This is just a compilations of some notes together with my own notes during the preparation for SCJP exam. Implementations

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1 SCJP Java Cllectins Cheat Sheet This is just a cmpilatins f sme ntes tgether with my wn ntes during the preparatin fr SCJP exam. Implementatins Interfaces Hash Table Resizable Array Implementatins Balanced Tree Linked List Hash Table + Linked List Set HashSet TreeSet LinkedHashSet List ArrayListVectr LinkedList Map HashMapHashtable TreeMap LinkedHashMap 1

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3 Cllectin Interfaces Cllectin - A grup f bjects. N assumptins are made abut the rder f the cllectin (if any), r whether it may cntain duplicate elements. Interfaces Set - Extends the Cllectin N duplicate elements permitted. At mst ne null element. May r may nt be rdered.interface. SrtedSet - Extends Set autmatically srted, either in their natural rdering (see the Cmparable interface), r by a Cmparatrbject prvided when a SrtedSet instance is created. first() and last() methds thrws NSuchElementExecptin when set is empty NavigableSet - Extends SrtedSet Navigatin methds reprting clsest matches fr given search targets. A NavigableSet may be accessed and traversed in either ascending r descending rder. All methds that receives a parameter can thrw exceptins: ClassCast if arg type are different NPE if arg is null - - Map- A mapping frm keys t values. Each key can map t at mst ne value. Main methds> blean add(e e) blean remve(object ) blean cntains(object ) isempty() size() Cmparatr<? super E> cmparatr() E first() E last() SrtedSet<E> headset(e telement) SrtedSet<E> subset(e frmelement, EtElement) SrtedSet<E> tailset(e frmelement) E lwer(e e) E flr(e e) E higher(e e) E ceiling(e e) E pllfirst() E plllast() SrtedSet<E> subset(e frmelement, EtElement) SrtedSet<E> headset(e telement, blean inclusive) - Methd withud inclusive flag available frm SrtedSet interface SrtedSet<E> tailset(e frmelement, blean inclusive) - Methd withud inclusive flag available frm SrtedSet interface V put(k key, V value) - replaces the ld value is replaced by the specified value returns the previus value assciated with key V get(object key) V remve(object key) returns the previus value assciated with key blean cntainskey(object key) SrtedMap- Extends Map 3 A map whse mappings are autmatically srted by key, either in the keys' natural rdering r by a cmparatr prvided when a SrtedMapinstance is created. Extends the Mapinterface. Set<Map.Entry<K,V>> entryset() Set<K> keyset() Cllectin<V> values() K firstkey() - firstentry nly in NavigableMap K lastkey() - lastentry nly in NavigableMap SrtedMap<K,V> headmap(k tkey) SrtedMap<K,V> tailmap(k frmkey) SrtedMap<K,V> submap(k frmkey, KtKey) Map.Entry<K,V> lwerentry(k key) Map.Entry<K,V> flrentry(k key) Map.Entry<K,V> higherentry(k key)

4 NavigableMap - Extends SrtedMap navigatin methds returning the clsest matches fr given search targets. May be accessed and traversed in either ascending r descending key rder. All methds that receives a parameter can thrw exceptins: ClassCast if arg type are different NPE if arg is null Map.Entry<K,V> ceilingentry(k key) K lwerkey(k key) K flrkey(k key) K higherkey(k key) K ceilingkey(k key) Map.Entry<K,V> firstentry() Map.Entry<K,V> lastentry() NavigableSet<K> descendingkeyset() NavigableMap<K,V> descendingmap() - - List - Extends the Cllectin. Ordered cllectin, als knwn as a sequence. Duplicates are generally permitted. Allws psitinal access. Queue - Extends Cllectin A cllectin designed fr hlding elements prir t prcessing. Prvide additinal insertin, extractin, and inspectin peratins. Each f these methds exists in tw frms: ne thrws an exceptin if the peratin fails, the ther returns a special value (either null r false, depending n the peratin). Map.Entry<K,V> pllfirstentry() Map.Entry<K,V> plllastentry() blean add(e e) blean remve(object ) E get(int index) blean cntains(object ) int indexof(object ) int size() List<E> sublist(int frmindex, int tindex) Thrws exceptin: blean add(e e) E remve() E element() Returns null r false: blean ffer(e e) E pll() E peek() Thrws exceptin Insert add(e) ffer(e) Remve remve() pll() Examine element() peek() Returns special value Deque - Extends Queue A linear cllectin that supprts element insertin and remval at bth ends. The name deque is shrt fr "duble ended queue" and is usually prnunced "deck". When a deque is used as a queue, FIFO (First-In-First-Out) behavir results. Deques can als be used as LIFO (Last-In-First-Out) stacks. First Element (Head) Last Element (Tail) Thrws exceptin Special value Thrws exceptin Special value Insert addfirst(e) fferfirst(e) addlast(e) fferlast(e) Remve remvefirst() pllfirst() remvelast() plllast() Examine getfirst() peekfirst() getlast() peeklast() 4

