Java Equivalent of C# Anonymous Arrays and Lists?
C# lets me make arrays on the fly when I need to pass them into functions. Let's say I have a method called findMiddleItem(String[] items)
. In C#, I can write code like:
findMiddleItem(new String[] { "one", "two", "three" });
It's awesome, because it means I don't have to write:
IList<String> strings = new List<String>();
strings.add("one");
strings.add("two");
strings.add("three");
findMiddleItem(strings.ToArray());
Which sucks, because I don't really care about strings
-- it's just a construct to let me pass a string array into a method that requires it. A method which I can't modify.
So how do you do this in Java? I need to know 开发者_JAVA百科this for array types (eg. String[]) but also generic types (eg. List).
A List and an Array are fundamentally different things.
A List
is a Collection
type, an implementation of an interface.
An Array is a special operating system specific data structure that can only be created through either a special syntax or native code.
Arrays
In Java, the array syntax is identical to the one you are describing:
String[] array = new String[] { "one", "two", "three" };
Reference: Java tutorial > Arrays
Lists
The easiest way to create a List is this:
List<String> list = Arrays.asList("one", "two", "three");
However, the resulting list will be immutable (or at least it won't support add()
or remove()
), so you can wrap the call with an ArrayList constructor call:
new ArrayList<String>(Arrays.asList("one", "two", "three"));
As Jon Skeet says, it's prettier with Guava, there you can do:
Lists.newArrayList("one", "two", "three");
Reference: Java Tutorial > The List Interface
, Lists
(guava javadocs)
VarArgs
About this comment:
It would be nice if we would be able to do findMiddleItem({ "one", "two", "three" });
Java varargs gives you an even better deal:
public void findMiddleItem(String ... args){
//
}
you can call this using a variable number of arguments:
findMiddleItem("one");
findMiddleItem("one", "two");
findMiddleItem("one", "two", "three");
Or with an array:
findMiddleItem(new String[]{"one", "two", "three"});
Reference: Java Tutorial > Arbitrary Number of Arguments
In Java you can construct an array in the same way:
findMiddleItem(new String[] { "one", "two", "three" });
You can't construct a List<T>
in quite the same way, but there are various ways of getting around that, e.g. wrapping an array, or using some of the Guava Lists.*
methods. (Your code trying to call findMiddleItem
with an argument of type IList<string>
wouldn't have compiled, as an IList<string>
isn't necessarily a string[]
.) For example, if findMiddleItem
actually had a parameter of type List<String>
you could use:
findMiddleItem(Lists.newArrayList("one", "two", "three"));
As well as not having collection initializers (or object initializers), Java also doesn't have implicitly typed arrays... your original C# code can be condensed in C# 3 and higher:
findMiddleItem(new[] { "one", "two", "three" });
You can do it the exact same way:
findMiddleItem(new String[] { "one", "two", "three" });
is valid in Java. Assuming that findMiddleItem
is defined as:
findMiddleItem(String[] array)
The same way as in C# findMiddleItem(new String[] { "one", "two", "three" })
;
Also, for future reference, you can construct a List in Java in a slightly less-verbose way*:
List<String> myStringList = new ArrayList<String>() {{
add("a");
add("b");
add("c");
}};
* As Sean pointed out, this can be considered bad practice since it does create an anonymous subclass of ArrayList
.
I think it's the exact same syntax in Java. This works:
public class Main
{
public static void main( String[] args )
{
method1( new String[] {"this", "is", "a", "test"} );
}
private static void method1( String[] params )
{
for( String string : params )
System.out.println( string );
}
}
I think this will work on non-static methods, too.
Apart from using vargs like
findMiddleItem("one", "two", "three", "four", "five");
you can do
findMiddleItem("one,two,three,four,five".split(","));
EDIT: to turn a String into a List you can use a helper method.
public static List<String> list(String text) {
return Arrays.asList(text.split(","));
}
findMiddleItem(list("one,two,three,four,five"));
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