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?

Limbic

No. However, the Builder pattern can help. – Baron

@Jatin With the Eclipse "Change method signature" refactoring you can add a parameter and provide a default value that existing invokers will use. – Brutus

@ErwinBolwidt Thanks. I am using Android Studio, and it also has the option of refactoring the method and providing default values. Quite useful. – Ratchford

Use the static factory method! – Doak

@temporary_user_name public MyParameterizedFunction(String param1, int param2) is a constructor, not method, declaration. – Sixtieth

Answer 1

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.

Toastmaster

this was in 2009, static anything is pretty much considered harmful in 2015. Type safe fluent Builder instances that enforce complete and valid construction contracts are a much better solution now. – Bastogne

@JarrodRoberson: Static factory methods are no more harmful than new. They are used all the time in new code. Builders for simple value objects are often the result of over-engineering. – Negrito

Answer 2

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.

Spindell

Im curious though, why do we need a builder class when using the builder pattern. I was thinking of Student s1 = new Student().name("Spicolo").age(16).motto("Aloha, Mr Hand); – Unwearied

@ivanceras: It's relevant when classes have required fields, and you don't want to be able to instantiate those classes in an invalid state. So if you just said Student s1 = new Student().age(16); then that would leave you with a Student without a name, which might be bad. If it's not bad, then your solution is fine. – Spindell

@ivanceras: another reason is you might want your class to be immutable after construction, so you wouldn't want methods in it that change its values. – Slipslop

@ivanceras: I used Builders for 3 things - eliminating the multiple argument and fluent initialization, immutability and most importantly I feel to validate the domain object in the build() method.Why create an object instance if is invalid.You can also overload static factory methods like in the above case buildFreshman(), buildSenior() etc – Morna

The use of the Builder pattern here seems rather superfluous for what the OP is asking (default parameter values.) As a matter of fact, I don't see how this provides an answer to the original question at all. – Spite

That's the telescoping constructor. The Builder Pattern of the GoF is somewhat different. – Haller

Answer 3

There are several ways to simulate default parameters in Java:

1. 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.

1. Varargs.

a) All optional parameters are of the same type:

    void foo(String a, Integer... b) {
        Integer b1 = b.length > 0 ? b[0] : 0;
        Integer b2 = b.length > 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 > 0) {
          if (!(b[0] instanceof Integer)) { 
              throw new IllegalArgumentException("...");
          }
          b1 = (Integer)b[0];
        }
        if (b.length > 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.

1. 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.

1. 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 bOpt) { Integer b = bOpt.orElse(0); //... }

   foo("a", Optional.of(2)); foo("a", Optional.absent());

   Optional makes a method contract explicit for a caller, however, one may find such signature too verbose.

2. 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();
   

3. 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<String, Object> 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.<String, Object>of( "a", "a", "b", 2, "d", "value"));

Please note that you can combine any of these approaches to achieve a desirable result.

Marietta

Good explanation. I've never seen return values used like this. For 5) what do the return this do? Also, doesn't FooBuilder().setA("a").build(); since (by definition) the constructor is called first and FooBuilder() returns a value, doesn't this mean .setA("a"): doesn't get the chance to be called? – Tonneson

@Tonneson return this returns the same object on which the method was called (in the example, FooBuilder). This allows chaining of methods in one statement acting on the same object: new FooBuilder().setA(..).setB(..).setC(..) etc as opposed to calling each method in a separate statement. – Pergrim

@Tonneson new FooBuilder() returns a FooBuilder object on which the setA method is called. As setB is not called, this.b retains the default value. Finally the build method is called on this FooBuilder object. The build method creates and returns a Foo object which is set to the variable Foo foo. Notice that the FooBuilder object is not stored in any variable. – Pergrim

Annotation can also be used to create default parameters and is most useful when required for polymorphic collections. docs.oracle.com/javase/tutorial/java/annotations/declaring.html – Banerjee

Java now supports optionals in 1.8, no need for guava :) – Flashbulb

I thought that Optional was discouraged as a parameter type for methods in Java, and that it was primarily intended as a return type. I certainly get flagged on this in IntelliJ IDEA whenever I try to pass Optional<T> for some type T to a method. – Vaticination

Answer 4

Sadly, no.

