初始为10：

若先指定 ArrayList 大小，底层类似，不再赘述

Vector

注意事项

Vector 类的定义说明
Vector 底层也是一个对象数组，protected Object[] elementData;
Vector 是线程同步的，即线程安全，Vector 类的操作方法带有 synchronized
在开发中，需要线程同步安全时，考虑使用 Vector

Vector 和 ArrayList 比较

扩容源码分析

package com.bbedu.list_;

import java.util.Vector;

@SuppressWarnings({"all"})
public class Vector_ {
    public static void main(String[] args) {
        Vector vector = new Vector();
        for (int i = 0; i < 10; i++) {
           vector.add(i);
        }
        vector.add(100);
    }
}

LinkedList

LinkedList 底层实现了双向链表和双向队列特点
可以添加任意元素（元素可以重复），包括 null
线程不安全，没有实现同步

LinkedList 的底层操作机制

LinkedList 底层维护了一个双向链表
LinkedList 中维护了两个属性 first 和 last 分别指向首节点和尾节点
每个节点（Node对象）里又维护了 prev、next、item 三个属性，其中通过 prev 指向前一个，通过 next 指向后一个节点，最终实现双向链表
LinkedList 的元素的添加和删除，不是通过数组完成的，因此效率相对较高

package com.bbedu.list_;

import java.util.Iterator;
import java.util.LinkedList;
@SuppressWarnings({"all"})
public class LinkedListCRUD {
    public static void main(String[] args) {

        LinkedList linkedList = new LinkedList();
        linkedList.add(1);
        linkedList.add(2);
        linkedList.add(3);

        System.out.println("linkedList=" + linkedList);

        linkedList.remove();    // 默认删除第一个节点

        System.out.println("linkedList=" + linkedList);

        linkedList.set(0, 999);
        System.out.println("linkedList=" + linkedList);

        linkedList.get(1);
        System.out.println(linkedList.get(1));

        Iterator iterator = linkedList.iterator();
        while (iterator.hasNext()) {
            Object next =  iterator.next();
            System.out.println("next=" + next);
        }

        for (Object o : linkedList) {
            System.out.println("o=" + o);
        }

    }
}

jdk底层研究方法类似，请自行研究。

ArrayList 和 LinkedList 比较

如何选择ArrayList和LinkedList:

如果我们改查的操作多，选择ArrayList
如果我们增删的操作多，选择LinkedList
一般来说，在程序中，80%-90%都是查询，因此大部分情况下会选择ArrayList
在一个项目中，根据业务灵活选择，也可能这样，一个模块使用的是ArrayList, 另外一个模块是 LinkedList.

Set 接口

基本介绍

无序（添加和取出的顺序不一致），没有索引
不允许重复元素，所以最多包含一个null
JDK API 中 Set 接口的实现类有：

常用方法

和 List 接口一样，Set 接口也是 Collection 的子接口，因此常用方法和 Collection 接口一样

遍历方法

同 Collection 的遍历方式一样，因为 Set 接口是 Collection 接口的子接口：

可以使用迭代器
增强 for 循环
不能使用索引的方式来获取

package com.bbedu.set_;

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

@SuppressWarnings({"all"})
public class SetMethod {
    public static void main(String[] args) {
        Set set = new HashSet();

        set.add("john");
        set.add("lucy");
        set.add("john");
        set.add("job");
        set.add(null);

        System.out.println("set=" + set);

        System.out.println("====迭代器====");
        Iterator iterator = set.iterator();
        while (iterator.hasNext()) {
            Object next =  iterator.next();
            System.out.println("next=" + next);
        }

        set.remove(null);

        System.out.println("====增强for====");
        for (Object o : set) {
            System.out.println("o=" + o);
        }
    }
}

HashSet

HashSet 实现了 Set 接口
HashSet 底层实际上是是 HashMap
不能有重复的元素，可以存放 null，但是只能有一个 null
HashSet 不保证元素是有序的，取决于 Hash 后，再确定索引的结果

package com.bbedu.set_;

import java.util.HashSet;

public class HashSet01 {
    public static void main(String[] args) {
        HashSet hashSet = new HashSet();
        System.out.println(hashSet.add("John"));
        System.out.println(hashSet.add("Lucy"));
        System.out.println(hashSet.add("John"));
        System.out.println(hashSet.add("Jack"));
        System.out.println(hashSet.add("Tim"));

