Preopterećenje metode u JVM-u

Dobrodošli na novi blog Java Challengers ! Ovaj je blog posvećen izazovnim konceptima u Java programiranju. Svladajte ih i bit ćete na dobrom putu da postanete visoko vješt Java programer.

Za savladavanje tehnika na ovom blogu potreban je određeni napor, ali one će uvelike utjecati na vaše svakodnevno iskustvo kao Java programera. Izbjegavanje bugova je lakše kada znate kako pravilno primijeniti osnovne tehnike programiranja Java, a praćenje bugova je puno lakše kada točno znate što se događa na vašem Java kodu.

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Terminologija: Preopterećenje metode

Zbog izraza preopterećenost , programeri misle da će ova tehnika preopteretiti sustav, ali to nije istina. U programiranju, preopterećenje metode znači korištenje istog naziva metode s različitim parametrima. 

Što je preopterećenje metode?

Preopterećenje metode je tehnika programiranja koja omogućava programerima da isti naziv metode koriste više puta u istoj klasi, ali s različitim parametrima. U ovom slučaju kažemo da je metoda preopterećena. Popis 1 prikazuje jednu metodu čiji se parametri razlikuju po broju, vrsti i redoslijedu.

Popis 1. Tri vrste preopterećenja metoda

 Number of parameters: public class Calculator { void calculate(int number1, int number2) { } void calculate(int number1, int number2, int number3) { } } Type of parameters: public class Calculator { void calculate(int number1, int number2) { } void calculate(double number1, double number2) { } } Order of parameters: public class Calculator { void calculate(double number1, int number2) { } void calculate(int number1, double number2) { } } 

Preopterećenje metoda i primitivni tipovi

Na popisu 1 vidite primitivne tipove inti double. Radit ćemo više s tim i drugim vrstama, pa potrajte minutu da pregledate primitivne tipove u Javi.

Tablica 1. Primitivni tipovi u Javi

Tip Domet Zadano Veličina Primjer literala
 boolean  istina ili laž  lažno  1 bit  točno netočno
 byte  -128 .. 127  0  8 bitova  1, -90,128
 char  Unicode znak ili 0 do 65.536  \ u0000  16 bita  "a", "\ u0031", "\ 201", "\ n", 4
 short  -32.768 .. 32.767  0  16 bita  1, 3, 720, 22 000
 int  -2,147,483,648 .. 2,147,483,647  0  32 bita  -2, -1, 0, 1, 9
 long  -9,223,372,036,854,775,808 do 9,223,372,036,854,775,807  0  64 bita  -4000L, -900L, 10L, 700L
 float  3,40282347 x 1038, 1,40239846 x 10-45  0,0  32 bita  1,67e200f, -1,57e-207f, .9f, 10,4F
 double

 1,7976931348623157 x 10308, 4,9406564584124654 x 10-324

 0,0  64 bita  1.e700d, -123457e, 37e1d

Zašto bih trebao koristiti preopterećenje metode?

Preopterećenje čini vaš kod čišćim i lakšim za čitanje, a može vam pomoći i u izbjegavanju bugova u vašim programima.

Za razliku od popisa 1, zamislite program u kojem ste imali više calculate()metoda s imenima poput calculate1 calculate2,, calculate3. . . nije dobro, zar ne? Preopterećenje calculate()metode omogućuje vam da koristite isto ime metode, mijenjajući samo ono što treba promijeniti: parametre. Također je vrlo lako pronaći preopterećene metode jer su one grupirane u vašem kodu.

Ono što preopterećenje nije

Imajte na umu da promjena imena varijable nije preopterećenje. Sljedeći se kod neće kompajlirati:

 public class Calculator { void calculate(int firstNumber, int secondNumber){} void calculate(int secondNumber, int thirdNumber){} } 

Također ne možete preopteretiti metodu promjenom tipa povrata u potpisu metode. Ni sljedeći se kod neće kompajlirati:

 public class Calculator { double calculate(int number1, int number2){return 0.0;} long calculate(int number1, int number2){return 0;} } 

Preopterećenje konstruktora

Konstruktor možete preopteretiti na isti način kao i metodu:

 public class Calculator { private int number1; private int number2; public Calculator(int number1) {this.number1 = number1;} public Calculator(int number1, int number2) { this.number1 = number1; this.number2 = number2; } } 

Prihvatite izazov preopterećenja metode!

