Collections

Often you need to store many elements: a list of names, a list of scores, etc. Java provides collections in the java.util package. The most used is ArrayList, which implements the List interface.

List and ArrayList

List is an interface: it describes what operations a list supports. ArrayList is a concrete implementation that stores elements in a resizable array. You typically declare the variable as List and create an ArrayList:

java
import java.util.ArrayList;
import java.util.List;

List<String> names = new ArrayList<>();
names.add(\"Alice\");
names.add(\"Bob\");
names.add(\"Carol\");

System.out.println(names.size());   // 3
System.out.println(names.get(0));   // Alice

• List<String> means a list that can only hold String elements. This is a generic type; it gives you type safety and avoids casting.
• get(index) returns the element at that index. Indices start at 0.
• size() returns the number of elements.

The for-each loop

To iterate over every element in a list (or any collection) without using an index, use the enhanced for loop (for-each):

java
for (String name : names) {
    System.out.println(name);
}

Read this as: for each name in names, do something. The variable name holds the current element in each iteration.

Useful List methods

Method	Description
add(element)	Add at the end
add(index, element)	Insert at index
get(index)	Get element at index
set(index, element)	Replace element at index
remove(index)	Remove at index, return value
remove(object)	Remove first matching element
size()	Number of elements
contains(object)	true if the list contains it

Example:

java
names.set(1, \"Bob Smith\");   // replace second element
names.remove(2);              // remove third element
boolean hasAlice = names.contains(\"Alice\");

Other collections

Besides ArrayList, the Java standard library offers LinkedList, HashSet, HashMap, and others. Each has different performance and usage. For a simple ordered list of elements, ArrayList is usually the right choice.