Cases that we get NullPointerException are accessing/modifying the field of null object or accessing/modifying the slot of null as if it were an array or taking the length of null as if it were an array.
//Let us have a Person class
public class Person {
private String name;
private int age;
public Person(String name, int age) {
this.name = name;
this.age = age;
}
public String getName(){
return name;
}
public int getAge(){
return age;
}
public String toString(){
return "[Name->"+ getName() +" ,Age->"+getAge()+"]";
}
}
//The main class simulate collection of persons using array
import java.util.Arrays;
public class ListOfPersonIn {
public static void arrayManipulation()
{
Person[] persons=new Person[3]; // Array of Person to conatain 3 persons
Person titi=new Person("Titi", 35);
Person beti=new Person("Beti", 10);
Person nati=new Person("nati", 18);
// Display list of persons
for(Person person:persons){
System.out.println(person.toString());
}
//Double array size, copy the old value to the new array and add new persons
Person[]newPersons=copyArraySize(persons);
System.out.println("Loop through a new Array ");
for(Person person: newPersons){
System.out.println(person.toString());
}
}
// Private method to resize array, copy the old array to the new array and add new list of persons
private static Person [] copyArraySize(Person [] persons)
{
Person[]newPersons=Arrays.copyOf(persons, persons.length*2);
// newPersons[persons.length]=new Person("meti", 50); in this case we get NullPointerException because the new array has length 6 but only 4 data is populated the reaming 2 indices are not populated i.e newArray[4] and newArray[5] are null value so it raised NullPointerException. Not to get NullPointerException just populate all array indices with data
for(int i=persons.length;i< newPersons.length;i++){
newPersons[i]=new Person("meti", 50);//duplicate data, array can’t maintain uniqueness like set
}
return newPersons;
}
public static void main(String[] args) {
arrayManipulation();
}
}