CODEWITHSUNDEEP
javanullpointerexception
Rohit Sharma
Rohit Sharma

Reputation: 83

Why am I getting null pointer exception in this program when using enhanced for loop

I am getting a null pointer exception in the selectAllStudent Method. When I am using foreach loop but When I am using a normal loop it is working fine. Please explain the reason. Thank you

Driver Class

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;

public class Test {

    public static void main(String[] args) {
        Student[] sdb = new Student[2];
        try {
            for (Student s : sdb) {
                s = takeInput();
            }
        } catch (IOException ioe) {
            System.err.println(ioe.getMessage());
        }
        selectAllStudent(sdb);
    }

    static void selectAllStudent(Student[] sdb) {
        for (Student s : sdb) {
            s.printStudentDetails();               // Getting NullPOinterException here
        }
    }
    public static Student takeInput() throws IOException{
        BufferedReader in = new BufferedReader(new InputStreamReader(System.in));
        System.out.println("Enter the details ");
        System.out.print("ROLL      :"); int rollno = Integer.parseInt(in.readLine());
        System.out.print("NAME      :"); String name = in.readLine();
        System.out.print("BRANCH    :"); String branch = in.readLine();
        return (new Student(rollno,name,branch));
    }  
}

Student Class

public class Student {

    private int rollno;
    private String name;
    private String branch;

    Student(int rollno, String name, String branch) {
        this.rollno = rollno;
        this.name = name;
        this.branch = branch;
    }

    void printStudentDetails() {
        System.out.println("ROLLNO  :" + rollno);
        System.out.println("NAME    :" + name);
        System.out.println("BRANCH  :" + branch);
        System.out.println("-------------------");
    }
}

Upvotes: 0

Views: 186

Answers (1)

rgettman
rgettman

Reputation: 178293

You are not assigning a new Student to the array in this for loop.

for (Student s : sdb) {
    s = takeInput();
}

The takeInput method is returning a Student properly, but you've assigned it to a local reference s, not as an element in the array sdb. The elements of the array remain null, and the NullPointerException comes from attempting to call printStudentDetails on null in the selectAllStudent method.

You can convert the enhanced for loop to a standard for loop, assigning the Student with an array access expression.

for (int i = 0; i < sdb.length; i++)
{
    sdb[i] = takeInput();
}

Upvotes: 3

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