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Details Title: Java Cert Ch-7 Description: Objects and Collections Total Flash Cards: 62 Created: 02/19/2006 18:36:53	To study from this set of flash cards, or to create your own your own study flash cards, register HERE. If you are already a registered user, CLICK HERE.
Cards Term what is the default behavior of the toString() method? Definition The class of the object@unsigned hex representation of object's hash code Term What is the default behavior of equals method? Definition It uses the == to compare to see if two objects are equal. Term what are the implications of not overriding the equals() method. Definition they cant be used in hashmap as keys since the hashmap uses the equals method to see if search key is same as the object you provided. Term What is an important check that is required in an objects equals method? Definition the test to see if the Object is an instanceof the class. Term why must we caset the object into a moof before we call a getter method on it Definition because without the cast the compiler will try to call the given method on an Object and fail miserably Term can we use the default access modifier when overriding equals, hashCode() or toString()? Definition NO! they are default public and the default access modifier is more restrictive. Term what does it mean to say that equals method is reflexive? Definition it means x.equals(x) Term what does it mean to say that equals method is symmetric? Definition y.equals(x) then x.equals(y) Term What does it mean to say that the equals method is transitive? Definition x.equals(y) and y.equals(z) then x.equals(z) Term What does it mean to say that the equals method is consistent? Definition x.equals(y) will always return true if x and y dont change no matter weather it is called on sunday or monday. Term What is the fourth unamed rule of equals() Definition x.equals(null) should always be false. Term what is the relationship between the hashCode() method and the equals() method? Definition if equals() method says to objects are equal then their hashcodes should be the same. Term what happens if the hashcode method is implemented incorrectly? Definition objects that test equal using .eqauls() will not be found in Collections since hash code wont come out equal. Term does a hashcode that returns same value for all objects violate the exam contract? Definition No! since objects that are .equals() get the same hashCode since all objects get the same hashCode() Term What does a typical hashCode method do? Definition XORs instance variables and multiplies with a prime number Term what is the return type of the hashCode method. Definition the return type is always int. Term what is the requirement of hashCode changing when invoked on an object? Definition if no info changed in equals method is changed hashcode should return same integer. Term does the integer of hashcode need to remain same for all execution so it. Definition no need not be same between executions. Term if two objects are equal using .equals() what is the requirement of hashCode() Definition they must be equal ie equal objects must lie in same bucket. Term what if two hashcodes are not equal? Definition then they are not in same bucket and cannot be equal. Term why shouldnt we use transient variables in hashCode() Definition because when serialised their info will be lost. Term what can be done with objects in a collection? Definition add/remove , search/retrieve copy, iterate Term name the core collection interfaces? Definition Collection,List,Set,Map, SortedSet,SortedMap Term What is the Collections class? Definition Since it ends with s it is a class in java.util which provides static utility methods for use with collections Term Which collections class does not extend the Collection interface. Definition Map doesnt implement Collection interface. Term what is a list Definition a list of things Term what is diff between list an Set Definition set is unique things, while list can have duplicates Term what are Maps Definition things with unique ID Term What does the term Ordered mean? Definition it means iterating through the list or set will always produce its contents in a certain order . eg Hashtable will not have an order any one iteration may differ from the next. Term What does Sorted Collection mean? Definition it means that elements are stored in their natural order. I dont know about the compareTo() method wheather there is such a thing in Java. Term How do you iterate through a collection? Definition Iteratoer it = list.iterator(); while(it.hasNext()){ System.out.println(it.next()); } Term What are the general specialised methods of Lists Definition get(int index), indexOf(),add(int index,Object obj) Term What is a marker interface? Definition An interface with no methods implemented to give other classes information about the class. Term When should you choose ArrayList over LinkedList? Definition when you need fast iterations but wont do much iteration and deletion. eg. ArrayList implements RandomAccess interface Term Why do you want to use ArrayList Definition for fast iteration and retrieval while infrequent insertion and removal Term Does the Vector implement the RandomAccess Interface? Definition Vector is the only class apart from an ArrayList to implement the RandomAccess interface. Term When was ArrayList introduced into java language. Definition in 1.2 and 1.4 Term What are the thread issues in using ArrayList? Definition array list methods are not synchronised you have to use the collection methods to get synchronisations Term How are LinkedList when compared to ArrayList? Definition insertion deletion is fast but random access is slow. Underlying data structure is a doubly linked list. Term What determines the uniqueness of a Set? Definition the .equals() method is typically used to determine the uniqueness of a set. Term What is a HashSet> Definition it is a Set that is unordered and unsorted which is accessible using the elements hashCode() Term What is the difference between LinkedHashSet and HashSet? Definition LinkedHashSet is implemented with a with a doubly-linked list used to maintain the order of the elements. Term What is a TreeSet? Definition it is sorted collection implemented with red-black trees. Maintains the Natural order of elements Term Name two sorted Collection classes? Definition TreeSet amd TreeMap Term What is a Map? Definition creates a link between a key object and a value object. stored based on key object's hashCode() Term So what does HashMap do ? Definition Store unsorted unordered key value pairs, Term Can null be used as a key in HashMap? Definition Yes Null may be used as one of the keys Term is a map a collection? Definition nope only list and sets are collections. stands to reason as there is no concept of key in collectoins Term What is the difference between a HashMap and a Hashtable? Definition 1. that methods in Hashtable like Vector are synchronized. 1. that null cannot be used as key to Hashtable. 3. that t is little Term What is a LinkedHashMap? Definition it has an order ie maintains insertion order. Term What are the advantages of HashMap over LinkedHashMap? Definition Adding and removing elements is slower. Term what are the advantages of LinkedHashMap over HashMap? Definition iteration is faster and maintains insertion order. Term what is a TreeMap how does it compare with a HashMap? Definition it is a sorted map. sorted by natural order of the key and allows you to specify what that order is. Term What does teh garbage collector do? Definition it finds and deletes unreachable objects. Term can you ask for garbage collection to run at any point in time? Definition you could ask garbage collector to run at a particular point in time but you cant force this to happen Term How would you set up an object for garbage collection? Definition by setting its reference to null.reassigning the reference, Term is there any way to garbage collect even when there is a valid reference? Definition yes if there is no way to get to the reference ie ther eis loop of valid references but there is no way to to get to them Term What is the Runtime class in java? Definition a singleton class providing a way to communicate directly with the VM Term what is the command to request for a garbage collection Definition rt.gc() or System.gc(). Term When is the garbage collector guaranteed to run? Definition right before system throws an out of memory exception Term What is the advantage of finalize() method? Definition it can be used to close resources an object is using before it is deleted. Term what is the disadvantage of finalize() method? Definition it is not guaranteed to run