Pass by Reference vs. Pass by Value

Transcription

1 Pass by Reference vs. Pass by Value Most methods are passed arguments when they are called. An argument may be a constant or a varable. For example, n the expresson Math.sqrt(33) the constant 33 s passed to the sqrt() method of the Math class. In the expresson Math.sqrt(x) the varable x s passed. Ths s smple enough, however there s an mportant but smple prncple at work here. If a varable s passed, the method receves a copy of the varable's value. The value of the orgnal varable cannot be changed wthn the method. Ths seems reasonable because the method only has a copy of the value; t does not have access to the orgnal varable. Ths process s called pass by value. However, f the varable passed s an object, then the effect s dfferent. Ths dea s explored n detal n ths secton. But frst, let's clarfy the termnology. For brevty, we often say thngs lke, "ths method returns an object...", or "ths method s passed an object as an argument...". As noted earler, ths not qute true. More precsely, we should say, or ths method returns a reference to an object... ths method s passed a reference to an object as an argument... In fact, objects, per se, are never passed to methods or returned by methods. It s always "a reference to an object" that s passed or returned. The term "varable" also deserves clarfcaton. There are two types of varables n Java: those that hold the value of a prmtve data type and those that hold a reference to an object. A varable that holds a reference to an object s an "object varable" although, agan, the prefx "object" s often dropped for brevty. Returnng to the topc of ths secton, pass by value refers to passng a constant or a varable holdng a prmtve data type to a method, and pass by reference refers to passng an object varable to a method. In both cases a copy of the varable s passed to the method. It s a copy of the "value" for a prmtve data type varable; t s a copy of the "reference" for an object varable. So, a method recevng an object varable as an argument receves a copy of the reference to the orgnal object. Here's the clncher: If the method uses that reference to make changes to the object, then the orgnal object s changed. Ths s reasonable because both the orgnal reference and the copy of the reference "refer to" to same thng the orgnal object. There s one excepton: strngs. Snce Strng objects are mmutable n Java, a method that s passed a reference to a Strng object cannot change the orgnal object. Ths dstncton s also brought out n ths secton. Let's explore pass by reference and pass by value through a demo program (see Fgure 1). As a self test, try to determne the output of ths program on your own before proceedng. If understood the precedng dscusson and f you guessed the correct output, you can confdently skp the rest of ths secton. Java Prmer PassByReference-1 Scott MacKenze

2 1 publc class DemoPassByReference 2 { 3 publc statc vod man(strng[] args) 4 { 5 // declare and ntalze varables and objects 6 nt = ; 7 Strng s = ""; 8 StrngBuffer sb = new StrngBuffer("Hello, world"); 9 10 // prnt varable and objects s and sb 11 System.out.prntln(); // prnt t (1) 12 System.out.prntln(s); // prnt t (2) 13 System.out.prntln(sb); // prnt t (3) 14 System.out.prntln(" "); // attempt to change, s, and sb usng methods 17 Method(); 18 smethod(s); 19 sbmethod(sb); 20 System.out.prntln(" "); // prnt varable and objects s and sb (agan) 23 System.out.prntln(); // prnt t (7) 24 System.out.prntln(s); // prnt t (8) System.out.prntln(sb); // prnt t (9) 26 } publc statc vod Method(nt Test) 29 { 30 Test = 9; // change t 31 System.out.prntln(Test); // prnt t (4) 32 return; 33 } publc statc vod smethod(strng stest) 36 { 37 stest = stest.substrng(8, 11); // change t 38 System.out.prntln(sTest); // prnt t (5) 39 return; 40 } publc statc vod sbmethod(strngbuffer sbtest) 43 { 44 sbtest = sbtest.nsert(7, "Java "); // change t 45 System.out.prntln(sbTest); // prnt t (6) 46 return; 47 } 48 } Ths program generates the followng output: Fgure 1. DemoPassByReference.java Java Prmer PassByReference-2 Scott MacKenze

