As I understand an array consists of fixed number of elements and a variable length argument takes as many number of arguments as you pass (of the same type). But are they same? Can I pass one where the other is expected?
Yes, if you have a method with a varargs parameter like this:
public void foo(String... names)
and you call it like this:
foo("x", "y", "z");
then the compiler just converts that into:
foo(new String[] { "x", "y", "z"});
The type of the names parameter is String[], and can be used just like any other array variable. Note that it could still be null:
String[] nullNames = null;
foo(nullNames);
See the documentation for varargs for more information.
This does not mean that varargs are interchangeable with arrays - you still need to declare the method to accept varargs. For example, if your method were declared as:
public void foo(String[] names)
then the first way of calling it would not compile.
They are the same, array is internally used by JVM when creating varargs methods. So you can treat vararg argument in the same way you treat array so use for instance enhanced for loop
public void method(String... args) {
for(String name : args) {
//do something
}
}
and call it like this or even pass an array
method("a", "b", "c");
method(new String[] {"a", "b", "c"});
See this nice article for further explanation.
A simple test would suggest that they are the same:
public class test {
public static void varArgs(String... strings) {
for (String s : strings) {
System.out.println(s);
}
}
public static void main(String[] args) {
String[] strings = {"string1", "string2", "string3"};
varArgs(strings);
varArgs("string4", "string5", "string6");
}
}
Outputs:
string1
string2
string3
string4
string5
string6
Related
In python, you can do this:
for item in [a, b, c, d]:
some-code
Is something similar possible in java, where you declare the array in the for loop condition area?
My gut reaction is to do this:
public static void main(String[] args) {
for (String string : String myArr[] = {a, b, c, d}) {
some-code
}
}
But that does not work
Note: I did a preliminary search before asking, the similar-seeming question I found (Initializing an array in Java using the 'advanced' for each loop [duplicate]) is different.
Well, you learn something new everyday. Apparently you can initialize an array but you must define the type and not just use an array initializer.
This works
for (String string : new String[] { "a", "b", "c" }) {
//code
}
This doesn't work because it's unaware of type.
for (String string : { "a", "b", "c" }) {
//code
}
You can directly initialize array with the help of Arrays.asList() method call in java.
A sample code snippet is shown as follows:
for(String data: Arrays.asList("a","b","c"){
// code here
}
I am trying to write a generic method printAll which prints an array of integer or character.
Here's the code:
public static void main(String[] args) {
char cArray[] = {'a','b','c','d'};
int iArray[] = {1,2,3,4};
printAll(iArray); // Error at this line--refer below the code
}
public static <T> void printAll(T[] t){
for(T x:t) {
System.out.println(x);
}
}
Error: Exception in thread "main" java.lang.RuntimeException: Uncompilable source code - Erroneous tree type: <.any>
printAll(T[] t) will not accept primitive type arrays. You need to pass arrays of the respective wrapper types:
Character cArray[] = {'a','b','c','d'};
Integer iArray[] = {1,2,3,4};
But, you don't need to frame your own method. Just use the already existing - Arrays.toString() method, which is overloaded for different types of primitive arrays, and Object[] array.
Do not reinvent the wheel, use Arrays.toString or Arrays.deepToString. The former already is overloaded to support arrays of primitive (as noted in the first link that receives a char[]), the latter works only on arrays of class references objects.
You cannot use primitive types with generics. Use Integer (or the corresponding reference type)
Integer iArray[] = {1,2,3,4};
printAll(iArray);
You can always overload the printAll method for each of the primitive types.
I don't believe in doing peoples work for them, more to inform and teach how it can be done.
one easy and simple way to print arrays in java will be done through converting the array to a string then print. This works best as array lists. This can then be manipulated to print in a better format.
for example make an array list and add some values:
ArrayList<String> Array = new ArrayList<String>();
Array.add(1)
Array.add(2)
Array.add(3)
Array.add(4)
Array.add(5)
Then from here this can be printed by changing it to a string:
String str = Array.toString()
System.out.println(str);
this will print the following:
[1,2,3,4,5]
you can then change the format of this by using replaceALL on the string before you print it:
String str = Array.toString().replaceAll("[\\[\\]]","")
This will now give:
1,2,3,4,5
That is pretty much it, from here you could add extra things like System.lineSeparator which you would use to replace thhe "," with a new line.
String str = Array.toString().replaceAll("[\\[\\]]","").replaceAll(",",System.lineSeparator());
This would make it print out like:
1
2
3
4
5
class Generic {
public static<T> void printArray(T[] list) {
for(T in : list) {
System.out.println(in);
}
}
public static void main(String[] args) {
String[] lit=new String[3];
lit[0]="aiq";
lit[1]="abusov";
lit[2]="java";
printArray(lit);
}
}
In java we can define main() method as both these ways.
public static void main(String[] args) {
System.out.println("Hello World");
}
.
public static void main(String... args) {
System.out.println("Hello World");
}
Both method takes array of String arguments. Now consider following scenario.