5 Bth in a queue r stack view, the elements are always read/remved frm the head f the list: element/peek/pll fr queue, pp/peek fr the stack. In the queue elements are added t the end f the list: add/ffer In the stack elements are added t the head f the list: push The methds inherited frm the Queue interface are precisely equivalent t Deque Queue Methd Equivalent Deque Methd add(e) addlast(e) ffer(e) fferlast(e) remve() remvefirst() pll() pllfirst() element() getfirst() peek() peekfirst() Stack methds are precisely equivalent t Deque methds as indicated in the table belw: Stack Methd Equivalent Deque Methd push(e) addfirst(e) pp() remvefirst() peek() peekfirst() 5

6 General-Purpse Implementatins HashSet Implements Cllectin<E>, Set<E> LinkedHashSet Implements Cllectin<E>, Set<E> TreeSet - Implements Cllectin<E>, NavigableSet<E>, Set<E>, SrtedSet<E> - - ArrayList Implements Cllectin<E>, List<E> Vectr Implements Cllectin<E>, List<E> LinkedList Implements Cllectin<E>, Deque<E>, List<E>, Queue<E> PrirityQueue Implements Cllectin<E>, Queue<E> Stack - Extends Cllectin<E>, List<E> Deque interface shuld be used in preference t the legacy Stack class HashMap Implements Map<K,V> Hashtable Implements Map<K,V> LinkedHashMap Implements Map<K,V> TreeMap Implements Map<K,V>, NavigableMap <K,V>, SrtedMap<K,V> permits the null element. Fast access, assures n duplicates, prvides n rdering. Runs nearly as fast as HashSet. N duplicates; iterates by insertin rder. nt syncrnized N duplicates; iterates in srted rder. Fast iteratin and fast randm access synchrnised It's like a slwer ArrayList, but it has synchrnized methds. May prvide better perfrmance than the ArrayListimplementatin if elements are frequently inserted r deleted within the list. Gd fr adding elements t the ends, i.e., stacks and queues. The elements f the pririty queue are rdered accrding t their natural rdering, r by a Cmparatr prvided at queue cnstructin time unbunded. its capacity grws autmatically. des nt permit null elements The head f this queue is the least element The Iteratr prvided in methd iteratr() is nt guaranteed t traverse the elements f the pririty queue in any particular rder. E pp() E push(e item) E peek() Essentially an unsynchrnized Hashtable that supprts nullkeys and values. Fastest updates (key/values); allws ne null key, many null values. Like a slwer HashMap (as with Vectr, due t its synchrnized methds). N null values r null keys allwed Predictable iteratin rder. Runs nearly as fast as HashMap. Allws ne null key, many null values. A srted map Ascending element rder 6