Escamilla

Is it so sad? Doing so would introduce potentially ambiguous function signatures. – Coverall

@Trey: languages with default parameters often ditch function overloading since it is then less compelling. So no ambiguity. Beside, Scala added the feature in 2.8, and somehow solved the ambiguity issue (since they kept the overloading for compatibility reasons). – Leavening

I believe parameter defaults are bad practice and overloading is a much cleaner solution. You can even hide the default constructor and use static factory methods to return initialized instances. This is also useful for IOC, where you determine the implementation of the interface to return. If you want to optionally assign a property, create a public property, or overload a constructor Having another way to perform the same function does not make it better. It just makes the language more cryptic and is simply lazy programming when this is one of the reasons for function overloading. – Tamie

I fail to see how parameter defaults prevent function overloading. C# for instance allows overrides and also allows for default initializes. Seems like arbitrary choice, not restriction is the reason. – Algy

Works in C# for many years now without problems, even combined with method overloading. Compiler shows a warning on conflicts and that's it. Just do something like writeToFile(String content, boolea append=false); – Hydrophyte

Answer 5

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 <= 1;
    bool param3 = params.length > 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 <= 2;
    assert l < 1 || Boolean.class.isInstance(p[0]);
    assert l < 2 || Integer.class.isInstance(p[1]);
    bool param3 = l > 0 && p[0] != null ? ((Boolean)p[0]).booleanValue() : false;
    int param4 = l > 1 && 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.

Occasionalism

One should never use an assert in production code. Throw an exception. – Reckoner

Boolean != boolean... Boolean.class.isInstance(p[0]); can be written as p[0] instanceof Boolean... – Inurn

-1 This really isn't what varargs is for. This is a hack. -- in this case, using overloads will be way more readable (which is unfortunate, since three extra characters is more readable than 5 extra lines of source...). -- but Java does not support default parameters. – Featheredge

Answer 6

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.

Getz

The main idea of C# default value notation is precisely to avoid this boilerplate coding and have just one constructor instead of many. – Velez

@KolyaIvankov I don't know C#, but I know C++ where the reasoning is the same. I don't know what is the better, but I think, actually the same boilerplate code is generated by the compiler in the case of C++/C# and it is going into the final binary. – Getz

Every programming language is (in particular) a means to avoid an Assembler boilerplate, am I wrong? The question is only whether it gives a handy functionality, or not. – Velez

Being more specific: languages are tools that allow us to write programs in a way we can have control over what is written, compiling is a way to tell a machine what we want from it. A tool is more useful if it allows us to avoid boilerplate. In effect, gnavi asked, if they can avoid precisely the type of boilerplate code you propose as an answer, since C# allows for it. – Velez

Answer 7

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)  {}

Inconveniency

Bring whole new language to get one not so common feature? – Revolute

that has nothing to do with the OP's question – Potaufeu

Answer 8

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.

Rapier

i tried but still need to pass params3 into function for avoid warning error from android studio – Hildehildebrand

What's the necessary imports for Nullable and Optional? – Tolbert

import java.util.Optional; and import javax.annotation.Nullable; – Talavera

Answer 9

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);
}

Faradism

Answer 10

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);

Emigration

The poster asked if this (rather ugly) pattern can be avoided ... ;-) In more modern languages (like c#, Scala) you don't need this extra overloads which only creates more lines of code. Up to some point you can use varargs in the meantime (static int max( int... array ) { }), but they are only a very ugly workaround. – Ewens

Overloading is not ugly and has many benefits, such as different method calls with different signatures can perform different functionality. //This is better public class Foo() { /* This does something */ public void func(String s){ //do something } /* This does something else with b */ public void func(String s, boolean b){ // b was passed } } //Than this public class Foo() { /* This does something unless b = value, then it does something else */ public void func(String s, boolean b = value){ If (b){ // Do Something } else{ // Do something else } } } – Tamie

@Ewens default parameters have nothing to do with "modern language". I used them in Delphi 20 years ago and they probably already existed in Turbo Pascal. – Blanca

Answer 11

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

Allomerism

Answer 12

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<Byte>, off: Int = 0, len: Int = b.size) {
    ...
}

Like Scala, Kotlin runs on the JVM and can be easily integrated into existing Java projects.

Dyslogistic

Answer 13

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.

Kuo

Answer 14

No. In general Java doesn't have much (any) syntactic sugar, since they tried to make a simple language.

