Why do I get "java.lang.ArrayIndexOutOfBoundsException: Index 1 out of bounds for length 1"?

Question

student[nStudent] = new Student(arr[0], Integer.parseInt(arr[1]), Integer.parseInt(arr[2]), Integer.parseInt(arr[3])); This line cause "java.lang.ArrayIndexOutOfBoundsException: Index 1 out of bounds for length 1".

How can I solve this problem?

public Student(String name, int physic, int chemistry, int math) {
    /*this.name = name;
    this.physic = physic;
    this.chemistry = chemistry;
    this.math = math;*/
    setName(name);
    setPhysic(physic);
    setChemistry(chemistry);
    setMath(math);
    getName();
    getPhysic();
    getChemistry();
    getMath();

    int sum = this.physic + this.chemistry + this.math;
    this.ave = sum / 3;
}

private static Student[] readData(String filename){
    int nStudent = 0;
    Student[] student = new Student[21];
    String string = null;

    try {
        FileReader fileReader = new FileReader(filename);
        BufferedReader bufferedReader = new BufferedReader(fileReader);

        while((string = bufferedReader.readLine()) != null){
            String[] arr = string.split(" ");
            student[nStudent] = new Student(arr[0], Integer.parseInt(arr[1]), Integer.parseInt(arr[2]), Integer.parseInt(arr[3]));
            nStudent++;
        }
        bufferedReader.close();
    } catch (Exception e) {
        e.printStackTrace();
    }
    return student;
}