记录我遇到过的Java面试技术问题

浏览数：59 / 时间：2015年06月08日

1. Java Memory model

execution engineer: java stack/pc registers/native|direct memory area/java heap/method area

volatile: thread = its own stack + memory cache inside CPU, volatile can make sure read/write directly to main memory data, not local cpu cache

2. Java GC

serial gc collector/parallel Scavenge/ConcurrentMS/G1

java heap:

new generation/old generation

new: eden+from/to survivor

xmn=xmx, survivorratio, newratio

Copy/Mark-Sweep/Mark-Sweep-Compact

parallel new

throughput first:

prallel old

response time first:

CMS

3. Inner Class and Static Inner Class

inner class contains a reference to its outer class, so it can access its outers‘ class member

Static inner class can only access static filed in its outer class, not require for outer class‘s instance to access static inner class

4. Java constructor

父类静态初始化块
子类静态初始化块
父类初始化块
父类构造函数
子类初始化块
子类构造函数

5. Collection (Wha‘ts the difference between List and Set)

java的集合是一种特别有用的工具类，大体上分为：Set、List和Map三种体系。其中Set代表无序、不可重复的集合；List代表有序、重复的集合；而Map则代表具有映射关系的集合（键值对）。JDK 1.5以后，Java增加了Queue体系集合，代表一种队列集合实现。

6. JUnit (setup/teardown)如何写case，如何测试exception

@Test (expected = org.springframework.dao.InvalidDataAccessApiUsageException.class)

7. Mock

@Before

	public void setUp() {



		context = new Mockery();

		/* need to consider security testing in service layer */

		SecurityContextHolder.getContext().setAuthentication(

				new UsernamePasswordAuthenticationToken("Admin", "password"));



		riskRankDAO = context.mock(IRiskRankDAO.class);
context.checking(new Expectations() {

			{

				oneOf(riskRankDAO).findAll();

				will(returnValue(list));

				oneOf(riskRankDAO).save(riskRank);

				will(returnValue(riskRank));

			}

		});



		riskRankSaved = riskRankService.save(riskRank);



		context.assertIsSatisfied();

		assertSame("riskRankSaved: " + riskRankSaved.toString() + " riskRank: " + riskRank.toString(), riskRank, riskRankSaved);

8. TDD/Scrum

9. java xml 读写

DOM, DOM4j, JDOM, SAX

10. Java HashMap implementation & Map interface

hashmap internal data storage: array + link (String/enum is also stored in array), key and its hashcode is used to get value‘s storage index

internal stroage data structure: entry: final key, value, Entry<key, value> next, final hashcode

method: put/get/contains/putall/delete etc

put: use hashcode to calculate hash value and index it to [0,array.length-1],if array[i] is empty, point array[i] to new added entry, increase map size; if array[i] is not empty, check whether key exists in link, if yes, modify entry with new value, if not, set array[i] to new added entry and set new added entry‘s next to previous entry, increase hashmap‘s size, if array‘s capacity exceeds its‘ capacity factor (initialized length * factor),resize length = 2 * length. resize在多线程没保护的时候，不止会导致put/get失败，还要小心死锁死循环

remove: need to modify pervious entry‘s next if elements is not the first one in the link

11. Java hashmap and treemap的区别

hashmap is faster than treemap(red-black tree), treemap contains order

12. linux 常用命令

file related: ls, du/df, cat, vi

user related: chown,chmod,useradd,groupadd

network related: tcpdump, ifcfg,route,netstat

others: top, find,rpm,tar,grep, etc

13. JMS实现，Queue, Topic的区别

producer: destination.send(Message) destination: queue

consumer MDB: onMessage queue

Queue: point to point (1对1)