3 Hello, world fun DemoPassByReference begns by declarng and ntalzng three varables: an nt varable named, a Strng object varable named s, and a StrngBuffer object varable named sb (see lnes 6-8). The three are prnted n lnes Then, each varable s passed as an argument to a method. Wthn each method, the copy of the varable exsts as a local varable. The value of the varable or the value of the object referred to by the varable, n the case of the Strng and StrngBuffer object varables s changed and prnted wthn each method (lnes 31, 38, and 45). The prnt statements are numbered to show the order of prntng. Back n the man() method, the three values are prnted agan (lnes 23-). Have a look at the output and see f t s consstent wth our prevous dscusson. It s. Let's examne the treatment of each of the three types of varables used n DemoPassByReference. We'll begn wth the StrngBuffer objects. The pass-by-reference concept s llustrated by the object varables sb and sbtest. In the man() method, a StrngBuffer object s nstantated and ntalzed wth "Hello, world" and a reference to t s assgned to the StrngBuffer object varable sb (lne 8). The memory assgnments for the object and the object varable are llustrated n Fgure 2a. Ths corresponds to the state of the sb and sbtest varables just after lne 8 n Fgure 1. In lne 19, the sbmethod() method s called wth sb as an argument. The method s defned n lnes Wthn the method, a copy of sb exsts as a local varable named sbtest. The memory assgnments just after the sbmethod() begns executon are shown n Fgure 2b. Note that sb and sbtest refer to the same object. Java Prmer PassByReference-3 Scott MacKenze

4 sb sb Hello, world Hello, world sbtest (a) (b) sb sb sbtest (c) (d) Fgure 2. Memory assgnments for call-by-reference example (StrngBuffer objects) In lne 44, the object s modfed usng the nsert() method of the StrngBuffer class. The method s called through the nstance varable sbtest. The memory assgnments just after lne 44 executes are shown n Fgure 2c. After the sbmethod() fnshes executon, control passes back to the man() method and sbtest ceases to exst. The memory assgnments just after lne 19 n Fgure 1 are shown n Fgure 2d. Note that the orgnal object has changed, and that the orgnal object varable, sb, now refers to the updated object. The scenaro played out above, s a typcal example of pass by reference. It demonstrates that methods can change the objects nstantated n other methods when they pass a reference to the object as an argument. A smlar scenaro could be crafted for objects of any of Java's numerous classes. The StrngBuffer class s a good choce for the example, because StrngBuffer objects are tangble, prntable enttes. Other objects are less tangble (e.g., objects of the Random class); however, the same rules apply. A major excepton s, of course, the Strng class, because Strng objects, once nstantated, cannot change. For completeness, let's walk through the memory assgnments for the Strng objects n the DemoPassByReferernce program. A Strng object s nstantated n lne 7 and a reference to t s assgned to the Strng object varable s. The memory assgnments at ths pont are shown n Fgure 3a. Java Prmer PassByReference-4 Scott MacKenze

5 s s stest (a) (b) s s stest (c) fun (d) fun Fgure 3. Memory assgnments for call-by-reference example (Strng objects) In lne 18 the smethod() s called wth s as an argument. The method s defned n lnes Wthn the method, a copy of s exsts as a local varable named stest. The memory assgnments just after the smethod() begns executon are shown n Fgure 3b. Note that s and stest refer to the same object. At ths pont, there s no dfference n how memory was assgned for the Strng or StrngBuffer objects. However, n lne 37, Java's unque treatment of Strng objects surfaces. The substrng() method of the Strng class s called to extract the strng "fun" from the orgnal strng. The result s the nstantaton of a new Strng object. A reference to the new object s assgned to stest. The memory assgnments just after lne 37 executes are shown n Fgure 3c. After the smethod() fnshes executon, control passes back to the man() method and stest ceases to exst. The memory assgnments just after lne 18 n Fgure 1 are shown n Fgure 3d. Note that the orgnal object dd not change. The Strng object contanng "fun" s now redundant and ts space s eventually reclamed. Fnally, let's examne the memory assgnments for the nteger varables n DemoPassByReference. When an nteger varable s passed as an argument to a method, t s passed by value. In the demo program, an nteger varable named s declared and ntalzed wth n lne 6. The memory assgnment s llustrated n Fgure 4a. (a) (b) Test (c) Test Fgure 4. Memory assgnments for call-by-value example (nt varables) In lne 17, Method() s called wth as an argument. The method s defned n lnes Wthn the method, a copy of exsts as a local varable named Test. The memory 9 (d) Java Prmer PassByReference-5 Scott MacKenze

6 assgnments just after Method() begns executon are shown n Fgure 3b. Note that and Test are dstnct: They are two separate varables that happen to hold the same value. Wthn the Method() method, Test s re-assgned the value 9. The memory assgnments just after lne 30 executes are shown n Fgure 4c. After the Method() fnshes executon, control passes back to the man() method and Test ceases to exst. The memory assgnments just after lne 17 n Fgure 1 are shown n Fgure 4d. Note that the value of the orgnal varable dd not change. Java Prmer PassByReference-6 Scott MacKenze