Java基础之双列集合Map

Day18

 

18.01_集合框架(Map集合概述和特点)

A:Map接口概述           

查看API可以知道：

将键映射到值的对象

一个映射不能包含重复的键

每个键最多只能映射到一个值

B:Map接口和Collection接口的不同

Map是双列的,Collection是单列的

Map的键唯一,Collection的子体系Set是唯一的

Map集合的数据结构值针对键有效，跟值无关;Collection集合的数据结构是针对元素有效

C:hashSet底层是双列的,隐藏掉了值就是一个Object,依靠于hashMap,隐藏掉一个是可以的,但是凭空创造就比较难了,hashMap和

hashSet底层其实都是hash算法,搞了一套算方法.

18.02_集合框架(Map集合的功能概述)

重点:熟练.

A:Map集合的功能概述

a:添加功能

V      put(K key,V value):添加元素。

如果键是第一次存储，就直接存储元素，返回null

如果键不是第一次存在，就用值把以前的值替换掉，返回以前的值

b:删除功能

void    clear():移除所有的键值对元素

V      remove(Object key)：根据键删除键值对元素，并把值返回,键和值都取消了

c:判断功能

boolean   containsKey(Object key)：判断集合是否包含指定的键

boolean   containsValue(Object value):判断集合是否包含指定的值

boolean   isEmpty()：判断集合是否为空

d:获取功能

Set<Map.Entry<K,V>>   entrySet():拿到所有的键值集合

V                    get(Object key):根据键获取值

Set<K>               keySet():获取集合中所有键的集合

Collection<V>          values():获取集合中所有值的集合

e:长度功能

int size()：返回集合中的键值对的个数

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HashMap<String, Integer> map = new HashMap<String, Integer>();

map.put("张三", 33);

map.put("李四", 44);

map.put("王五", 55);

boolean a = map.containsKey("张三");

boolean b = map.containsValue(44);

boolean c = map.isEmpty();

System.out.println(a);

System.out.println(b);

System.out.println(c);

int x = map.size();

System.out.println(x);

int y = map.remove("张三");

System.out.println(y);

System.out.println(map);

map.clear();

System.out.println(map.size());

 

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18.03 Map集合的遍历之键找值

重点:掌握第一种遍历方式.

A:键找值思路：

获取所有键的集合

遍历键的集合，获取到每一个键

根据键找值

B:案例演示

Map集合的遍历之键找值

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HashMap<String, Integer> map = new HashMap<String, Integer>();

map.put("张三", 33);

map.put("李四", 44);

map.put("王五", 55);

int a = map.get("张三");

System.out.println(a);

for(String key : map.keySet()) {

Integer value = map.get(key);

System.out.println(key + "..." + value);

}

Set<String> keys = map.keySet(); //与单列集合不同,应该是先获得key的集合

Iterator<String> it = keys.iterator();

while (it.hasNext()) {

String key = it.next(); //两次用键,指针会后移所以就是固定起来

Integer value = map.get(key);

System.out.println(key + "..." + value);

}

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18.04 Map集合的遍历之键值对对象找键和值

重点:第二种遍历方式.

A:键值对对象找键和值思路：

获取所有键值对对象的集合

遍历键值对对象的集合，获取到每一个键值对对象

根据键值对对象找键和值

B:案例演示

Map集合的遍历之键值对对象找键和值

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HashMap<String, Integer> hm = new HashMap<>();

hm.put("张三", 23);

hm.put("李四", 24);

hm.put("王五", 25);

hm.put("赵六", 26);

/*Set<Map.Entry<String, Integer>> entrySet = hm.entrySet(); //获取所有的键值对象的集合

Iterator<Entry<String, Integer>> it = entrySet.iterator(); //获取迭代器

while(it.hasNext()) {

Entry<String, Integer> en = it.next(); //获取键值对对象

String key = en.getKey(); //根据键值对对象获取键

Integer value = en.getValue(); //根据键值对对象获取值

System.out.println(key + "=" + value);

}*/

for(Entry<String,Integer> en : hm.entrySet()) {

System.out.println(en.getKey() + "=" + en.getValue());

}

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C:源码分析,就是说entry是map下边的内部类.  导包的话就是将外部类进行了打开里边可以直接的进行了创建对象.

interface Inter {

interface Inter2 {

public void show();

}

}

 

class Demo implements Inter.Inter2 {

 

@Override

public void show() {

}

}

 

 

18.05 HashMap集合键是Student值是String的案例

重点:引用类型.

A:案例演示

HashMap集合键是Student值是String的案例(因为键是hash算法,必须重写自定义对象的hashCode和equts方法,不然的话

他们就只不同的对象,也就是说不能进行覆盖的),这个才是运用最多的,但是一般情况下键却是基本数据类型.

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public static void main(String[] args) {

HashMap<Student, String> hm = new HashMap<>();

hm.put(new Student("张三", 23), "北京");

hm.put(new Student("张三", 23), "上海");

hm.put(new Student("李四", 24), "广州");

hm.put(new Student("王五", 25), "深圳");

System.out.println(hm); //打印出了每一个对象和值,因为其父类重写了toString方法.

