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Module 6 Arrays and Strings - Part 1

2026-03-25 20:34

Tags: #java

Author: Duke Hsu

Arrays

Arrays are used to store multiple values in a single variable, instead of declaring separate variables for each value.

  • One dimensional Array (one row of data )
  • Multidimensional Array( multidimensional of data)
  • In Java , arrays are designed to be homogeneous, meaning they can only store values of a single, consistent data type.
  • To declare an array, define the variable type with square brackets [ ]
  • {} store elements
  • [] declare array or array size
  • [][] multidimensional array
  • , use commas to separate the elements

Three way to declaring Arrays

Declaring and Allocating Memory (Fixed Size)

  • You can declare an array variable and then allocate memory for a specific, fixed number of elements using the new keyword.
  • Elements are automatically initialized to default values
  • 0 for int, null for objects, false for boolean

Important Note

In Java, once an array is defined, its size cannot be changed.

Example: 

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//declaration; type[] arrayName;
int[] myNumbers;
//allocation: new type[size]
myNumbers = new int[5];
//combine declaration and cllocation in one line
String[] friends = new String[5];

//assign values to individual elements using their index (which starts at 0)
friends[0]="Alice";
friends[1]="Trump";
friends[2]="Elon";
friends[3]="KimChiu";
friends[4]="DukeHsu";

Using an Array Initializer (Shortcut Syntax)

  • You can use a shortcut syntax with curly braces{} .
  • The size is automatically determined by the number of values provided .

Example:

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//the most comon and concise way
int[] grades = {90,80,75,66,85,92}; //array of length 6 or size

String[] seasons = {"Winter","Spring","Summer","Autumn"};// size 4


//u can also use the new keyword with the initializer list
double[] money = new double[] {55.4,63.2,100.0,25.5};

Creating Multidimensional Arrays

  • Use multiple sets of square brackets [] []

Example: 

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//declare and allocate a 2D array (3rows, 4columns)
int[][] martix = new int [3][4];

//assign values to martix
martix[0][0] = 1;
martix[0][1] = 2;
martix[0][2] = 3;
martix[0][3] = 4;
martix[1][0] = 5;
martix[1][1] = 6;
martix[1][2] = 7;
martix[1][3] = 8;
martix[2][0] = 9;
martix[2][1] = 10;
martix[2][2] = 11;
martix[2][3] = 12;

//use for loop acces elements
  for(int i=0; i<martix.length; i ++){  //first loop is rows
    for(int j=0; j<martix[i].length; j++){
        System.out.println(martix[i][j]); //second loop is columns
    }

}//output 1-12



//Initialize a 2D array with velus
int[][] twoDarrays ={
    {1,2,3,4}, //use commas to separate the elements.
    {5,6,7,8},
    {9,10,11,12}
};

System.out.println(twoDarrays[0][2]); //output 3

Array table

Columns indx 0 1 2 3
Rows 0 1 2 3 4
1 5 6 7 8
2 9 10 11 12

Create a Array of Object

Steps and Example:

Step 1: 

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//Student class
class Student{

    //attribute
    public String name;
    public int id;

    //constructor to initialize object attributes
    public Student(int id, String name){
        this.id = id;
        this.name = name;
    }
}

Step 2 - 4:

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public class ArrayOfObjectExample{
    public static void main(String[] args){
        //step 2 , declare and instantiate the array of object references
        //all set to null initially
        Student[] students = new Student[3] //4 is array students size 

        //step 3 , Initialize each element with an actual object
        students[0]=new Student(1,"DukeHsu"); //int id, String name
        students[1]=new Student(2,"Steven");
        students[2]=new Student(3,"Jayvee");

        // step 4, access and use the objects - for loop
        for(Student student: students){
            System.out.println("Student ID: "+student.id+ ", Student Name: " +student.name+" .")

        }
    }

}

References