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>());