Does Java support default parameter values

Question

I came across some Java code that had the following structure   public MyParameterizedFunction String param1  int param2        this param1  param2  false      public MyParameterizedFunction String param1  int param2  boolean param3          use all three parameters here     I know that in C   I can assign a parameter a default value   For example   void MyParameterizedFunction String param1  int param2  bool param3 false     Does Java support this kind of syntax   Are there any reasons why this two step syntax is preferable

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Unfortunately  yes   void MyParameterizedFunction String param1  int param2  bool param3 false       could be written in Java 1 5 as   void MyParameterizedFunction String param1  int param2  Boolean    params        assert params length  lt   1      bool param3   params length  gt  0   params 0  booleanValue     false      But whether or not you should depend on how you feel about the compiler generating a  new Boolean       for each call   For multiple defaultable parameters   void MyParameterizedFunction String param1  int param2  bool param3 false  int param4 42       could be written in Java 1 5 as   void MyParameterizedFunction String param1  int param2  Object    p        int l   p length      assert l  lt   2      assert l  lt  1    Boolean class isInstance p 0        assert l  lt  2    Integer class isInstance p 1        bool param3   l  gt  0  amp  amp  p 0     null     Boolean p 0   booleanValue     false      int param4   l  gt  1  amp  amp  p 1     null     Integer p 1   intValue     42      This matches C   syntax  which only allows defaulted parameters at the end of the parameter list     Beyond syntax  there is a difference where this has run time type checking for passed defaultable parameters and C   type checks them during compile

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I might be stating the obvious here but why not simply implement the  default  parameter yourself   public class Foo             public void func String s                   func s  true                     public void func String s  boolean b                     your code here               for the default  you would either use  func  my string      and if you wouldn t like to use the default  you would use  func  my string   false

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No  In general Java doesn t have much  any  syntactic sugar  since they tried to make a simple language

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No  but the simplest way to implement this is   public myParameterizedFunction String param1  int param2  Boolean param3         param3   param3    null   false   param3     public myParameterizedFunction String param1  int param2         this param1  param2  false       or instead of the ternary operator  you can use if   public myParameterizedFunction String param1  int param2  Boolean param3         if  param3    null            param3   false           public myParameterizedFunction String param1  int param2         this param1  param2  false

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There are half a dozen or better issues such as this  eventually  you arrive at the static factory pattern     see the crypto API for that  Sort difficult to explain  but think of it this way  If you have a constructor  default or otherwise  the only way to propagate state beyond the curly braces is either to have a Boolean isValid    along with the null as default value v failed constructor   or throw an exception which is never informative when getting it back from field users   Code Correct be damned  I write thousand line constructors and do what I need  I find using isValid at object construction - in other words  two-line constructors - but for some reason  I am migrating to the static factory pattern  I just seem you can do a lot if you in a method call  there are still sync   issues but defaults can be  substituted  better   safer    I think what we need to do here is address the issue of null as default value vis-a-vis something String one new String      as a member variable  then doing a check for null before assigning string passed to the constructor    Very remarkable the amount of raw  stratospheric computer science done in Java   C   and so on has vendor libs  yes  Java can outrun them on large scale servers due to it s a massive toolbox  Study static initializer blocks  stay with us

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You can use the following-   public void mop Integer x         Define default values         x   x    null   200   x

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NO  But we have alternative in the form of function overloading   called when no parameter passed  void operation     int a   0  int b   0        called when  a  parameter was passed  void operation int a    int b   0    code       called when parameter b passed  void operation int a   int b     code

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No  the structure you found is how Java handles it  that is  with overloading instead of default parameters    For constructors  See Effective Java  Programming Language Guide s Item 1 tip  Consider static factory methods instead of constructors  if the overloading is getting complicated  For other methods  renaming some cases or using a parameter object can help  This is when you have enough complexity that differentiating is difficult  A definite case is where you have to differentiate using the order of parameters  not just number and type

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Instead of using   void parameterizedMethod String param1  int param2        this param1  param2  false      void parameterizedMethod String param1  int param2  boolean param3          use all three parameters here     You could utilize java s Optional functionality by having a single method   void parameterizedMethod String param1  int param2   Nullable Boolean param3        param3   Optional ofNullable param3  orElse false         use all three parameters here     The main difference is that you have to use wrapper classes instead of primitive Java types to allow null input Boolean instead of boolean  Integer instead of int and so on