Bicollateral

Not quite. The bitter truth is that the team was on a tight schedule and had no time for syntactic sugar. Why else would const and goto be reserved keywords which no implementation? — Especially const is something I miss bitterly — final is no replacement and they knew it. — And if you made the concious decision to never implement goto you won't need to reserve the keyword. — And later in the Java Team cheated by making the Label based break and continue as powerful as a Pascal goto. – Rheims

"Simple, Object-Oriented and Familiar" was indeed a design goal - see oracle.com/technetwork/java/intro-141325.html – Sappanwood

tomjen said: "No. In general Java doesn't have much (any) syntactic sugar, since they tried to make a simple language". So you are saying that removing a lot of unnecessary features from C++ makes a Java a simple language, then tell me why Java does have variadic methods? Why does it have varargs? It is not needed if you can simply use an array of objects instead, am I right? So varargs can be removed from the language, because it is unnecessary. This will make Java more simple than what it is now. Am I right? Overloading also can be removed, because you have infinite names for each method. – Allograph

Answer 15

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";}

Borghese

Answer 16

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);

Edwyna

Answer 17

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.

Subtropics

Answer 18

Try this solution:

public int getScore(int score, Integer... bonus)
{
    if(bonus.length > 0)
    {
        return score + bonus[0];
    }

    return score;
}

Ruvalcaba

Answer 19

constructor like for method

static void popuping() {
    popuping("message", "title");
}
static void popuping(String message) {
    popuping(message, "title");
}
static void popuping(String message, String title){
    JOptionPane.showMessageDialog(null, message,
            title, JOptionPane.INFORMATION_MESSAGE);
}

Matheny

Answer 20

A similar approach to https://mcmap.net/q/44980/-does-java-support-default-parameter-values 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; }
        });
    }
}

Quickfreeze

Answer 21

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
}

Muraida

Answer 22

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.

Genovese

Answer 23

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.

Elma

Answer 24

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
} 

Cameleer

Answer 25

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("ram", "rahim", "robert");
      new Sample().demoMethod("krishna", "kasyap");
      new Sample().demoMethod();
   }
}

Output

ram
rahim
robert
krishna
kasyap

from https://www.tutorialspoint.com/Does-Java-support-default-parameter-values-for-a-method

Kiyohara

Answer 26

You can use the following-

public void mop(Integer x) {
  // Define default values
        x = x == null ? 200 : x;
}

Trevortrevorr

Answer 27

The Straight answer is no.

These answers are pretty old and mainly discuss the alternatives we can have even if java doesn't have a straight way of the default method parameter.

we can deal with this scenario easily with functional Programming in java.

we can utilize the method currying of java functional programming and I think it can be best utilized for such scenarios where we need to have default parameters.

we may need a default parameter in the following situations... Example: Create a method that works on Two different objects and returns the computation

int mutiply(int a,int b){

      return a*b;

  }

if we want to have a method to get multiply by 2, we will have to Override the method like below

int mutiply(int a){

      mutiply(a, 2); // as can not write mutiply(int a,b=2)

  }

Above, 2 is a default value for b, but it's going to be fixed. what if we want to have multiplied by three, now we cannot overload this further and it does feel odd to have multiple methods like this.

This is when we can utilize the method currying technique.

method Currying is the way of converting multi-parameter functions into multiple functions each accepting a single parameter.

Steps to create a Curried method

1. Write a function that accepts a single parameter and returns another function, like below.

    Function<Integer,Function<Integer,Integer>> mutiply = a->{
    return  b -> a*b;
    };
   

2. Let's Simplify and understand the above method.

The Above method accepts only One integer Argument and returns another function with one parameter a as the default value and b as a variable. In case you are confused and start thinking how the method can use a variable from the caller function and don’t know about closures …

"A closure is a function that refers to a free variable in its lexical context." So here, in this case, closure is the returned function when operated on variable multiply and it has a free variable a that has the lexical context of the caller function.

3. Return the multiply method or fix the default value for one of the parameters.

    //multiply is defined in the first step
    mutiplyByTwo = mutiply.apply(2)
   
    mutiplyByThree = mutiply.apply(3);
   
    mutiplyByFour = mutiply.apply(4);;
   

   And so on …

4. Pass the second variable to the returned function to get the result.

   mutiplyByTwo.apply(2);//results 
   mutiplyByTwo.apply(3);//results 6
   
   
   
    mutiplyByThree.apply(2);//results 6
    mutiplyByThree.apply(3);//results 9
   
    mutiplyByFour.apply(2);//results 8
    mutiplyByFour.apply(3);//results 124
   

You can find my original post here ... https://www.linkedin.com/pulse/can-we-have-default-method-parameter-java-rajat-singh/?trackingId=yW84I1fzT6WGStrNPAWn4w%3D%3D

Obligee