Getting error : Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: Index 0 out of bounds for length 0

Question

I am trying to take name input from another class (i.e. from "StudentMarks.class"). I am taking all the user inputs from main class (i.e. from "StudentMarks.class"), but the declaration is done in another class (i.e. value.class).

Although I have initialized the variable size which will be the size of my string array, while taking input for user name it is putting error

Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: Index 0 out of bounds for length 0
    at studentmarks.StudentMarks.main(StudentMarks.java:53)

System.out.printf("\nEnter names : ");
for(int i=0; i<(mar.size); i++){
    mar.name[i]= sc.nextLine();          // Getting error here
}

I know this code is messy because I am in learning phase and I want to try all possible way to take input generate output etc. Generally, I am experimenting with my code.

My code here

package studentmarks;

import java.util.*;

class value{
    
    int size;   // variable to declare size of my array
    
    String name[]=new String[size];   //take name input from user
    float marks[]= new float[size];   //take marks input from user
    float sum=0;
    
    double total(){     
        for(float i: marks) 
            sum+=i;
        return sum;
    }
    
    double average(){
        return sum/(marks.length);
    }
    
    char grade(){
        if(average()>=90)
            return 'A';
        else if(average()>=80 && average()<90)
            return 'B';
        else
            return 'C';    
    }
}

public class StudentMarks {

       public static void main(String[] args) {
        // TODO code application logic here
        
        Scanner sc=new Scanner(System.in);
        value mar=new value();
        
        System.out.printf("Enter size : ");
        mar.size= sc.nextInt();        // taking user input to define array size declared in value.class
        
        System.out.println(mar.size); // checking size was correctly intitalized or not
        
        System.out.printf("\nEnter names : ");//taking name input here (array was declared in value.class)
        for(int i=0; i<(mar.size); i++){
            mar.name[i]= sc.nextLine();   //Getting array index error here
        }
        
        System.out.printf("\nEnter marks : ");//taking marks input here (array was declared in value.class)

        for(int i=0; i<mar.size; i++){
            mar.marks[i]= sc.nextFloat();
        }
        
        //printing result
        for(String i: mar.name){
            System.out.println(i);
        }
        System.out.println(mar.average());
        System.out.println(mar.grade());
    }
    
}

When `value` class is initialised by calling: `value mar=new value();` field size in `value` class is initialised by zero, and `name` array created with 0 length. After that you assigning `mar.size` to value that you read from system input, but it has no any effect on `mar.name` array length, since it is already initialised. — Eliahu, Mar 10 '23 at 11:22

score 3 · Accepted Answer · answered Mar 10 '23 at 11:21

The initialization of your value class¹ happens once. And at that time, size is 0.

int size;

String name[]=new String[size];
float marks[]= new float[size];

This is why you would normally put the size into a constructor. Without doing so, you could do

value mar=new value();

System.out.printf("Enter size : ");
mar.size= sc.nextInt();
mar.marks = new float[mar.size];
mar.name = new String[mar.size];

or, with a constructor

int size;
String[] name;
float[] marks;
public value(int size) {
    this.size = size;
    this.name = new String[size];
    this.marks = new float[size];
}

And then

System.out.print("Enter size : ");
int size = sc.nextInt();
value mar = new value(size);

¹ By Java naming conventions, class names start with a capital letter. value should be MyValue (or similar).

Getting error : Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: Index 0 out of bounds for length 0

1 Answers1