Chapter 21. Arrays as Objects Array Parameters

Transcription

1 253 Chpter 21 Arrys s Objects An rry is simply n object reference (see Section 20.1). As such, it cn be used like ny object reference; n rry cn be pssed s n ctul prmeter to method nd returned from method s return vlue. In the previous chpter we sw how ccess nd work with the elements within n rry. In this chpter we investigte how mnipulte n rry itself s whole Arry Prmeters Arry prmeters. An rry is specified s forml prmeter in method definition with the sme syntx s vrible declrtion: // Method f ccepts two rrys nd Boolen public int f(int[] numlist, double[] vector, flg) { /*... Detils omitted... */ Clling code simply psses the rry s it would ny vrible: int[] list = new int[100]; double[] v = new double[3]; /* Initilize list nd v (detils omitted) */ //... then cll f() list[3] = f(list, v, true); The following method sums the elements of n integer rry: public sttic int sumup(int[] ) { int sum = 0;

2 21.2. COPYING AN ARRAY 254 for ( int vlue : ) { sum += vlue; return sum; Since the method ccepts ny integer rry with no dditionl informtion bout its size, the number of elements in the rry must be determined from the rry s length ttribute. The for/ech sttement implicitly exmines the length field to iterte the correct number of times. The prmeter pssing mechnism for rry vribles works just like it does for other reference vribles. Suppose the supup() method is clled pssing n integer rry nmed list: int[] list; /* Allocte nd initilize list (detils omitted) */ // Disply the sum of ll the elements in list System.out.println(sumUp(list)); While in the supup() method, is n lis of list. This mens tht modifying within sumup() lso ffects list in the clling environment. It is not necessry to use nmed rry s n ctul prmeter. The following exmple mkes up n nonymous rry nd psses it to the sumup() method: // Disply the sum of ll the elements the nonymous rry specified here System.out.println(sumUp(new int[] { 10, 20, 30, 40, 50 )); 21.2 Copying n Arry It is importnt to note tht n rry is simply n object reference. It my seem plusible to mke copy of n rry s follows: int[], b; // Declre two rrys = new int[] { 10, 20, 30 ; // Crete one b = ; // Mke copy of rry? Since n rry is n object reference, nd since b hs been ssigned to, nd b refer to the sme rry. Arry b is n lis of, not copy of. Figure 21.1 illustrtes this rry lising. The following code cn be used to mke copy of : int[], b; // Declre two rrys = new int[] { 10, 20, 30 ; // Crete one // Relly mke copy of rry b = new int[.length]; // Allocte b for ( int i = 0; i <.length; i++ ) { b[i] = [i]; A new rry must be creted, nd then ech element is copied over into the new rry, s shown in Figure 21.2.

3 21.2. COPYING AN ARRAY 255 int[], b;? b? First, declre the rry vribles = new int[] { 10, 20, 30 ; b? Next, ssign to the newly creted rry b = ; b Finlly, nd b refer to the sme rry Figure 21.1: Arry lising Since copying n rry is such common ctivity, the jv.lng.system clss hs method nmed rrycopy() tht mkes it convenient to copy ll or prt of the elements of one rry to nother rry. It requires five prmeters: 1. The source rry; tht is, the rry from which the elements re to be copied 2. The strting index in the source rry from which the elements will be copied 3. The destintion rry; tht is, the rry into which the elements re to be copied 4. The strting index in the destintion rry into which the elements will be copied 5. The number of elements to be copied Notice tht this method is quite flexible. All or prt of n rry my be copied into nother rry. The simplest cse is complete copy, s in the exmple bove: int[], b; // Declre two rrys = new int[] { 10, 20, 30 ; // Crete one // Mke copy of rry b = new int[.length]; // Allocte b System.rrycopy(, 0, b, 0, b.length);