        // 指定删除对象
        hashSet.remove("John");
        System.out.println("set=" + hashSet);

        hashSet = new HashSet();
        System.out.println("set=" + hashSet);

        hashSet.add("lucy");
        hashSet.add("lucy");
        hashSet.add(new Dog("coco"));   // OK
        hashSet.add(new Dog("coco"));   // OK
        System.out.println("set=" + hashSet);

        // 看源码理解
        // 不能添加重复元素的真正含义？
        hashSet.add(new String("bbchen"));  // ok
        hashSet.add(new String("bbchen"));  // no
        System.out.println("set=" + hashSet);
    }
}

class Dog {
    private String name;

    public Dog(String name) {
        this.name = name;
    }

    @Override
    public String toString() {
        return "Dog{" +
                "name='" + name + '\'' +
                '}';
    }
}

HashSet 底层机制说明

分析 HashSet 底层是 HashMap, HashMap 底层是（数组 + 链表 + 红黑树）

数组+链表=邻接表，理解邻接表：

package com.bbedu.set_;

/**
 * 模拟邻接表
 */
public class HashSetStructure {
    public static void main(String[] args) {
        // Node[] 数组 称为 表
        Node[] table = new Node[16];

        Node john = new Node("john", null);
        table[2] = john;

        Node jack = new Node("jack", null);
        john.next = jack;

        Node rose = new Node("Rose", null);
        jack.next = rose;
    }
}

class Node {
    Object item;
    Node next;

    public Node(Object item, Node next) {
        this.item = item;
        this.next = next;
    }
}

分析 HashSet 的添加元素底层是如何实现的

同样，先结论，后Debug

源码分析：

初始化HashSet()

/**
 * Constructs a new, empty set; the backing {@code HashMap} instance has
 * default initial capacity (16) and load factor (0.75).
 */
public HashSet() {
    map = new HashMap<>();
}

add()

/**
 * Adds the specified element to this set if it is not already present.
 * More formally, adds the specified element {@code e} to this set if
 * this set contains no element {@code e2} such that
 * {@code Objects.equals(e, e2)}.
 * If this set already contains the element, the call leaves the set
 * unchanged and returns {@code false}.
 *
 * @param e element to be added to this set
 * @return {@code true} if this set did not already contain the specified
 * element
 */
public boolean add(E e) {
    return map.put(e, PRESENT)==null;
}

put()，先执行 hash(key)，得到对应的 hash 值，并不直接是 hashCode()

/**
 * Associates the specified value with the specified key in this map.
 * If the map previously contained a mapping for the key, the old
 * value is replaced.
 *
 * @param key key with which the specified value is to be associated
 * @param value value to be associated with the specified key
 * @return the previous value associated with {@code key}, or
 *         {@code null} if there was no mapping for {@code key}.
 *         (A {@code null} return can also indicate that the map
 *         previously associated {@code null} with {@code key}.)
 */
public V put(K key, V value) {
    return putVal(hash(key), key, value, false, true);
}

hash()

/**
 * Computes key.hashCode() and spreads (XORs) higher bits of hash
 * to lower.  Because the table uses power-of-two masking, sets of
 * hashes that vary only in bits above the current mask will
 * always collide. (Among known examples are sets of Float keys
 * holding consecutive whole numbers in small tables.)  So we
 * apply a transform that spreads the impact of higher bits
 * downward. There is a tradeoff between speed, utility, and
 * quality of bit-spreading. Because many common sets of hashes
 * are already reasonably distributed (so don't benefit from
 * spreading), and because we use trees to handle large sets of
 * collisions in bins, we just XOR some shifted bits in the
 * cheapest possible way to reduce systematic lossage, as well as
 * to incorporate impact of the highest bits that would otherwise
 * never be used in index calculations because of table bounds.
 */
static final int hash(Object key) {
    int h;
    return (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16);
}

