set implementation based on an array

Question

I was supposed to implement set without using any implemented structures. I would love to know what can I correct in my code, because I am not that much satisfied with its quality, so if you have any ideas I will be so grateful!

import java.util.Objects;
abstract class Set {
 Object elements[] = null;
 abstract Set sum(Set set);
 abstract Set antivalence(Set set);
 abstract Set union(Set set);
 abstract void add(Object x);
 abstract void delete(Object x);
 void print() {
 if(this.elements == null) System.out.print("Set is empty.");
 else for (Object element : this.elements) System.out.print(element);
 System.out.println();
 }
}
class NumSet extends Set {
 private int countReps(Set set) {
 int counter = 0;
 for (Object element1 : this.elements)
 for (Object element2 : set.elements)
 if(Objects.equals(element1, element2)) counter++;
 return counter;
 }
 @Override
 public Set sum(Set set) {
 if(this.elements == null) return set;
 if(set.elements == null) return this;
 Set S = new NumSet();
 boolean flag = false;
 S.elements = new Integer[set.elements.length + this.elements.length - this.countReps(set)];
 for(int i = 0; i < this.elements.length; i++) {
 S.elements[i] = this.elements[i];
 for(int j = 0; j < set.elements.length; j++) {
 for (Object element : this.elements)
 if (set.elements[j] == element) flag = true;
 if(!flag) S.elements[i + 1] = set.elements[j];
 flag = false;
 }
 }
 return S;
 }
 @Override
 public Set antivalence(Set set) {
 if(this.elements == null) return set;
 if(set.elements == null) return this;
 Set A = new NumSet();
 boolean flag = false;
 int counter = 0;
 A.elements = new Integer[set.elements.length + this.elements.length - 2*this.countReps(set)];
 for(int i = 0; i < this.elements.length; i++) {
 for (Object element : set.elements)
 if (this.elements[i] == element) flag = true;
 if(!flag) {
 A.elements[counter] = this.elements[i];
 counter++;
 }
 flag = false;
 }
 for(int i = 0; i < set.elements.length; i++) {
 for (Object element : this.elements)
 if (set.elements[i] == element) flag = true;
 if(!flag) {
 A.elements[counter] = set.elements[i];
 counter++;
 }
 flag = false;
 }
 return A;
 }
 @Override
 public Set union(Set set) {
 Set U = new NumSet();
 if(this.elements == null || set.elements == null) {
 U.elements = null;
 return U;
 }
 boolean flag = false;
 int counter = 0;
 U.elements = new Integer[this.countReps(set)];
 for(int i = 0; i < this.elements.length; i++) {
 for (Object element : set.elements)
 if (this.elements[i] == element) flag = true;
 if(flag) {
 U.elements[counter] = this.elements[i];
 counter++;
 }
 flag = false;
 }
 return U;
 }
 @Override
 public void add(Object x) {
 Integer new_elements[];
 if(this.elements == null) {
 new_elements = new Integer[1];
 new_elements[0] = (Integer)x;
 } else {
 int len = this.elements.length;
 new_elements = new Integer[len + 1];
 for(int i = 0; i < len; i++) new_elements[i] = (Integer) this.elements[i];
 new_elements[len] = (Integer)x;
 }
 this.elements = new_elements;
 }
 @Override
 public void delete(Object x) {
 Integer new_elements[];
 boolean flag = false;
 int i;
 if(this.elements == null) {
 System.out.println("Set is empty! Cannot delete anything.");
 } else {
 int len = this.elements.length;
 for (Object element : this.elements) {
 if (Objects.equals(element, x)) flag = true;
 }
 if(flag) {
 if(len == 1) {
 this.elements = null;
 } else {
 new_elements = new Integer[len - 1];
 for(i = 0; i < len; i++) {
 if (Objects.equals(this.elements[i], x)) break;
 new_elements[i] = (Integer) this.elements[i];
 }
 for(int j = i+1; j < len - 1; j++) new_elements[i] = (Integer) this.elements[j];
 this.elements = new_elements;
 }
 } else System.out.println("No such element!");
 }
 }
}

• yuri: What's with the inconsistent use of space after flow control keywords? Did your IDE do this?
• nuxie: yeah that's my IDE, I haven't noticed that, thanks a lot!

Answer 1

• Use java.lang.System.arraycopy() instead of copying array elements one by one.
• Introduce some private methods, for example one checking if array contains an object
• Either start using generics, remove cast to Integer or actually replace Object with Integer in interface if your implementation can't work on other types of elements
• Consider storing Set elements as sorted array/list, that will allow for faster implementation of operations other than add()