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No    You can achieve the same behavior by passing an Object which has smart defaults  But again it depends what your case is at hand

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It is not supported in java as in other language for ex  Kotlin

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This is how I did it     it s not as convenient perhaps as having an  optional argument  against your defined parameter  but it gets the job done   public void postUserMessage String s boolean wipeClean        if wipeClean                userInformation setText s     n              else               postUserMessage s            public void postUserMessage String s        userInformation appendText s     n        Notice I can invoke the same method name with either just a string or I can invoke it with a string and a boolean value  In this case  setting wipeClean to true will replace all of the text in my TextArea with the provided string  Setting wipeClean to false or leaving it out all together simply appends the provided text to the TextArea   Also notice I am not repeating code in the two methods  I am merely adding the functionality of being able to reset the TextArea by creating a new method with the same name only with the added boolean   I actually think this is a little cleaner than if Java provided an  optional argument  for our parameters since we would need to then code for default values etc  In this example  I don t need to worry about any of that  Yes  I have added yet another method to my class  but it s easier to read in the long run in my humble opinion

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I ve now spent quite some time to figure out how to use this with methods that return values  and I haven t seen any examples so far  I thought it might be useful to add this here   int foo int a           do something with a     return a     int foo         return foo 0      here  0 is a default value for a

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There are several ways to simulate default parameters in Java    Method overloading   void foo String a  Integer b                 void foo String a        foo a  0      here  0 is a default value for b    foo  a   2   foo  a      One of the limitations of this approach is that it doesn t work if you have two optional parameters of the same type and any of them can be omitted  Varargs   a  All optional parameters are of the same type   void foo String a  Integer    b        Integer b1   b length  gt  0   b 0    0      Integer b2   b length  gt  1   b 1    0               foo  a    foo  a   1  2     b  Types of optional parameters may be different   void foo String a  Object    b        Integer b1   0      String b2           if  b length  gt  0          if    b 0  instanceof Integer                throw new IllegalArgumentException                       b1    Integer b 0             if  b length  gt  1            if    b 1  instanceof String                  throw new IllegalArgumentException                           b2    String b 1                                    foo  a    foo  a   1   foo  a   1   b2      The main drawback of this approach is that if optional parameters are of different types you lose static type checking  Furthermore  if each parameter has different meaning you need some way to distinguish them  Nulls  To address the limitations of the previous approaches you can allow null values and then analyse each parameter in a method body   void foo String a  Integer b  Integer c        b   b    null   b   0      c   c    null   c   0               foo  a   null  2     Now all arguments values must be provided  but the default ones may be null  Optional class  This approach is similar to nulls  but uses Java 8 Optional class for parameters that have a default value   void foo String a  Optional lt Integer gt  bOpt        Integer b   bOpt isPresent     bOpt get     0               foo  a   Optional of 2    foo  a   Optional  lt Integer gt absent       Optional makes a method contract explicit for a caller  however  one may find such signature too verbose  Builder pattern  The builder pattern is used for constructors and is implemented by introducing a separate Builder class    class Foo        private final String a        private final Integer b        Foo String a  Integer b           this a   a         this b   b                          class FooBuilder      private String a           private Integer b   0      FooBuilder setA String a         this a   a       return this           FooBuilder setB Integer b         this b   b       return this           Foo build          return new Foo a  b             Foo foo   new FooBuilder   setA  a   build     Maps  When the number of parameters is too large and for most of them default values are usually used  you can pass method arguments as a map of their names values   void foo Map lt String  Object gt  parameters        String a            Integer b   0      if  parameters containsKey  a               if    parameters get  a   instanceof Integer                  throw new IllegalArgumentException                           a    String parameters get  a          else if  parameters containsKey  b                                         foo ImmutableMap  lt String  Object gt of       a    a        b   2        d    value         Please note that you can combine any of these approaches to achieve a desirable result

