JAVA学习--集合Map的使用

* Map接口
 *         |-----HashMap:Map的主要实现类
 *         |-----LinkedHashMap:使用链表维护添加进Map中的顺序。故遍历Map时，是按添加的顺序遍历的。
 *         |-----TreeMap:按照添加进Map中的元素的key的指定属性进行排序。要求：key必须是同一个类的对象！
 *                 针对key：自然排序   vs 定制排序
 *         |-----Hashtable:古老的实现类，线程安全，不建议使用。
 *             |----Properties:常用来处理属性文件。键和值都为String类型的



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     Object put(Object key,Object value):向Map中添加一个元素
      Object remove(Object key):按照指定的key删除此key-value
      void putAll(Map t)
      void clear():清空
      Object get(Object key)：获取指定key的value值。若无此key，则返回null
      boolean containsKey(Object  key)
      boolean containsValue(Object value)
      int size():返回集合的长度
      boolean isEmpty() boolean equals(Object obj)
      HashMap： 1.key是用Set来存放的，不可重复。value是用Collection来存放的，可重复
     一个key-value对，是一个Entry。所有的Entry是用Set存放的，也是不可重复的。
     2.向HashMap中添加元素时，会调用key所在类的equals()方法，判断两个key是否相同。若相同 则只能添加 进后添加的那个元素。

 

@Test
    public void test1() {
        Map map = new HashMap();
        map.put("AA", 213);
        map.put("BB", 456);
        map.put("BB", 45);
        map.put(123, "CC");
        map.put(null, null);
        map.put(new Person("DD", 23), 89);
        map.put(new Person("DD", 23), 87);
        System.out.println(map.size());
        System.out.println(map);
        map.remove("BB");
        System.out.println(map);
        Object value = map.get(1234);
        System.out.println(value);
    }

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    * 如何遍历Map Set keySet() Collection values() Set entrySet()

 

@Test
    public void test2() {
        Map map = new HashMap();
        map.put("AA", 213);
        map.put("BB", 45);
        map.put(123, "CC");
        map.put(null, null);
        map.put(new Person("DD", 23), 89);

        // 1.遍历key集。
        Set set = map.keySet();
        for (Object obj : set) {
            System.out.println(obj);
        }
        // 2.遍历value集
        Collection values = map.values();
        Iterator i = values.iterator();
        while (i.hasNext()) {
            System.out.println(i.next());
        }
        // 3.如何遍历key-value对。
        // 方式一：
        Set set1 = map.keySet();
        for (Object obj : set1) {
            System.out.println(obj + "----->" + map.get(obj));
        }
        // 方式二：
        Set set2 = map.entrySet();
        for (Object obj : set2) {
            Map.Entry entry = (Map.Entry) obj;
            // System.out.println(entry.getKey() + "---->" + entry.getValue());
            System.out.println(entry);
        }
    }

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**LinkedHashMap用法

@Test
    public void test3() {
        Map map = new LinkedHashMap();
        map.put("AA", 213);
        map.put("BB", 45);
        map.put(123, "CC");
        map.put(null, null);
        map.put(new Person("DD", 23), 89);

        Set set1 = map.keySet();
        for (Object obj : set1) {
            System.out.println(obj + "----->" + map.get(obj));
        }
    }

// 自然排序
    @Test
    public void test4() {
        Map map = new TreeMap();
        map.put(new Person("AA", 23), 89);
        map.put(new Person("MM", 22), 79);
        map.put(new Person("GG", 23), 99);
        map.put(new Person("JJ", 13), 69);

        Set set1 = map.keySet();
        for (Object obj : set1) {
            System.out.println(obj + "----->" + map.get(obj));
        }
    }

 // 定制排序
    @Test
    public void test5() {
        Comparator com = new Comparator() {
            public int compare(Object o1, Object o2) {
                if (o1 instanceof Customer && o2 instanceof Customer) {
                    Customer c1 = (Customer) o1;
                    Customer c2 = (Customer) o2;
                    int i = c1.getId().compareTo(c2.getId());
                    if (i == 0) {
                        return c1.getName().compareTo(c2.getName());
                    }
                    return i;
                }
                return 0;
            }
        };
        TreeMap map = new TreeMap(com);
        map.put(new Customer("AA", 1001), 87);
        map.put(new Customer("CC", 1001), 67);
        map.put(new Customer("MM", 1004), 77);
        map.put(new Customer("GG", 1002), 97);
       
        Set set1 = map.keySet();
        for (Object obj : set1) {
            System.out.println(obj + "----->" + map.get(obj));
        }
    }

-----------------------------------------------------------------------------------以下为填充集合的类

class Customer {
 
   private String name;
    private Integer id;
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public Integer getId() {
        return id;
    }
    public void setId(Integer id) {
        this.id = id;
    }
    public Customer(String name, Integer id) {
        super();
        this.name = name;
        this.id = id;
    }
    public Customer() {
        super();
    }
    @Override
    public String toString() {
        return "Customer [name=" + name + ", id=" + id + "]";
    }
    @Override
    public int hashCode() {
        final int prime = 31;
        int result = 1;
        result = prime * result + ((id == null) ? 0 : id.hashCode());
        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;
        Customer other = (Customer) obj;
        if (id == null) {
            if (other.id != null)
                return false;
        } else if (!id.equals(other.id))
            return false;
        if (name == null) {
            if (other.name != null)
                return false;
        } else if (!name.equals(other.name))
            return false;
        return true;
    }
   
}

class Person implements Comparable{
    private String name;
    private Integer age;
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public Integer getAge() {
        return age;
    }
    public void setAge(Integer age) {
        this.age = age;
    }
    public Person() {
        super();
    }
    public Person(String name, Integer age) {
        super();
        this.name = name;
        this.age = age;
    }
    @Override
    public String toString() {
        return "Person [name=" + name + ", age=" + age + "]";
    }
    //static int init = 1000;
    @Override
    public int hashCode() {//return age.hashCode() + name.hashCode();没下述的健壮性好。
        final int prime = 31;
        int result = 1;
        result = prime * result + ((age == null) ? 0 : age.hashCode());
        result = prime * result + ((name == null) ? 0 : name.hashCode());
        return result;
        //return init++;//不能这样用
    }
    @Override
    public boolean equals(Object obj) {
        if (this == obj)
            return true;
        if (obj == null)
            return false;
        if (getClass() != obj.getClass())
            return false;
        Person other = (Person) obj;
        if (age == null) {
            if (other.age != null)
                return false;
        } else if (!age.equals(other.age))
            return false;
        if (name == null) {
            if (other.name != null)
                return false;
        } else if (!name.equals(other.name))
            return false;
        return true;
    }
    //当向TreeSet中添加Person类的对象时，依据此方法，确定按照哪个属性排列。
    @Override
    public int compareTo(Object o) {
        if(o instanceof Person){
            Person p = (Person)o;
            //return this.name.compareTo(p.name);
            //return -this.age.compareTo(p.age);
            int i = this.age.compareTo(p.age);
            if(i == 0){
                return this.name.compareTo(p.name);
            }else{
                return i;
            }
        }
        return 0;
    }
   
}