4 21.2. COPYING AN ARRAY 256 int[], b;? b? First, declre the rry vribles = new int[] { 10, 20, 30 ; b? Next, ssign to the newly creted rry b = new int[.length]; Then, crete new rry for vrible b b for ( int i = 0; i <.length; i++ ) { b[i] = [i]; Finlly, copy the contents of rry to rry b b Figure 21.2: Mking true copy of n rry In the following cse prt of one rry is copied into prt of nother rry: int[] = { 10, 20, 30, 40, 50, 60, 70, 80, 90 ; int[] b = { 11, 22, 33, 44, 55, 66, 77, 88, 99 ; System.rrycopy(b, 2,, 4, 3); After executing this code the contents of re: 10,20,30,40,33,44,55,80,90 The System.rrycopy() method is implemented in such wy to be more efficient thn equivlent code written in the Jv lnguge. Tble 21.1 shows the results of five runs of ArryCopyBenchmrk ( 21.1) on n rry of 5,000,000 elements. On verge, System.rrycopy() is only 2% fster thn the pure Jv copy code. public clss ArryCopyBenchmrk { public sttic void min( String [] rgs ) {

5 21.2. COPYING AN ARRAY 257 Run for rrycopy Percent Number Loop (msec) Speedup (msec) Averge Tble 21.1: Empiricl dt compring strightforwrd rry copy to System.rrycopy(). The rry size is 5,000,000. if ( rgs. length < 1 ) { System.out.println("Usge:"); System. out. println(" jv ArryCopyBenchmrk < size >"); System.exit (1); int size = Integer. prseint( rgs [0]); // Initilize the source rry with rndom vlues int [] src = new int[ size ]; jv.util.rndom rnd = new jv.util.rndom (); for ( int i = 0; i < size ; i ++ ) { src[i] = rnd.nextint(size ); int [] dest = new int[ size ]; Stopwtch timer = new Stopwtch (); // Copy rry by hnd timer.strt (); for ( int i = 0; i < size ; i ++ ) { dest[i] = src[i]; timer.stop (); System. out. println(" Elpsed time : " + timer. elpsed ()); // Copy rry using System. rrycopy () timer.strt (); System.rrycopy (src, 0, dest, 0, size ); timer.stop (); System. out. println(" Elpsed time : " + timer. elpsed ()); Listing 21.1: ArryCopyBenchmrk compres strightforwrd rry copy to System.rrycopy()

6 21.3. ARRAY RETURN VALUES Arry Return Vlues Arry return vlues. An rry cn be returned by method, s cn ny reference type. PrimesList ( 21.2) returns n rry of prime numbers over given rnge. import jv.util.scnner; public clss PrimesList { public sttic boolen isprime( int n) { boolen result = true ; // Assume no fctors unless we find one for ( int trilfctor = 2; trilfctor <= Mth. sqrt(n); trilfctor ++ ) { if ( n % trilfctor == 0 ) { // Is trilfctor fctor? result = flse; brek ; // No need to continue, we found fctor return result; // Returns list ( rry ) of primes in the rnge strt... stop public sttic int [] generteprimes ( int strt, int stop ) { // First, count how mny there re int vlue = strt ; // Smllest potentil prime number int count = 0; // Number of primes in the list while ( vlue <= stop ) { // See if vlue is prime if ( isprime(vlue ) ) { count ++; vlue ++; // Try the next potentil prime number // Next, crete n rry exctly the right size int [] result = new int[ count ]; // Next, populte the rry with the primes count = 0; vlue = strt ; // Smllest potentil prime number while ( vlue <= stop ) { // See if vlue is prime if ( isprime(vlue ) ) { result[ count ++] = vlue; vlue ++; // Try the next potentil prime number return result; public sttic void min( String [] rgs ) { int mxvlue ; Scnner scn = new Scnner( System. in); System. out. print(" Disply primes up to wht vlue? "); int [] primes = generteprimes (2, scn. nextint ());

