Difference between Collection.toArray() and Collection.toArray(Object obj[])

Question

According to java doc for toArray() Returns an array containing all of the elements in this collection.

and toArray(Object obj[]). Returns an array containing all of the elements in this collection; the runtime type of the returned array is that of the specified array.

first toArray() i understand but second toArray(Object obj[]) i can't understand.Please explain with example.

Answer 1

One is generic, the other isn't. toArray() will return Object[] while toArray(T[]) will return an array of type T[].

Sample:

public static void main(String[] args) {
    Object[] baseArray = new ArrayList<String>().toArray();
    System.out.println(baseArray.getClass().getCanonicalName());

    String[] improvArray = new ArrayList<String>().toArray(new String[5]);
    System.out.println(improvArray.getClass().getCanonicalName());
}

Output:

java.lang.Object[]
java.lang.String[]

Answer 2

First of all, the second method is toArray(T[] a) instead of toArray(Object[] a). The T in the first function header is called a type parameter, which means that the actual class it is referring to changes depending on how you call the method. The type parameter can be used by the toArray method to do things using the type T, without knowing what T actually is.

In this example, T would be String:

x.toArray(new String[0])

In this example, T would be Integer:

x.toArray(new Integer[0])

In this example, T would be MyClass:

x.toArray(new MyClass[0])

The method toArray(T[] a) uses the provided type information to return an array of the given type. For example, the first example would return an array of type String, while the second example would return an array of type Integer.

As a result, a call to toArray() produces the same result (An array of Object) as a call to toArray(new Object[0]).

For more information on generics, you can have a look at this tutorial.