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Započnite pažljivim pregledom sljedećeg koda.

Popis 2. Izazov preopterećenja napredne metode

 public class AdvancedOverloadingChallenge3 { static String x = ""; public static void main(String... doYourBest) { executeAction(1); executeAction(1.0); executeAction(Double.valueOf("5")); executeAction(1L); System.out.println(x); } static void executeAction(int ... var) {x += "a"; } static void executeAction(Integer var) {x += "b"; } static void executeAction(Object var) {x += "c"; } static void executeAction(short var) {x += "d"; } static void executeAction(float var) {x += "e"; } static void executeAction(double var) {x += "f"; } } 

Dobro, pregledali ste kôd. Koji je izlaz?

  1. befe
  2. bfce
  3. efce
  4. aecf

Odgovor provjerite ovdje.

Što se upravo dogodilo? Kako JVM sastavlja preopterećene metode

Da biste razumjeli što se dogodilo na popisu 2, morate znati neke stvari o tome kako JVM sastavlja preopterećene metode.

Prije svega, JVM je inteligentno lijen : uvijek će uložiti najmanji mogući napor da izvrši metodu. Stoga, kada razmišljate o tome kako JVM rješava preopterećenje, imajte na umu tri važne tehnike kompajlera:

  1. Proširivanje
  2. Boks (autoboks i unboxing)
  3. Varargovi

Ako se nikada niste susreli s ove tri tehnike, nekoliko primjera trebalo bi vam ih pojasniti. Imajte na umu da ih JVM izvršava prema zadanom redoslijedu .

Evo primjera proširenja :

 int primitiveIntNumber = 5; double primitiveDoubleNumber = primitiveIntNumber ; 

Ovo je redoslijed primitivnih tipova kada se prošire:

Rafael del Nero

Evo primjera automatskog boksanja :

 int primitiveIntNumber = 7; Integer wrapperIntegerNumber = primitiveIntNumber; 

Obratite pažnju na to što se događa iza kulisa kada se sastavi ovaj kôd:

 Integer wrapperIntegerNumber = Integer.valueOf(primitiveIntNumber); 

I evo primjera  raspakiranja :

 Integer wrapperIntegerNumber = 7; int primitiveIntNumber= wrapperIntegerNumber; 

Evo što se događa iza kulisa kada se sastavi ovaj kôd:

 int primitiveIntNumber = wrapperIntegerNumber.intValue(); 

I evo primjera vararga ; napomena koja varargsje uvijek zadnja koja se izvršava:

 execute(int… numbers){} 

Što su varargi?

Used for variable arguments, varargs is basically an array of values specified by three dots (…) We can pass however many int numbers we want to this method.

For example:

execute(1,3,4,6,7,8,8,6,4,6,88...); // We could continue… 

Varargs is very handy because the values can be passed directly to the method. If we were using arrays, we would have to instantiate the array with the values.

Widening: A practical example

When we pass the number 1 directly to the executeAction method, the JVM automatically treats it as an int. That’s why the number doesn't go to the executeAction(short var) method.

Similarly, if we pass the number 1.0, the JVM automatically recognizes that number as a double.

Of course, the number 1.0 could also be a float, but the type is pre-defined. That’s why the executeAction(double var) method is invoked in Listing 2.

When we use the Double wrapper type, there are two possibilities: either the wrapper number could be unboxed to a primitive type, or it could be widened into an Object. (Remember that every class in Java extends the Object class.) In that case, the JVM chooses to wided the Double type to an Object because it takes less effort than unboxing would,  as I explained before.

The last number we pass is 1L, and because we've specified the variable type this time, it is long.