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One idea is to use String    args public class Sample      void demoMethod String    args          for  String arg   args             System out println arg                   public static void main String args             new Sample   demoMethod  quot ram quot    quot rahim quot    quot robert quot          new Sample   demoMethod  quot krishna quot    quot kasyap quot          new Sample   demoMethod            Output ram rahim robert krishna kasyap  from https   www tutorialspoint com Does-Java-support-default-parameter-values-for-a-method

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Sadly  no

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No  but you can very easily emulate them  What in C   was   public  void myFunction int a  int b 5  string c  test             In Java  it will be an overloaded function   public void myFunction int a  int b  string c           public void myFunction int a  int b        myFunction a  b   test       public void myFunction int a        myFunction a  5       Earlier was mentioned  that default parameters caused ambiguous cases in function overloading  That is simply not true  we can see in the case of the C    yes  maybe it can create ambiguous cases  but these problem can be easily handled  It simply wasn t developed in Java  probably because the creators wanted a much simpler language as C   was - if they had right  is another question  But most of us don t think he uses Java because of its simplicity

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If you really want to  you can check it manually by using null  public MyParameterizedFunction String param1  int param2  boolean param3        if param3    null            param3   false           However i heavily recommend using something else  like overloading or a static factory  Maybe you can get away with this  but it can lead to unexcspected behavior  For example you could have an error in your code  so that your boolean never gets a value  In this case you would not get a NullPointerException  Instead it will look like it was set to false  which can be very confusing to debug

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Try this solution   public int getScore int score  Integer    bonus        if bonus length  gt  0                return score   bonus 0              return score

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A similar approach to https   stackoverflow com a 13864910 2323964 that works in Java 8 is to use an interface with default getters  This will be more whitespace verbose  but is mockable  and it s great for when you have a bunch of instances where you actually want to draw attention to the parameters   public class Foo         public interface Parameters           String getRequired            default int getOptionalInt    return 23            default String getOptionalString    return  Skidoo                public Foo Parameters parameters                            public static void baz             final Foo foo   new Foo new Person                  Override public String getRequired    return  blahblahblah                  Override public int getOptionalInt    return 43

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It is not supported but there are several options like using parameter object pattern with some syntax sugar   public class Foo         private static class ParameterObject           int param1   1          String param2                  public static void main String   args            new Foo   myMethod new ParameterObject      param1   10  param2    bar                  private void myMethod ParameterObject po              In this sample we construct ParameterObject with default values and override them in class instance initialization section   param1   10  param2    bar

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As Scala was mentioned  Kotlin is also worth mentioning  In Kotlin function parameters can have default values as well and they can even refer to other parameters   fun read b  Array lt Byte gt   off  Int   0  len  Int   b size                Like Scala  Kotlin runs on the JVM and can be easily integrated into existing Java projects

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No  but you can use the Builder Pattern  as described in this Stack Overflow answer   As described in the linked answer  the Builder Pattern lets you write code like  Student s1   new StudentBuilder   name  Eli   buildStudent    Student s2   new StudentBuilder                     name  Spicoli                     age 16                    motto  Aloha  Mr Hand                     buildStudent      in which some fields can have default values or otherwise be optional

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You may use Java Method Invocation Builder to automatically generate the builder with default values   Just add  GenerateMethodInvocationBuilder to the class  or interface  and the  Default to parameters in methods where you want default values  A builder will be generated at compile time  using the default values that you specified with your annotations    GenerateMethodInvocationBuilder public class CarService    public CarService          public String getCarsByFilter        Default  Color BLUE   Color color         Default  new ProductionYear 2001    ProductionYear productionYear        Default  Tomas   String owner          return  Filtering        color   productionYear   owner         And then you can invoke the methods   CarService instance   new CarService    String carsByFilter   CarServiceGetCarsByFilterBuilder getCarsByFilter        invoke instance     Or set any of the default values to something else   CarService instance   new CarService    String carsByFilter   CarServiceGetCarsByFilterBuilder getCarsByFilter        withColor Color YELLOW       invoke instance

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You can do this is in Scala  which runs on the JVM and is compatible with Java programs  http   www scala-lang org   i e   class Foo var prime  Boolean   false  val rib  String

[java] Does Java support default parameter values?

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