7 21.4. COMMAND LINE ARGUMENTS 259 for ( int p : primes ) { System.out.print(p + " "); System. out. println (); // Move cursor down to next line Listing 21.2: PrimesList Uses method to generte list (rry) of prime numbers over given rnge The generteprimes() method first counts the number of prime numbers so tht n rry of the proper size cn be llocted. Then it the repets the process gin filling in ech position of the newly llocted rry with prime number. Note tht the sttement result[count++] = vlue; uses the postincrement opertor; thus, the current vlue of count is used s the index, then count is incremented Commnd Line Arguments The min() method for n executble clss hs the generl structure: public sttic void min(string[] rgs) { /* Body goes here... */ The min() method specifies single prmeter n rry of strings. The identifier rgs my be replced with ny vlid identifier nme, but ll the other nmes (public, sttic, void, min, nd String) must pper s presented here. To this point we hve ignored this prmeter of min(), but now, rmed with the knowledge of rrys, we cn write even more flexible Jv progrms. The commnd line is the sequence of text user enters in commnd shell. On Unix/Linux system, the shell (typiclly bsh or csh) is the commnd shell; on Windows system, the DOS shell is commnd shell. The commnd to see the files nd subdirectories (subfolders) in the current directory (folder) is ls on Unix/Linux system, nd dir on Windows system. Both ls nd dir re considered simple commnd lines. Additionl informtion cn be provided to produce more complicted commnd lines. For exmple, ls -l

8 21.4. COMMAND LINE ARGUMENTS 260 on Unix/Linux system gives long listing providing extr informtion. The commnd line dir /w on Windows system provides wide listing of files. We sy tht -l is n rgument to ls nd /w is n rgument to dir. To execute the Jv progrm ArryAverge ( 20.3) from the commnd shell, the file ArryAverge.jv would first be compiled to produce the clss file ArryAverge.clss. The Jv interpreter (normlly nmed jv) would then be invoked s jv ArryAverge This is how Jv ppliction is typiclly executed from the commnd line. It is possible to pss dditionl informtion to Jv progrms using commnd line rguments. The commnd shell (OS) uses the string rry prmeter in the min() to pss commnd line rguments to Jv progrm. CmdLineAverge ( 21.3) ccepts the vlues to verge from the commnd line insted of hving the user enter them during the progrm execution. public clss CmdLineAverge { public sttic void min( String [] rgs ) { double sum = 0.0; // Get commnd line rguments nd compute their verge for ( String s : rgs ) { sum += Double. prsedouble (s); System.out.print("The verge of "); for ( int i = 0; i < rgs. length - 1; i ++ ) { System.out.print(rgs[i] + ", "); // No comm following lst element System.out.println(rgs[rgs.length - 1] + " is " + sum/rgs.length ); Listing 21.3: CmdLineAverge Averges vlues provided on the commnd line Issuing the commnd line jv CmdLineAverge yields the output

9 21.5. VARIABLE ARGUMENT LISTS 261 The verge of 9, 3.5, 0.2, 100, 15.3 is 25.6 Figure 21.3 shows the correspondence of commnd line rguments to the rgs rry elements. jv CmdLineAverge rgs[0] rgs[1] rgs[2] rgs[3] rgs[4] Figure 21.3: Correspondence of commnd line rguments to rgs rry elements It is importnt to note tht commnd line rguments re whitespce delimited nd ech rgument is presented to the JVM s String object. As shown in CmdLineAverge, if number is provided on the commnd line nd the rgument is to be processed s number by the Jv progrm, then the rgument must be converted from its string form to its numeric form. In CmdLineAverge, the Double.prseDouble() clss method is used to convert string rgument to double. At lst, ll spects of the declrtion of the min() method hve been reveled: public the min() method cn be invoked by methods outside the clss sttic no object need be creted to execute the min() method void min() returns no vlue min the nme of the method invoked when the JVM executes the clss String[] rgs min() ccepts n rry of String objects s prmeter; these strings correspond to commnd line rguments In IDEs developers typiclly run Jv progrm through menu item or hotkey. In these development environments, commnd line rguments re often specified in dilog box Vrible Argument Lists It is esy to write method tht dds two integers together nd returns their sum. So too, three integers, four integers, etc. up to some smll prcticl limit. Since Jv permits methods to be overloded, the tsk is esy, lthough time consuming. OverlodedSumOfInts ( 21.4) provides good strting point. public clss OverlodedSumOfInts { public sttic int sum( int, int b) { return + b; public sttic int sum( int, int b, int c) { return + b + c;

