HashSet和HashMap的区别比较

HashSet 实现的Set接口，集合中不允许出现重复的值（如果重复会覆盖）:

package com.wlf.base;

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

private String name;
private int 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;
}

public String toString()
{
return "{" + name + ", " + age + "}";
}

}

public static void main(String[] args)
{
Collection<Person> set = new HashSet<Person>();
Person object = new Person("张三",22);
set.add(new Person("李四", 19));
set.add(new Person("王五", 22));
set.add(object);
object.setAge(34);
set.add(object);
print(set);
}

private static void print(Collection<Person> set)
{
Iterator<Person> it = set.iterator();
while (it.hasNext())
{
Person p = it.next();
System.out.println(p.toString());
}
}

输出结果是：

{张三, 34}

{李四, 19}

{王五, 22}

HashSet内部使用HashMap实现， 通过

if (e.hash == hash && ((k = e.key) == key || key.equals(k)))来判断加入的元素是否相同。这里的hash是对象的hashCode，默认是通过地址计算的hashCode, 这里加入过的object相同，第二次加入的会覆盖前一次加入的

为了避免出现下面这种输出：

public static void main(String[] args)
{
Collection<Person> set = new HashSet<Person>();
Person object = new Person("张三",22);
set.add(new Person("李四", 19));
set.add(new Person("王五", 22));
set.add(new Person("张三",22));
set.add(object);
print(set);
}

private static void print(Collection<Person> set)
{
Iterator<Person> it = set.iterator();
while (it.hasNext())
{
Person p = it.next();
System.out.println(p.toString());
}
}

输出

{王五, 22}

{张三, 22}

{张三, 22}

{李四, 19}

输出了两个张三，这两个加入的对象不一样，本意是不想出现这种情况，所以在加入HashSet前需要重写HashCode和equals方法：

Person类增加重写方法：

public int hashCode()
{
return name.hashCode() + age;
}

public boolean equals(Object obj)
{
if (!(obj instanceof Person))
throw new ClassCastException("类型不匹配");
Person p = (Person)obj;
return this.name.equals(p.getName()) && this.age == p.getAge();
}

HashMap 实现了Map接口,Map中不允许重复的 键值（如果重复会覆盖），HashMap中允许key和value为null

//测试HashMap
HashMap<Integer,Person> tmpMap = new HashMap<Integer,Person>();
Integer intObj = new Integer(3);
tmpMap.put(intObj, new Person("李四",14));
tmpMap.put(intObj, new Person("李四",16));
tmpMap.put(null, null);
printMap(tmpMap);

hashMap的三种输出方法:

private static void printMap(HashMap<Integer,Person> tmpHash)
{
//Iterator<Entry<Integer,Person>> result = tmpHash.entrySet().iterator();
//1.通过Entry来遍历HashMap
for(Entry<Integer,Person> result:tmpHash.entrySet())
{
System.out.println("key="+result.getKey()+"value="+result.getValue());
}

//2.通过KeySet来获取
for(Integer key:tmpHash.keySet())
{
System.out.println("key="+key+"value="+tmpHash.get(key));
}
//3.通过entrySet的iterator来获取
Iterator<Entry<Integer, Person>> it = tmpHash.entrySet().iterator();
while (it.hasNext()) {
Entry<Integer, Person> entry = it.next();
System.out.println("key= " + entry.getKey() + " and value= " + entry.getValue());
}
}

输出结果：

key=nullvalue=null

key=3value={李四, 16}

HashMap是非线程安全的，但是Collection提供了方法转成线程安全：

Map<Integer, Person> tmpMap = Collections.synchronizedMap(new HashMap<Integer,Person>());