Java common face question 2015 (continuous update ... ）

1. The difference between overwrite and overload

overload is an overload that declares a property or method with the same name as an existing member, but parameter list differs from original member . Override is primarily used for method overrides between the parent and child classes, that is, the specified property or method can be overridden in a derived class with the same argument list requirements . 

2. Class and Object differences  

Seems to like to ask, for example, the class represents a computer, his basic properties include CPU, memory, input, and objects represent some type of computer, such as Dell, Lenovo, HP, Apple Computer. A class is a common, abstract, or object that is specific to something.

3, HashMap and Hashtable differences

1) HashMap Inherits the parent class  Abstractmap,hashtable inherits the parent class dictionary

   
  
 

   
  

  
public      class    hashtable     
   
  

  
    extends    dictionary     
   
  

  
    implements   map, cloneable,  Java.io.Serializable     
   
  

  

  
 

However HashMap is

  
 

   
  

  
public      class    hashmap     
   
  

  
    extends    abstractmap     
   
  

  
    implements   map, cloneable,  Serializable     
  
 

2) HashMap Key/value can be empty, so thread is unsafe, key must be unique, value can not be unique; HashTable Key/value are not allowed to be empty, so threads are safe.

3) HashMap is not synchronized, while Hashtable is synchronous, multiple threads do not need to sync manually when accessing Hashtable, and HashMap need to provide external synchronization (COLLECTIONS.SYNCHRONIZEDMAP) 

 4 "hashtable   The Hash/rehash algorithm used by HashMap is probably the same, so there is no big difference in performance. 

4. Private default protected public scope

location Private default protected public
The same class Yes Yes Yes Yes
classes within the same package no Yes Yes Yes
subclasses in different packages no no no yes Yes
different packages and not subclasses no no no no Yes

To be continued in .....

Java common face question 2015 (continuous update ... ）