*putVal，核心代码！！！

/**
 * Implements Map.put and related methods.
 *
 * @param hash hash for key
 * @param key the key
 * @param value the value to put
 * @param onlyIfAbsent if true, don't change existing value
 * @param evict if false, the table is in creation mode.
 * @return previous value, or null if none
 */
final V putVal(int hash, K key, V value, boolean onlyIfAbsent,
               boolean evict) {
    Node<K,V>[] tab; Node<K,V> p; int n, i;
    if ((tab = table) == null || (n = tab.length) == 0)
        n = (tab = resize()).length;
    if ((p = tab[i = (n - 1) & hash]) == null)
        tab[i] = newNode(hash, key, value, null);
    else {
        Node<K,V> e; K k;
        if (p.hash == hash &&
            ((k = p.key) == key || (key != null && key.equals(k))))
            e = p;
        else if (p instanceof TreeNode)
            e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
        else {
            for (int binCount = 0; ; ++binCount) {
                if ((e = p.next) == null) {
                    p.next = newNode(hash, key, value, null);
                    if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st
                        treeifyBin(tab, hash);
                    break;
                }
                if (e.hash == hash &&
                    ((k = e.key) == key || (key != null && key.equals(k))))
                    break;
                p = e;
            }
        }
        if (e != null) { // existing mapping for key
            V oldValue = e.value;
            if (!onlyIfAbsent || oldValue == null)
                e.value = value;
            afterNodeAccess(e);
            return oldValue;
        }
    }
    ++modCount;
    if (++size > threshold)
        resize();
    afterNodeInsertion(evict);
    return null;
}

练习

package com.bbedu.set_;

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

@SuppressWarnings({"all"})
public class HashSetExercise {
    public static void main(String[] args) {
        HashSet hashSet = new HashSet();
        hashSet.add(new Employee("Jack", 20));
        hashSet.add(new Employee("Mary", 18));
        hashSet.add(new Employee("Jack", 20));

        Iterator iterator = hashSet.iterator();
        while (iterator.hasNext()) {
            Object next =  iterator.next();
            System.out.println("next=" + next);
        }
    }
}
@SuppressWarnings({"all"})
class Employee {
    private String name;
    private int age;

    public Employee(String name, int age) {
        this.name = name;
        this.age = age;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public int getAge() {
        return age;
    }

    public void setAge(int age) {
        this.age = age;
    }

    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (o == null || getClass() != o.getClass()) return false;
        Employee employee = (Employee) o;
        return age == employee.age && Objects.equals(name, employee.name);
    }

    @Override
    public int hashCode() {
        return Objects.hash(name, age);
    }

    @Override
    public String toString() {
        return "Employee{" +
                "name='" + name + '\'' +
                ", age=" + age +
                '}';
    }
}

package com.bbedu.set_;

import java.util.Date;
import java.util.HashSet;
import java.util.Iterator;
import java.util.Objects;

public class HashSetExercise02 {
    public static void main(String[] args) {
        HashSet hashSet = new HashSet();
        hashSet.add(new Employee02("Tim", 20000, new MyDate(1990, 1, 1)));
        hashSet.add(new Employee02("Rock", 15000, new MyDate(1998, 4, 29)));
        hashSet.add(new Employee02("Tim", 23333, new MyDate(1990, 1, 1)));

        Iterator iterator = hashSet.iterator();
        while (iterator.hasNext()) {
            Object next =  iterator.next();
            System.out.println("next=" + next);
        }
    }
}

class Employee02{
    private String name;
    private double sal;
    private MyDate birthday;

    public Employee02(String name, double sal, MyDate birthday) {
        this.name = name;
        this.sal = sal;
        this.birthday = birthday;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public double getSal() {
        return sal;
    }

    public void setSal(double sal) {
        this.sal = sal;
    }

    public MyDate getBirthday() {
        return birthday;
    }

    public void setBirthday(MyDate birthday) {
        this.birthday = birthday;
    }

    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (o == null || getClass() != o.getClass()) return false;
        Employee02 that = (Employee02) o;
        return Objects.equals(name, that.name) && Objects.equals(birthday, that.birthday);
    }

    @Override
    public int hashCode() {
        return Objects.hash(name, birthday);
    }

    @Override
    public String toString() {
        return "Employee02{" +
                "name='" + name + '\'' +
                ", sal=" + sal +
                ", birthday=" + birthday +
                '}';
    }
}

class MyDate {
    private int year;
    private int month;
    private int day;

    public MyDate(int year, int month, int day) {
        this.year = year;
        this.month = month;
        this.day = day;
    }

    public int getYear() {
        return year;
    }

    public void setYear(int year) {
        this.year = year;
    }

    public int getMonth() {
        return month;
    }

    public void setMonth(int month) {
        this.month = month;
    }

    public int getDay() {
        return day;
    }

    public void setDay(int day) {
        this.day = day;
    }

    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (o == null || getClass() != o.getClass()) return false;
        MyDate myDate = (MyDate) o;
        return year == myDate.year && month == myDate.month && day == myDate.day;
    }