7 Utility methds Methd Class Explanatin static? return reverse(list<?> list) Cllectins reverses the rder f elements in a List. static vid reverseorder() Cllectins Returns a cmparatr that impses the reverse f the natural rdering static Cmparatr<T> aslist(t... a) Arrays Returns a fixed-size list backed by the specified array. static List<T> tarray() Cllectin interface Returns an array cntaining all f the elements in this cllectin. n Object[] tarray(t[] a) Cllectin interface Returns an array cntaining all f the elements in this cllectin; the runtime type f the returned array is that f the specified array. n T[] LinkedList add,remve and get thrws an exceptin (fr instance if element nt fund) ffer, pll and peek returns null (fr instance if element nt fund) stack interface is nly: push, pp and peek queue interface is: ffer, pll and peek deque interface is: ffer First and Last, pll First and Last, peek First and Last. It's a duble linked list!! sublist des nt affect the list given as parameter. It returns the mdified list. TreeSet / TreeMap 7 frm SrtedSet interface: first(), last(), tailset(), headset() and subset(). Frm a natural rder sequence!! headset() wrk exclusive until the given e. tailset() wrk inclusive. lwer() returns the element less than the given element, and flr() returns the element less than r equal t the given element. Similarly, higher() returns the element greater than the given element, and ceiling() returns the element greater than r equal t the given element. subset wrk inclusive at begin at exclusive and the end. submap() frm NavigableMap returns an instance f SrtedMap, nt NavigableMap as expected. subset(a, z) thrws a IllegalArgumentExceptin if z is greater than a backed cllectins: adding elements t the subset will add the element als t the main data set adding a element ut f range will thrw a java.lang.illegalargumentexceptin

8 lwer,flr,ceiling, andhigher are methds frm navigableset and frm NavigableMap TreeMap desn't implement Iterable, s cannt it's nt pssible t iterate ver the entries, nly frm the keyset r valuesset add() thrws NPE with a null arg if set is wrking with natural rder, r if the cmparatr desn't allws null bjects. This nly happens at secnd insert, because nly that time the cmparatr is invked. LinkedHashMap As in the TreeMap and ther Map implementatin, it's nt pssible t iterate ver the entries. First we need t get the values and then we already can iterate since this is a Cllectin. Of curse with the LinkedHashMap (that is rder, and nt srted), the iteratin ver the values gives elements rdered by the insertin rder. PrirityQueue PrirityQueue it's iterable, it's a queue and it's a cllectin The iteratin rder given by the iteratr is undefined, depends f the implmentatin. pll and peek shuld be used It have the main methd frm a queue: ffer(), pll(), peek(). Als the ne frm cllectin: add, remve, get... Can be srted by natural rder r by any ther custm rder: The cnstructr culd receive a Cmparatr. 8

9 Arrays, Srt and Search dn't frget t check if the cmparet() methd is public. Natural Order Implement Cmparable using cmparet(); prvides nly ne srt rder. cmparet must implement the ascending rder: return this.value - arg.value; a cmparatr in a natural rder is: cmpare(a, b) { return a.cmparet(b)) reverse rder: public int cmparet(human h) { return h.age.cmparet(this.age); } Srt vid srt(object[] a) - srts by natural rder Srts the specified array f bjects int ascending rder, accrding t the natural rdering f its elements. All elements in the array must implement the Cmparableinterface. Cmparable interface: int cmparet(t ) Can thrw a ClassCastExceptin if there are different element types in the array vid srt(object[] a, Cmparatr c) - srts by the given cmpratr Cmparatr interface: int cmpare(t 1, T 2) Search binarysearch(object[] a, Object key) // Searches using the binary search algrithm. l. If it is nt srted, the results are undefined. returns the bject index binarysearch(object[] a, Object key, Cmparatr c) Cmparatr interface: int cmpare(t 1, T 2) Attempting t search an array r cllectin, which is nt srted will cause an unpredictable search result. The binarysearch() methd gives meaningful results nly if it uses the same Cmparatr as the ne used t srt the array. Other way the result is: -1 9

10 Equals and Hashcde default (Object) implementatin fr equals and hashcde methds supprts their cntracts. public vid equals(object ) Cllectins Cllectins methds srt(), reverse(), and binarysearch() d nt wrk n Sets. all f these have List as args, and s gives cmpilatin errr fr Setsmimder Fur basic flavrs f cllectins include Lists, Sets, Maps, Queues. Map IS NOT a Cllectin, all thers are Dn't frget t verify that fr Cllectin - add() fr Map - put() LinkedList is a (implements) List and a Queue Queue is a interface Surces: - SCJP Study Guide, McGrawHill - racle.cm - Cllectins Framewrk - JnathanGiles.net 10

To over come these problems collections are recommended to use. Collections Arrays

To over come these problems collections are recommended to use. Collections Arrays Q1. What are limitatins f bject Arrays? The main limitatins f Object arrays are These are fixed in size ie nce we created an array bject there is n chance f increasing r decreasing size based n ur requirement.