Video challenge! Debugging method overloading

Debugging is one of the easiest ways to fully absorb programming concepts while also improving your code. In this video you can follow along while I debug and explain the method overloading challenge:

Common mistakes with overloading

By now you’ve probably figured out that things can get tricky with method overloading, so let’s consider a few of the challenges you will likely encounter.

Autoboxing with wrappers

Java is a strongly typed programming language, and when we use autoboxing with wrappers there are some things we have to keep in mind. For one thing, the following code won't compile:

 int primitiveIntNumber = 7; Double wrapperNumber = primitiveIntNumber; 

Autoboxing will only work with the double type because what happens when you compile this code is the same as the following:

 Double number = Double.valueOf(primitiveIntNumber); 

The above code will compile. The first int type will be widened to double and then it will be boxed to Double. But when autoboxing, there is no type widening and the constructor from Double.valueOf will receive a double, not an int. In this case, autoboxing would only work if we applied a cast, like so:

 Double wrapperNumber = (double) primitiveIntNumber; 

Remember that Integer cannot be Long and Float cannot be Double. There is no inheritance. Each of these types--Integer, Long, Float, and Double--is a Number and an Object.

When in doubt, just remember that wrapper numbers can be widened to Number or Object. (There is a lot more to explore about wrappers but I will leave it for another post.)

Hard-coded number types in the JVM

When we don’t specify a type to a number, the JVM will do it for us. If we use the number 1 directly in the code, the JVM will create it as an int. If you try to pass 1 directly to a method that is receiving a short, it won’t compile.

For example:

 class Calculator { public static void main(String… args) { // This method invocation will not compile // Yes, 1 could be char, short, byte but the JVM creates it as an int calculate(1); } void calculate(short number) {} } 

The same rule will be applied when using the number 1.0; although it could be a float, the JVM will treat this number as a double:

 class Calculator { public static void main(String… args) { // This method invocation will not compile // Yes, 1 could be float but the JVM creates it as double calculate(1.0); } void calculate(float number) {} } 

Another common mistake is to think that the Double or any other wrapper type would be better suited to the method that is receiving a double. In fact, it takes less effort for the JVM to widen the Double wrapper to an Object instead of unboxing it to a double primitive type.

To sum up, when used directly in Java code, 1 will be int and 1.0 will be double. Widening is the laziest path to execution, boxing or unboxing comes next, and the last operation will always be varargs.

As a curious fact, did you know that the char type accepts numbers?

 char anyChar = 127; // Yes, this is strange but it compiles 

What to remember about overloading

Overloading is a very powerful technique for scenarios where you need the same method name with different parameters. It’s a useful technique because having the right name in your code makes a big difference for readability. Rather than duplicate the method and add clutter to your code, you may simply overload it. Doing this keeps your code clean and easy to read, and it reduces the risk that duplicate methods will break some part of the system.

What to keep in mind: When overloading a method the JVM will make the least effort possible; this is the order of the laziest path to execution:

  • First is widening
  • Second is boxing
  • Third is Varargs

What to watch out for: Tricky situations will arise from declaring a number directly: 1 will be int and 1.0 will be double.

Also remember that you can declare these types explicitly using the syntax of 1F or 1f for a float or 1D or 1d for a double.

That concludes our first Java Challenger, introducing the JVM’s role in method overloading. It is important to realize that the JVM is inherently lazy, and will always follow the laziest path to execution.

 

Answer key

The answer to the Java Challenger in Listing 2 is: Option 3. efce.

More about method overloading in Java

  • Java 101: Classes and objects in Java: A true beginner’s introduction to classes and objects, including short sections on methods and method overloading.
  • Java 101: Elementary Java language features: Learn more about why it matters that Java is a strongly typed language and get a full introduction to primitive types in Java.
  • Previše parametara u Java metodama, 4. dio: Istražite ograničenja i nedostatke preopterećenja metoda i kako se oni mogu ispraviti integriranjem prilagođenih vrsta i objekata parametara.

Ovu priču, "Preopterećenje metoda u JVM-u" izvorno je objavio JavaWorld.