java se面试题(经常出现却又容易被忽略的题目).doc

java se面试题(经常出现却又容易被忽略的题目) 1) runtime exception, non runtime exception. The runtime exception can be processed or not processed. Non runtime exceptions must be handled (either by try-catch statements or by keyword throws). Non runtime exceptions inherit from Exception, but do not inherit from RuntimeException. 2) (equals) difference method and = = (1) for packaging type string variables and basic data types: = = two reference variables are equal, that is variable in the memory of the first address is the same. The equals () compares the content of the entity (memory space) because the String class redefines the equals () method and compares its worth rather than its memory address. Note: if the StringBuffer s1=new StringBuffer (a); StringBuffer s2=new StringBuffer (a); s1.equals (S2); for false, because equals is not defined in the StringBuffer class (RE) method, so the equals () method comes from the Object class, Object class (equals) method is used to compare. So, for false. (2) for non string variables: = = and (equals) is the same, are used to compare the first address object in the heap memory, which is used to compare if two reference variables refer to the same entity (memory). Note: (1) if is the basic data types can only use = = not equals (). (2) for the package type basic data types = is entity references (equals) is the entity content. 3) is it possible to inherit the String class? The String class is the final class, so it cannot be inherited. 4) constructor Constructor can be override? The constructor Constructor cannot be inherited, so it cannot be overridden (override) but can be overload (overloaded). 5) the difference between HashMap and Hashtable. 1.HashMap is a lightweight implementation of Hashtable and is more efficient. 2.Hashtable is thread safe and HashMap is non thread safe, that is, the method in the former is useful for synchronized modification, and the latter is not. 3.HashMap allows the empty (null) key value (key), and Hashtable does