}

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public class Student {

private String name;

private int age;

public Student() {

super();

}

public Student(String name, int age) {

super();

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 "Student [name=" + name + ", age=" + age + "]";

}

@Override

public int hashCode() {

final int prime = 31;

int result = 1;

result = prime * result + age;

result = prime * result + ((name == null) ? 0 : name.hashCode());

return result;

}

@Override

public boolean equals(Object obj) {

if (this == obj)

return true;

if (obj == null)

return false;

if (getClass() != obj.getClass())

return false;

Student other = (Student) obj;

if (age != other.age)

return false;

if (name == null) {

if (other.name != null)

return false;

} else if (!name.equals(other.name))

return false;

return true;

}

}

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18.06_集合框架(LinkedHashMap的概述和使用)

A:案例演示

LinkedHashMap的特点

底层是链表实现的可以保证怎么存就怎么取

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LinkedHashMap<String, Integer> lhm = new LinkedHashMap<>();

lhm.put("张三", 23);

lhm.put("李四", 24);

lhm.put("赵六", 26);

lhm.put("王五", 25);

System.out.println(lhm);

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18.07 TreeMap集合键是Student值是String的案例

重点:为了保证键值是唯一的,当键值是自定义对象的时候应该进行重写hashcode和equals方法.与hashMap是一样的

A:案例演示

TreeMap集合键是Student值是String的案例,除了保证唯一性他还得重写自定义类的comparable比较器接口,因为

人家是排序的集合,你不传比较算法人家就不让你存.

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public static void main(String[] args) {

TreeMap<Student, String> map = new TreeMap<Student, String>(

new Comparator<Student>() {

 

@Override

public int compare(Student o1, Student o2) {

int num = o1.getName().compareTo(o2.getName());   //按照姓名进行比较

return num == 0 ? o1.getAge() - o2.getAge() : num;

}

});

 

map.put(new Student("张三", 33), "北京");

map.put(new Student("张三", 33), "北京");

map.put(new Student("李四", 44), "上海");

map.put(new Student("王五", 55), "西安");

System.out.println(map);

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public class Student implements Comparable<Student> {

private String name;

private int age;

public Student() {

super();

}

public Student(String name, int age) {

super();

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 "Student [name=" + name + ", age=" + age + "]";

}

@Override

public int hashCode() {

final int prime = 31;

int result = 1;

result = prime * result + age;

result = prime * result + ((name == null) ? 0 : name.hashCode());

return result;

}

@Override

public boolean equals(Object obj) {

if (this == obj)

return true;

if (obj == null)

return false;

if (getClass() != obj.getClass())

return false;

Student other = (Student) obj;

if (age != other.age)

return false;

if (name == null) {

if (other.name != null)

return false;

} else if (!name.equals(other.name))

return false;

return true;

}

@Override

public int compareTo(Student o) {

int num = this.age - o.age; //以年龄为主要条件

return num == 0 ? this.name.compareTo(o.name) : num;

}

}

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案例--18.08_集合框架(统计字符串中每个字符出现的次数)

A:案例演示

需求：统计字符串中每个字符出现的次数

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HashMap<Character, Integer> map = new HashMap<Character, Integer>();

String s = "aaaaaaaaaaaabbbbbbbbbbbbbcccccccccccc";

char[] chs = s.toCharArray();

for (char c : chs) {

/*if (!map.containsKey(c)) {

map.put(c, 1);

} else {

map.put(c, map.get(c) + 1);

}*/

map.put(c, map.containsKey(c) ? map.get(c) + 1 : 1);

}

//System.out.println(map);

// 打印到的方式.

for (Entry<Character, Integer> c : map.entrySet()) {

System.out.println(c.getKey() + "..." + c.getValue());

}

for (char  c : map.keySet()) {

System.out.println(c + "..." + map.get(c));

}

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重点:18.09_集合框架(集合嵌套之HashMap嵌套HashMap)

重点:注意取费分层取到键值和相应的键值的entry.KeySet.

A:案例演示

集合嵌套之HashMap嵌套HashMap

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HashMap<Student,String> map = new HashMap<Student, String>();

map.put(new Student("张三",33), "北京");

map.put(new Student("张三",33), "北京");

map.put(new Student("李四",44), "上海");

map.put(new Student("王五",55), "西安");

HashMap<Student,String> map1 = new HashMap<Student, String>();

map1.put(new Student("张三1",331), "北京1");

map1.put(new Student("张三1",331), "北京1");

map1.put(new Student("李四1",441), "上海1");

map1.put(new Student("王五1",551), "西安1");

HashMap<HashMap<Student, String>, String> hm =

new HashMap<HashMap<Student,String>, String>();

hm.put(map, "88期学员");

hm.put(map1, "99期学员");

/*for (HashMap<Student, String> s : hm.keySet()) {

String value1 = hm.get(s);

for (Student str : s.keySet()) {

String value2 = s.get(str);

