Why calling methods from container class instead of instances of it in Java

Question

I am trying to learn java collections. But there is a particular syntax that i can't understand about calling collection methods. Like .toString() or .sort() eg ;

I don't understand the reason why we are calling the below methods from Arrays class or Collections class. For printing arrays this is called inside a println() .

Arrays.toString(myArray);

For sorting lists

Collections.sort(myList);

And why not ?

myArray.toString();

myList.sort();

Can any one walk me through it?

Answer 1

The reasons are different for your two examples:

There's no custom implementation of toString in array types. For MyType[].toString() , the developer doesn't get to "override" toString() for the array type. This is why the helper Arrays.toString is provided. The toString implementation on array types is the one inherited from Object , which is almost always not very helpful.

List.sort is a default method in List , introduced with Java 8. Before then, it was possible to sort only by using Collections.sort . If you look at the source of Collections.sort , it delegates the sorting itself to List.sort . As for why this was not in List from the beginning, that can only be answered by the API designers. One reason I can think of, though, is consistency ( Collections has a number of static collections methods, and Java didn't support static methods in interfaces until version 8).

Answer 2

From the first line of the docs :

The collections framework is a unified architecture for representing and manipulating collections, enabling them to be manipulated independently of the details of their representation. It reduces programming effort while increasing performance. It enables interoperability among unrelated APIs, reduces effort in designing and learning new APIs, and fosters software reuse. The framework is based on more than a dozen collection interfaces. It includes implementations of these interfaces and algorithms to manipulate them.

The objective is to reduce code duplication. In the case of myList.sort() , all implementations of the List interface would have to implement the sort function, and each implementation's sort may vary in behaviour. But as a static method on the Collections class, List implementations do not need to implement the sort function. From a user's perspective, you can pass whatever List you have to Collections.sort() and know what is going to happen.

Since Java 8, this functionality could have been implemented with default methods, which is what List.sort(comparator) does. Default methods allow all implementations an interface to receive the implementation defined in the interface, so from Java 8 it is possible to do do myList.sort() , with the caveat that you have to tell it how to sort by providing a comparison function. For example, assuming you have a List<Integer> : mylist.sort(Integer::compareTo) . But the Collections Framework pre-dates default methods, hence the need for Collections.sort() .

As well as Collections.sort() and List.sort(comparator) , you can also use Stream.sorted() : myList.stream().sorted().collect(Collectors.toList());

Answer 3

You cannot call these static methods directly on the object as the arguments won't match. The methods are bound to the class not to a specific instance of it so you have to give the object as an argument. Also, the methods do different things:

Arrays.toString(myArray);

is a convinience method to print the content of an array. If you call myArray.toString() the hashcode of the object is printed.

Answer 4

It is better to use

Arrays.toString(myArray);

instead of

myArray.toString();

Because, if the myArray will be null, then myArray.toString() will throw java.lang.NullPointerException but Arrays.toString(myArray) will return null. Same as with myArray.sort()

Answer 5

Java's toString() for an array is to print [ , followed by a character representing the type of the array's elements , followed by @ then the " identity hash code " of the array (think of it like you would a "memory address").

To get something meaningful in human understandable you need Arrays.toString(myArray);

Arrays don't override toString , There's a static method: java.util.Arrays.toString to rescue.