String[] arr=new String[10]; // valid
String... arr=new String[10];// invalid
Java never allows to create an array like this wayString... arr=new String[10];. But in above method implementation java allows to do so. My question is how java achieve this two different behavior for two scenarios?
...
is a syntax for method arguments and not for variable definitions. The name of this notation is varargs, which is self explanatory i.e variable number of arguments.
Variable argument or varargs(...) in Java used to write more flexible methods which can accept as many argument as you need not for initialization.
... refers to varargs and its main intention to make method more readable.
void method(String... args) {
}
can be called as
method("a"); OR method("a", "b"); OR method("a", "b", "c");
I see no point in using it in variable declaration, we can't do much with
String... a = {"a", "b"}
An array can anyways be declared with dynamic size
String[] arr = {"a"};
OR
String[] arr = {"a", "b"};
You can use varargs in main because a method declared with varargs (...) is bytecode compatible with a declaration of a method with an array argument (for backwards compatibility). That does not mean that the same syntax is allowed for type declarations.
How come the first call to someMethod doesn't compile without being explicit that it's String[]?
It's fine to use an array initializer to create a String[] array but you can't use it to pass an argument. Are the curly braces used in some other fashion for passing arguments that derails how I'd expect this to behave?
public void someMethod(String[] arr){
//do some magic
}
public void makeSomeMagic(){
String[] arr = {"cat", "fish", "cow"};
//Does not compile!
someMethod({"cat", "fish", "cow"});
//This compiles!
someMethod(new String[]{"cat", "fish", "cow"});
//This compiles!
someMethod(arr);
}
The compiler error is the following:
The method someMethod(String[]) in the type Moo is not applicable for the arguments (String, String, String)
You can only use the { "hello", "world" } initialization notation when declaring an array variable or in an array creation expression such as new String[] { ... }.
See Section 10.6 Array Initializers in the Java Language Specification:
An array initializer may be specified in a declaration, or as part of an array creation expression (§15.10), creating an array and providing some initial values
If you don't want to use explicit String[], use:
public void someMethod(String... arr){
//do some magic
}
…
someMethod("cm", "applicant", "lead");
The three periods after the final parameter's type indicate that the final argument may be passed as an array or as a sequence of arguments.
Read more.
Or you can use varargs:
public void someMethod(String... arr){
//do some magic
}
public void makeSomeMagic(){
someMethod("cat", "fish", "cow");
}
It's basically a fancy syntax for an array parameter (vararg must be the last parameter in method signature).
You can use the curly braces to initialize an array. In every else case it is used to define blocks of statments.
Consider the method declaration:
String.format(String, Object ...)
The Object ... argument is just a reference to an array of Objects. Is there a way to use this method with a reference to an actual Object array? If I pass in an Object array to the ... argument - will the resultant argument value be a two-dimensional array - because an Object[] is itself an Object:
Object[] params = ....; // Make the array (for example based on user-input)
String s = String.format("%S has %.2f euros", params);
So the first component of the array (Which is used in the String.format method), will be an array and he will generate:
[class.getName() + "#" + Integer.toHexString(hashCode())]
and then an error because the array size is 1.
The bold sequence is the real question.
This is a second question: Does a ... array/parameter have a name?
From the docs on varargs:
The three periods after the final
parameter's type indicate that the
final argument may be passed as an
array or as a sequence of arguments.
So you can pass multiple arguments or an array.
The following works just fine:
class VarargTest {
public static void main(String[] args) {
Object[] params = {"x", 1.2345f};
String s = String.format("%s is %.2f", params);
System.out.println(s); // Output is: x is 1.23
}
}
You can just pass an array:
public void foo(String... args) {
}
String args[] = new String[10];
foo(args);
The situation you are describing is going to be fairly rare: most of the time, your varargs items will be Strings, or numbers, or Widgets... it will be unusual for them to be Objects (which could be anything) or arrays.
But if the varargs argument is a bunch of Objects or an array type, then your question does arise: you can pass it a single array and then how will the compiler know whether you meant to pass an array (the one you provided), or an series of 1 item which it should PUT into an array for you?
A quick test shows the answer:
public class TestClass {
public static void main(String[] args) {
Object anObject = new Object();
Object[] anArray = new Object[] {anObject, anObject};
System.out.println("object1 = " + anObject);
System.out.println("array1 = " + anArray);
takesArgs();
takesArgs(anObject, anObject);
takesArgs(anArray); // is this the same as array1?
takesArgs(anArray, anArray);
}
public static void takesArgs(Object... stuff) {
System.out.println("The array was " + stuff);
}
}
The result of executing (your exact numbers will vary:
object1 = java.lang.Object#3e25a5
array1 = [Ljava.lang.Object;#19821f
The array was [Ljava.lang.Object;#addbf1
The array was [Ljava.lang.Object;#42e816
The array was [Ljava.lang.Object;#19821f
The array was [Ljava.lang.Object;#9304b1
So the answer is that in ambiguous cases it treats what you passed as the array instead of creating a new array to wrap it. This makes sense as you could always wrap it in an array yourself if you wanted the other interpretation.