    @Override
    public int hashCode() {
        return Objects.hash(year, month, day);
    }

    @Override
    public String toString() {
        return "MyDate{" +
                "year=" + year +
                ", month=" + month +
                ", day=" + day +
                '}';
    }
}

LinkedHashSet

LinkedHashSet 是 HashSet 的子类
LinkedHashSet 底层是一个 LinkedHashMap，底层维护了一个数组 + 双向链表
LiniedHashSet 根据元素的 hashCode 值来决定元素的存储位置，同时使用链表维护元素的次序，这使得元素看起来是以插入顺序保存的
LinkedHashSet 不允许添加重复元素

Map 接口

Map 与 Collection 并列存在，用于保存具有映射关系的数据：Key-Value (双列元素)
Map 中的 key 和 value 可以是任何引用类型的数据，会封装到 HashMap$Node 对象中
Map 中的 key不允许重复，当有相同的 key 时，相当于替换
Map 中的 value 可以重复
key 和 value 都可以为 null，但 key 只能有一个 null，而 value 可以有多个 null
常用 String 类来做 key，别的引用类型同样可以
key 和 value 存在单向一对一关系(满射关系，即一种映射)

常用方法

六种遍历方法：

package com.bbedu.map_;

import java.util.*;

public class MapFor {
    public static void main(String[] args) {
        HashMap hashMap = new HashMap();

        hashMap.put("刘慈欣", "三体");
        hashMap.put("张爱玲", "倾城之恋");
        hashMap.put("萧红", "呼兰河传");
        hashMap.put("托尔斯泰", "复活");
        hashMap.put("史铁生", "我与地坛");

        // 第一组
        Set set = hashMap.keySet();

        System.out.println("===增强for===");
        for (Object o : set) {
            System.out.println(o + "-" + hashMap.get(o));
        }

        System.out.println("===迭代器===");
        Iterator iterator = set.iterator();
        while (iterator.hasNext()) {
            Object next =  iterator.next();
            System.out.println(next + "-" + hashMap.get(next));
        }

        // 第二组
        Collection values = hashMap.values();

        System.out.println("===取value===");
        for (Object value : values) {
            System.out.println(value);
        }

        System.out.println("===迭代器===");
        Iterator iterator1 = values.iterator();
        while (iterator1.hasNext()) {
            Object next =  iterator1.next();
            System.out.println(next);
        }

        // 第三组
        Set set1 = hashMap.entrySet();
        System.out.println("===增强for===");
        for (Object o : set1) {
            Map.Entry o1 = (Map.Entry) o;
            System.out.println(o1.getKey() + "-" + o1.getValue());
        }

        System.out.println("===迭代器===");
        Iterator iterator2 = set1.iterator();
        while (iterator2.hasNext()) {
            Object next =  iterator2.next();
            Map.Entry next1 = (Map.Entry) next;
            System.out.println(next1.getKey() + "-" + next1.getValue());
        }
    }
}

练习

package com.bbedu.map_;

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

public class MapExercise {
    public static void main(String[] args) {
        HashMap<Integer,Employee> map = new HashMap();
        Employee tom = new Employee("Tom", 20000, 1);
        Employee bob = new Employee("Bob", 15000, 2);
        Employee sam = new Employee("Sam", 19000, 3);


        map.put(tom.getId(), tom);
        map.put(bob.getId(), bob);
        map.put(sam.getId(), sam);

        System.out.println("===方式1===");
        Set set = map.keySet();
        Iterator iterator = set.iterator();
        while (iterator.hasNext()) {
            Object next =  iterator.next();
            Employee employee = (Employee) map.get(next);
            if(employee.getSal() > 18000){
                System.out.println(employee);
            }
        }

