What do 3 dots next to a parameter type mean in Java?
What do the 3 dots following String in the following method mean?
public void myMethod(String... strings){
// method body
}
Solution 1:
It means that zero or more String objects (or a single array of them) may be passed as the argument(s) for that method.
See the "Arbitrary Number of Arguments" section here: http://java.sun.com/docs/books/tutorial/java/javaOO/arguments.html#varargs
In your example, you could call it as any of the following:
myMethod(); // Likely useless, but possible
myMethod("one", "two", "three");
myMethod("solo");
myMethod(new String[]{"a", "b", "c"});
Important Note: The argument(s) passed in this way is always an array - even if there's just one. Make sure you treat it that way in the method body.
Important Note 2: The argument that gets the ... must be the last in the method signature. So, myMethod(int i, String... strings) is okay, but myMethod(String... strings, int i) is not okay.
Thanks to Vash for the clarifications in his comment.
Solution 2:
That feature is called varargs, and it's a feature introduced in Java 5. It means that function can receive multiple String arguments:
myMethod("foo", "bar");
myMethod("foo", "bar", "baz");
myMethod(new String[]{"foo", "var", "baz"}); // you can even pass an array
Then, you can use the String var as an array:
public void myMethod(String... strings){
for(String whatever : strings){
// do what ever you want
}
// the code above is equivalent to
for( int i = 0; i < strings.length; i++){
// classical for. In this case you use strings[i]
}
}
This answer borrows heavily from kiswa's and Lorenzo's... and also from Graphain's comment.
Solution 3:
It's Varargs :)
The varargs short for variable-length arguments is a feature that allows the method to accept variable number of arguments (zero or more). With varargs it has become simple to create methods that need to take a variable number of arguments. The feature of variable argument has been added in Java 5.
Syntax of varargs
A vararg is secified by three ellipsis (three dots) after the data type, its general form is
return_type method_name(data_type ... variableName){
}
Need for varargs
Prior to Java 5, in case there was a need of variable number of arguments, there were two ways to handle it
If the max number of arguments, a method can take was small and known, then overloaded versions of the method could be created. If the maximum number of arguments a method could take was large or/and unknown then the approach was to put those arguments in an array and pass them to a method which takes array as a parameter. These 2 approaches were error-prone - constructing an array of parameters every time and difficult to maintain - as the addition of new argument may result in writing a new overloaded method.
Advantages of varargs
Offers a much simpler option. Less code as no need to write overloaded methods.
Example of varargs
public class VarargsExample {
public void displayData(String ... values){
System.out.println("Number of arguments passed " + values.length);
for(String s : values){
System.out.println(s + " ");
}
}
public static void main(String[] args) {
VarargsExample vObj = new VarargsExample();
// four args
vObj.displayData("var", "args", "are", "passed");
//three args
vObj.displayData("Three", "args", "passed");
// no-arg
vObj.displayData();
}
}
Output
Number of arguments passed 4
var
args
are
passed
Number of arguments passed 3
Three
args
passed
Number of arguments passed 0
It can be seen from the program that length is used here to find the number of arguments passed to the method. It is possible because varargs are implicitly passed as an array. Whatever arguments are passed as varargs are stored in an array which is referred by the name given to varargs. In this program array name is values. Also note that method is called with different number of argument, first call with four arguments, then three arguments and then with zero arguments. All these calls are handled by the same method which takes varargs.
Restriction with varargs
It is possible to have other parameters with varargs parameter in a method, however in that case, varargs parameter must be the last parameter declared by the method.
void displayValues(int a, int b, int … values) // OK
void displayValues(int a, int b, int … values, int c) // compiler error
Another restriction with varargs is that there must be only one varargs parameter.
void displayValues(int a, int b, int … values, int … moreValues) // Compiler error
Overloading varargs Methods
It is possible to overload a method that takes varargs parameter. Varargs method can be overloaded by -
Types of its vararg parameter can be different. By adding other parameters. Example of overloading varargs method
public class OverloadingVarargsExp {
// Method which has string vararg parameter
public void displayData(String ... values){
System.out.println("Number of arguments passed " + values.length);
for(String s : values){
System.out.println(s + " ");
}
}
// Method which has int vararg parameter
public void displayData(int ... values){
System.out.println("Number of arguments passed " + values.length);
for(int i : values){
System.out.println(i + " ");
}
}
// Method with int vararg and one more string parameter
public void displayData(String a, int ... values){
System.out.println(" a " + a);
System.out.println("Number of arguments passed " + values.length);
for(int i : values){
System.out.println(i + " ");
}
}
public static void main(String[] args) {
OverloadingVarargsExp vObj = new OverloadingVarargsExp();
// four string args
vObj.displayData("var", "args", "are", "passed");
// two int args
vObj.displayData(10, 20);
// One String param and two int args
vObj.displayData("Test", 20, 30);
}
}
Output
Number of arguments passed 4
var
args
are
passed
Number of arguments passed 2
10
20
a Test
Number of arguments passed 2
20
30
Varargs and overloading ambiguity
In some cases call may be ambiguous while we have overloaded varargs method. Let's see an example
public class OverloadingVarargsExp {
// Method which has string vararg parameter
public void displayData(String ... values){
System.out.println("Number of arguments passed " + values.length);
for(String s : values){
System.out.println(s + " ");
}
}
// Method which has int vararg parameter
public void displayData(int ... values){
System.out.println("Number of arguments passed " + values.length);
for(int i : values){
System.out.println(i + " ");
}
}
public static void main(String[] args) {
OverloadingVarargsExp vObj = new OverloadingVarargsExp();
// four string args
vObj.displayData("var", "args", "are", "passed");
// two int args
vObj.displayData(10, 20);
// This call is ambiguous
vObj.displayData();
}
}
In this program when we make a call to displayData() method without any parameter it throws error, because compiler is not sure whether this method call is for displayData(String ... values) or displayData(int ... values)
Same way if we have overloaded methods where one has the vararg method of one type and another method has one parameter and vararg parameter of the same type, then also we have the ambiguity -
As Exp -
displayData(int ... values) and displayData(int a, int ... values)
These two overloaded methods will always have ambiguity.
Solution 4:
This is the Java way to pass varargs (variable number arguments).
If you are familiar with C, this is similar to the ... syntax used it the printf function:
int printf(const char * format, ...);
but in a type safe fashion: every argument has to comply with the specified type (in your sample, they should be all String).
This is a simple sample of how you can use varargs:
class VarargSample {
public static void PrintMultipleStrings(String... strings) {
for( String s : strings ) {
System.out.println(s);
}
}
public static void main(String[] args) {
PrintMultipleStrings("Hello", "world");
}
}
The ... argument is actually an array, so you could pass a String[] as the parameter.
Solution 5:
Arguably, it is an example of syntactic sugar, since it is implemented as an array anyways (which doesn't mean it's useless) - I prefer passing an array to keep it clear, and also declare methods with arrays of given type. Rather an opinion than an answer, though.