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Java Java Data Structures Exploring the Java Collection Framework Lists

Matthew Francis
Matthew Francis
6,967 Points
Posted by Matthew Francis
Matthew Francis
6,967 Points

What does the number in fruit.toArray[0] represent?

Documentation:

https://docs.oracle.com/javase/7/docs/api/java/util/List.html

Code:

        List<String> fruit = new ArrayList<String>();
        fruit.add((String) "pear");
        fruit.add((String)"avocado");
        System.out.printf("%s %n", fruit);
        String[] newFruit = fruit.toArray(new String[0]);/*This means "turn the Array(a String array) into a string array", but what does the number represent inside the []? I tried to read the documentation but I'm still puzzled*/

1 Answer

Evan Demaris
Evan Demaris
64,262 Points

Hi Matthew,

In String[] newFruit = fruit.toArray(new String[0]) the new String[0] is there to ensure that the correct type of object is returned. If the array is initialized to the wrong size, it's replaced with an array of the same type with the correct size by toArray().

Ideally, you would prevent the code from having to replace anything to maximize efficiency, and use something like String[] newFruit = fruit.toArray(new String[fruit.size()]);

Hope that helps!

J.D. Sandifer
J.D. Sandifer
18,813 Points
J.D. Sandifer
J.D. Sandifer
18,813 Points

Evan, it probably doesn't take long to check if the new array of size 0 is large enough to hold the elements from the list newFruit since any non-empty list would make it too small. Remember to avoid Premature Optimization.

However, I would agree with your suggested code based on code readability - it's much easier to understand what's happening there with your change. Thus saving precious brain power and time for whoever else has to look at the code. (And preventing or pre-answering questions like the one that started this conversation. Bravo!)