System.out.println(str + "..." + value2 + "..." + value1);

}

}*/

for (Entry<HashMap<Student, String>, String> en : hm.entrySet()) {

String value1 = en.getValue();

for (Entry<Student, String>  en1 : en.getKey().entrySet()) {

String value2 = en1.getValue();

System.out.println(en1.getKey() + "..." + value2 + "..." + value1);

}

}

/*for (Student s : map1.keySet()) {

System.out.println(s + "..." +map1.get(s));

}

for (Entry<Student,String> en : map1.entrySet()) {

System.out.println(en.getKey() + "..." + en.getValue());

}*/

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18.10_集合框架(HashMap和Hashtable的区别)经常面试！

A:面试题

HashMap和Hashtable的区别

Hashtable是JDK1.0版本出现的,是线程安全的,效率低,HashMap是JDK1.2版本出现的,是线程不安全的,效率高

Hashtable不可以存储null键和null值,HashMap可以存储null键和null值

hashTable和vertor的命运一样,都被替代了.

B:案例演示

HashMap和Hashtable的区别

 

18.11_集合框架(Collections工具类的概述和常见方法讲解)

A:Collections类概述

针对集合操作 的工具类

B:Collections成员方法

 

public static <T> void sort(List<T> list)

public static <T> int binarySearch(List<?> list,T key)

public static <T> T max(Collection<?> coll)

public static void reverse(List<?> list)

public static void shuffle(List<?> list)

 

 

18.12_集合框架(模拟斗地主洗牌和发牌)

A:案例演示

模拟斗地主洗牌和发牌，牌没有排序

 

//买一副扑克

String[] num = {"A","2","3","4","5","6","7","8","9","10","J","Q","K"};

String[] color = {"方片","梅花","红桃","黑桃"};

ArrayList<String> poker = new ArrayList<>();

for(String s1 : color) {

for(String s2 : num) {

poker.add(s1.concat(s2));

}

}

poker.add("小王");

poker.add("大王");

//洗牌

Collections.shuffle(poker);

//发牌

ArrayList<String> gaojin = new ArrayList<>();

ArrayList<String> longwu = new ArrayList<>();

ArrayList<String> me = new ArrayList<>();

ArrayList<String> dipai = new ArrayList<>();

for(int i = 0; i < poker.size(); i++) {

if(i >= poker.size() - 3) {

dipai.add(poker.get(i));

}else if(i % 3 == 0) {

gaojin.add(poker.get(i));

}else if(i % 3 == 1) {

longwu.add(poker.get(i));

}else {

me.add(poker.get(i));

}

}

//看牌

System.out.println(gaojin);

System.out.println(longwu);

System.out.println(me);

System.out.println(dipai);

 

18.13_集合框架(模拟斗地主洗牌和发牌并对牌进行排序的原理图解)

A:画图演示

画图说明排序原理

 

18.14_集合框架(模拟斗地主洗牌和发牌并对牌进行排序的代码实现)

A:案例演示

模拟斗地主洗牌和发牌并对牌进行排序的代码实现

 

//买一副牌

String[] num = {"3","4","5","6","7","8","9","10","J","Q","K","A","2"};

String[] color = {"方片","梅花","红桃","黑桃"};

HashMap<Integer, String> hm = new HashMap<>(); //存储索引和扑克牌

ArrayList<Integer> list = new ArrayList<>(); //存储索引

int index = 0; //索引的开始值

for(String s1 : num) {

for(String s2 : color) {

hm.put(index, s2.concat(s1)); //将索引和扑克牌添加到HashMap中

list.add(index); //将索引添加到ArrayList集合中

index++;

}

}

hm.put(index, "小王");

list.add(index);

index++;

hm.put(index, "大王");

list.add(index);

//洗牌

Collections.shuffle(list);

//发牌

TreeSet<Integer> gaojin = new TreeSet<>();

TreeSet<Integer> longwu = new TreeSet<>();

TreeSet<Integer> me = new TreeSet<>();

TreeSet<Integer> dipai = new TreeSet<>();

for(int i = 0; i < list.size(); i++) {

if(i >= list.size() - 3) {

dipai.add(list.get(i)); //将list集合中的索引添加到TreeSet集合中会自动排序

}else if(i % 3 == 0) {

gaojin.add(list.get(i));

}else if(i % 3 == 1) {

longwu.add(list.get(i));

}else {

me.add(list.get(i));

}

}

//看牌

lookPoker("高进", gaojin, hm);

lookPoker("龙五", longwu, hm);

lookPoker("冯佳", me, hm);

lookPoker("底牌", dipai, hm);

}

public static void lookPoker(String name,TreeSet<Integer> ts,HashMap<Integer, String> hm) {

System.out.print(name + "的牌是:");

for (Integer index : ts) {

System.out.print(hm.get(index) + " ");

}

System.out.println();

}

18.15_集合框架(泛型固定下边界)

? super E

 

18.16_day18总结

把今天的知识点总结一遍。