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Arrays

Quick Reference

Array Declaration and Initialization

int[] arr = new int[5];        // Fixed-size declaration
int[] arr = {1, 2, 3};         // Initialization
int[][] matrix = new int[3][]; // Jagged array
When to Use Arrays
  • Use arrays when you need a fixed-size collection of elements of the same type.
  • Ideal for scenarios where you need fast access to elements using an index.
  • Arrays are memory-efficient and provide better performance for fixed-size data.
Drawbacks of Arrays
  • Fixed Size: Size cannot be changed after declaration.
  • Homogeneous Data: Only stores elements of the same type.
  • Limited Methods: No built-in methods for resizing, searching, or sorting.

Jagged Arrays

What are Jagged Arrays?

Arrays of arrays where sub-arrays can have different sizes, useful for non-rectangular data like triangles or sparse matrices.

int[][] jaggedArray = new int[3][];
jaggedArray[0] = new int[]{1, 2};
jaggedArray[1] = new int[]{3, 4, 5};
jaggedArray[2] = new int[]{6};

for (int[] row : jaggedArray) {
    for (int num : row) {
        System.out.print(num + " ");
    }
    System.out.println();
}
// Output:
// 1 2
// 3 4 5
// 6

Arrays of Objects

Arrays of Objects
  • Arrays of objects store references to the objects, not the objects themselves.
  • Each object in the array must be manually created and initialized.
  • If an object is not initialized, its reference will be null. In this case, trying to access its properties or methods will result in a NullPointerException.
class Person {
    String name;
    Person(String name) {
        this.name = name;
    }
}

Person[] people = new Person[3];
people[0] = new Person("Alice");
people[1] = new Person("Bob");
people[2] = new Person("Charlie");

for (Person person : people) {
    System.out.println(person.name);
}
// Output:
// Alice
// Bob
// Charlie

Person[] people = new Person[3];
// people[0] is null, accessing it will throw NullPointerException
System.out.println(people[0].name); // Throws NullPointerException