24. Enumerable Data Structures

[!NOTE] In chapter 6, we learned about standard Arrays (e.g., String[] cars = new String[5];). Standard arrays are incredibly fast, but they have a fatal flaw: their size cannot be changed once created.

If you don't know how many users will sign up for your application today, you cannot use a strictly sized array. You need a data structure that dynamically grows or shrinks instantly as data is added or removed.

Introducing ArrayList

The ArrayList class is a resizable array, which is completely built-in to the java.util package.

Under the hood, an ArrayList is secretly just a standard [] array. When it gets full, Java silently creates a brand new array double the size in memory, copies all the old data over rapidly, and destroys the old array without bothering you.

import java.util.ArrayList;

public class Main {
  public static void main(String[] args) {
    // 1. Creation. We MUST use the primitive wrapper class (Integer, not int)!
    ArrayList<String> cars = new ArrayList<String>();
    
    // 2. Dynamic Addition. We didn't specify a size limit!
    cars.add("Volvo");
    cars.add("BMW");
    cars.add("Ford");
    cars.add("Mazda");
    
    // 3. Retrieval. (We use .get() instead of standard [0] brackets)
    System.out.println(cars.get(0)); // Outputs "Volvo"
    
    // 4. Modification
    cars.set(0, "Opel");
    
    // 5. Deletion. All elements automatically shift to fill the gap safely!
    cars.remove(1); // Deletes BMW
    
    // 6. Checking dynamic size
    System.out.println(cars.size()); // Outputs 3
  }
}

  

Primitive Wrapper Classes

An annoying quirk of ArrayList is that it can only store fully-featured Objects on the Heap. It physically cannot store stack-based primitive variables like int, double, or boolean.

To fix this, Java provides Object wrappers for all primitives. They act identically, but cost slightly more memory.

• Use Integer instead of int
• Use Double instead of double
• Use Boolean instead of boolean
• Use Character instead of char

// ERROR: Syntax error on token "int", Dimensions expected!
// ArrayList<int> myNumber = new ArrayList<int>();

// CORRECT!
ArrayList<Integer> myNumbers = new ArrayList<Integer>();
myNumbers.add(5);
myNumbers.add(10);

  

Looping through ArrayLists

Just like arrays, you can use a classic loop or an enhanced for-each loop. Notice how we use .size() instead of .length!

for (String car : cars) {
    System.out.println(car);
}