        System.out.println("===方式2===");
        Set set1 = map.entrySet();
        for (Object o : set1) {
            Map.Entry o1 = (Map.Entry) o;
            Employee value = (Employee) o1.getValue();
            if(value.getSal() > 18000){
                System.out.println(value);
            }
        }
    }
}

class Employee {
    private String name;
    private double sal;
    private int id;

    public Employee(String name, double sal, int id) {
        this.name = name;
        this.sal = sal;
        this.id = id;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public double getSal() {
        return sal;
    }

    public void setSal(double sal) {
        this.sal = sal;
    }

    public int getId() {
        return id;
    }

    public void setId(int id) {
        this.id = id;
    }

    @Override
    public String toString() {
        return "Employee{" +
                "name='" + name + '\'' +
                ", sal=" + sal +
                ", id=" + id +
                '}';
    }
}

HashMap

底层机制及源码分析

先结论：

Hashtable

存放的元素是键值对，即 K-V
Hashtable 的键和值都不能为null，否则会抛出 NullPointerException
Hashtable 使用的方法基本上和 HashMap一样
Hashtable 是线程安全的，HashMap 是线程不安全的

扩容机制

Hashtable 和 HashMap 对比

Properties

Properties 类继承自 Hashtable 类并且实现了 Map 接口，也是使用一种键值对的形式来保存数据
它的特点和 Hashtable 类似
Properties 还可以用于从 xxx.properties 文件中加载数据到Properties 类对象，并进行读取和修改

开发中如何选择集合实现类

TreeSet

TreeMap

TreeMap 是 TreeSet 的底层

Collections 工具类

常用方法

package com.bbedu.collections_;

import java.util.ArrayList;
import java.util.Collections;
import java.util.Comparator;


public class Collections_ {
    public static void main(String[] args) {
        // ArrayList 集合，用于测试
        ArrayList list = new ArrayList();
        list.add("tom");
        list.add("jack");
        list.add("sam");
        list.add("mike");
        list.add("smith");
        System.out.println(list);
        System.out.println("===reverse===");
        Collections.reverse(list);
        System.out.println(list);

        System.out.println("===shuffle===");
        for (int i = 0; i < 5; i++) {
            Collections.shuffle(list);
            System.out.println(list);
        }

        System.out.println("===sort====");
        Collections.sort(list);
        System.out.println(list);

        System.out.println("===自定义sort===");
        Collections.sort(list, new Comparator<>() {
            @Override
            public int compare(Object o1, Object o2) {
                if(o1 instanceof String && o2 instanceof String){
                    return ((String) o1).length() - ((String) o2).length();
                }
                return 0;
            }
        });
        System.out.println(list);

        System.out.println("===swap===");
        Collections.swap(list,0, 1);
        System.out.println(list);

        System.out.println("===max===");
        System.out.println(Collections.max(list));

        System.out.println("===自定义max===");
        Object max = Collections.max(list, new Comparator() {
            @Override
            public int compare(Object o1, Object o2) {
                return ((String) o1).length() - ((String) o2).length();
            }
        });
        System.out.println(max);

        System.out.println("===frequency===");
        System.out.println(Collections.frequency(list, "tom"));

        System.out.println("===copy===");
        ArrayList dest = new ArrayList();
        // 需要先给 dest 赋值，大小和 list相同
        for (int i = 0; i < list.size(); i++) {
            dest.add("");
        }
        Collections.copy(dest, list);
        System.out.println(dest);

        System.out.println("===replaceAll===");
        Collections.replaceAll(list, "tom", "汤姆");
        System.out.println(list);
    }
}

本章作业

package com.bbedu.homework;

import java.util.ArrayList;

@SuppressWarnings({"all"})
public class Homework01 {
    public static void main(String[] args) {
        ArrayList arrayList = new ArrayList();
        arrayList.add(new News("广东中山发现8例无症状感染者，均在隔离管控的密接人员中发现"));
        arrayList.add(new News("经济衰退阴云笼罩 惠誉下调美国2023年GDP增速预期"));

        for (int i = arrayList.size() - 1; i >= 0; i--) {
            News news = (News) arrayList.get(i);
            String s = processTitle(news.getTitle());
            System.out.println(s);
        }
    }

    public static String processTitle(String title) {
        if (title == null) {
            return "";
        }
        if (title.length() > 15){
            return title.substring(0, 15) + "...";
        }else{
            return title;
        }
    }
}

class News {
    private String title;
    private String content;

    public News(String title) {
        this.title = title;
    }

    public String getTitle() {
        return title;
    }

    public void setTitle(String title) {
        this.title = title;
    }

    public String getContent() {
        return content;
    }

    public void setContent(String content) {
        this.content = content;
    }

    @Override
    public String toString() {
        return "News{" +
                "title='" + title + '\'' +
                '}';
    }
}

package com.bbedu.homework;

import java.util.ArrayList;

public class Homework02 {
    public static void main(String[] args) {
        Car car1 = new Car("宝马", 300000);
        Car car2 = new Car("奔驰", 400000);
        Car car3 = new Car("宾利", 500000);

