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?
public MyParameterizedFunction(String param1, int param2)
is a constructor, not method, declaration.
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.
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.
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.
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 > 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. 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
Please note that you can combine any of these approaches to achieve a desirable result.
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?
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.
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.
Sadly, no.
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.
assert
in production code. Throw an exception.
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.
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) {}
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);
}
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.
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);
//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 } } }
It is not supported in java as in other language for ex. Kotlin.
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.
No. In general Java doesn't have much (any) syntactic sugar, since they tried to make a simple language.
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
.
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.
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.
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";}
Try this solution:
public int getScore(int score, Integer... bonus)
{
if(bonus.length > 0)
{
return score + bonus[0];
}
return score;
}
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);
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; }
});
}
}
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
}
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.
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
}
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.
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
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);
}
You can use the following-
public void mop(Integer x) {
// Define default values
x = x == null ? 200 : x;
}
Success story sharing
static
anything is pretty much considered harmful in 2015. Type safe fluentBuilder
instances that enforce complete and valid construction contracts are a much better solution now.new
. They are used all the time in new code. Builders for simple value objects are often the result of over-engineering.