A stack (array style) data structure implementation done in java along with respective time complexities for each method

ArrayStack.java

/**
  * Array Stack Java Implementation
  * public void push(E value): O(1)
  * public E peek(): O(1)
  * public E pop(): O(1)
  */

import java.util.Objects;

public class ArrayStack<E> {
  private int top;
  private Object[] stack;

  public ArrayStack(final int n) throws Exception {
    if (n < 1) {
      throw new Exception("Cannot Initialize. Capacity to be greater than zero");
    }
    this.stack = new Object[n];
    this.top = -1;
  }

  public void push(final E obj) {
    if (!this.isFull() && !Objects.isNull(obj)) {
      this.stack[++this.top] = obj;
    }
  }

  public E peek() {
    return (E)(this.isEmpty() ? null : this.stack[this.top]);
  }

  public void pop() {
    if (!this.isEmpty()) {
      this.stack[this.top--] = null;
    }
  }

  public boolean isFull() {
    return this.size() == this.stack.length;
  }

  public boolean isEmpty() {
    return this.size() == 0;
  }

  public int size() {
    return this.top + 1;
  }

  @Override
  public String toString() {
    String string = "[";
    if (!this.isEmpty()) {
      for (int i = this.size() - 1; i >= 0; --i) {
        string += (i == 0) ? this.stack[i] : this.stack[i] + " --> ";
      }
    }
    return string += "]";
  }
}