        ArrayList<Car> cars = new ArrayList<>();
        cars.add(car1);
        cars.add(car2);
        cars.add(car3);
        System.out.println(cars);
        System.out.println("===增强for===");
        for (Car car : cars) {
            System.out.println(car);
        }

        cars.remove(car3);
        System.out.println(cars);

        System.out.println(cars.contains(car3));

        System.out.println(cars.isEmpty());

        cars.clear();
        System.out.println(cars);

        cars.add(car1);
        cars.add(car2);
        cars.add(car3);
        ArrayList<Car> cars1 = new ArrayList<>();

        cars1.addAll(cars);
        System.out.println(cars1);

        System.out.println(cars1.containsAll(cars));

        cars1.removeAll(cars);
        System.out.println(cars1);
    }

}

class Car {
    private String name;
    private double price;

    public Car(String name, double price) {
        this.name = name;
        this.price = price;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public double getPrice() {
        return price;
    }

    public void setPrice(double price) {
        this.price = price;
    }

    @Override
    public String toString() {
        return "Car{" +
                "name='" + name + '\'' +
                ", price=" + price +
                '}';
    }
}

package com.bbedu.homework;

import java.util.Collection;
import java.util.HashMap;
import java.util.Set;

public class Homework03 {
    public static void main(String[] args) {
        HashMap hashMap = new HashMap();
        hashMap.put("jack", 650);
        hashMap.put("tom", 1200);
        hashMap.put("smith", 2900);
        printAll(hashMap);

        System.out.println("===修改jack工资===");
        hashMap.replace("jack", 2600);
        printAll(hashMap);

        System.out.println("===所有人工资+100===");
        Set set = hashMap.keySet();
        for (Object o : set) {
            hashMap.put(o, (Integer) hashMap.get(o) + 100);
        }
        printAll(hashMap);
    }

    public static void printAll(HashMap hashMap){
        Set set = hashMap.keySet();
        for (Object o : set) {
            System.out.println(o + "-" + hashMap.get(o));
        }
    }
    public static void printAllSal(HashMap hashMap){
        Set set = hashMap.keySet();
        for (Object o : set) {
            System.out.println(hashMap.get(o));
        }
    }

}

package com.bbedu.homework;

import java.util.TreeSet;

public class Homework05 {
    public static void main(String[] args) {
        TreeSet treeSet = new TreeSet();
        treeSet.add(new Person());
        treeSet.add(new Person());
        treeSet.add(new Person());
        treeSet.add(new Person());
        treeSet.add(new Person());
        treeSet.add(new Person());
        System.out.println(treeSet);
    }
}

class Person implements Comparable{
    @Override
    public int compareTo(Object o) {
        return 0;
    }
}

输出

[Person2{id=1002, name='BB'}, Person2{id=1001, name='CC'}]
[Person2{id=1002, name='BB'}, Person2{id=1001, name='CC'}, Person2{id=1001, name='CC'}]
[Person2{id=1002, name='BB'}, Person2{id=1001, name='CC'}, Person2{id=1001, name='CC'}, Person2{id=1001, name='AA'}]

Chapter14_集合

集合

集合的框架体系

Collection

Collection 接口实现类的特点

常用方法

迭代器遍历

增强 for 循环

练习

List 接口

基本介绍

常用方法

List 的三种遍历

练习

ArrayList

注意事项

底层结构和源码分析

Vector

注意事项

Vector 和 ArrayList 比较

扩容源码分析

LinkedList

LinkedList 的底层操作机制

ArrayList 和 LinkedList 比较

Set 接口

基本介绍

常用方法

遍历方法

HashSet

HashSet 底层机制说明

分析 HashSet 的添加元素底层是如何实现的

练习

LinkedHashSet

Map 接口

常用方法

练习

HashMap

底层机制及源码分析

Hashtable

扩容机制

Hashtable 和 HashMap 对比

Properties

开发中如何选择集合实现类

TreeSet

TreeMap

Collections 工具类

常用方法

本章作业

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