10 21.5. VARIABLE ARGUMENT LISTS 262 public sttic int sum( int, int b, int c, int d) { return + b + c + d; public sttic int sum( int, int b, int c, int d, int e) { return + b + c + d + e; //... Overlod s mny times s you like! Listing 21.4: OverlodedSumOfInts A simple clss to sum integers Wht if the number of integers to dd exceeds the number of prmeters in ny of our overloded methods? We could lwys use functionl composition, s in int totl = OverlodedSumOfInts.sum(OverlodedSumOfInts.sum(3, 4, -1, 18, 6), OverlodedSumOfInts.sum(2, 2, 10, -3)); but this cumbersome. We could use n rry nd write just one method, s is ArrySumOfInts ( 21.5). public clss ArrySumOfInts { public sttic int sum( int [] ) { int sum = 0; for ( int i : ) { sum += i; return sum; Listing 21.5: ArrySumOfInts A better clss to sum integers using n rry The rry version is extremely flexible, but it is more wkwrd for the client to use. Consider the code required to sum five selected integers: int totl = ArrySumOfInts.sum(new int[] { 3, 4, -1, 18, 6 ); The explicit rry cretion is necessry, but it sure would be convenient to just pss the integer rguments by themselves without the rry wrpper. Before Jv 5.0, the number of rguments ccepted by method ws fixed t compile time. A method declred with three forml prmeters ccepted only three ctul prmeters. As we hve seen, overloded methods provide dditionl flexibility, but even then the number of prmeters is fixed. Jv 5.0 provides feture clled vrrgs (the nme cn be trced to the C progrmming lnguge) tht llows method writers to leve the number of prmeters open. The syntx is illustrted in VrrgsSumOfInts ( 21.6).

11 21.5. VARIABLE ARGUMENT LISTS 263 public clss VrrgsSumOfInts { public sttic int sum( int... rgs ) { int sum = 0; for ( int i : rgs ) { sum += i; return sum; Listing 21.6: VrrgsSumOfInts A truly better clss to sum integers using vrrgs public clss VrrgTester { public sttic void min( String [] rgs ) { System.out.println(OverlodedSumOfInts.sum (3, 4, 5, 6)); System.out.println(ArrySumOfInts.sum(new int [] { 3, 4, 5, 6 )); System.out.println(VrrgsSumOfInts.sum (3, 4, 5, 6)); Listing 21.7: VrrgTester A truly better clss to sum integers using vrrgs Note tht the body of VrrgsSumOfInts is functionlly identicl to ArrySumOfInts; only the method signtures differ. The two methods work the sme wy they process n rry of ints. The new prmeter specifiction hs the following form: typenme... prmeternme where typenme is ny stndrd or progrmmer-defined type (primitive or reference), nd prmeternme is progrmmer chosen prmeter nme. The... signifies tht within the method body the prmeter is to be treted s n rry of the type specified. In our VrrgsSumOfInts exmple, the prmeter rgs is treted s n int rry. The... lso mens tht the client will send ny number of prmeters of the specified type. The compiler will generte code in the context of the cller tht cretes n rry to hold these rguments nd sends tht new rry off to the method to be processed. The client s view is now much simpler: int totl = VrrgsSumOfInts.sum(3, 4, -1, 18, 6); The cll to VrrgsSumOfInts.sum() now looks like cll to norml method tht ccepts five prmeters. Behind the scenes, however, n rry is involved, nd only one true prmeter is pssed. The VrrgsSumOfInts.sum() method ccepts ny number of int rguments, but wht if you need to ccept ny number of rguments of mixed types? The solution is to expect ny number of Objects, s in public sttic void process(object... objs) { /* Do something with prmeters... */

12 21.6. SUMMARY 264 Primitive types pssed by the cller will be utoboxed into the pproprite wrpper objects. Of course, if nonuniform types re pssed, the method body will be more tricky to write, since ny type could pper t ny plce in the prmeter list. As n exmple, the System.out.printf() method cn ccept ny number of ny types of prmeters following its formt string. Its signture is public PrintStrem printf(string formt, Object... rgs) The code within the body of printf() scns the control codes within the formt string to properly ccess the rry of Objects tht follow. Tht is why printf() cn fil with runtime error if the progrmmer is inconsistent with plcement of control codes nd rrngement of triling prmeters. As shown in System.out.printf(), norml prmeters cn be mixed with vrrgs. In order for the compiler to mke sense of the cll of such method, ll norml prmeters must precede the single vrrgs prmeter Summry Add summry items here Exercises