Inheritance IV - Object Methods

List of class Object methods

• String toString()
• boolean equals(Object obj)
• Class <?> getClass()
• int hashCode()
• protected Objectclone()
• protected void finalize()
• void notify()
• void notifyAll()
• void wait()
• void wait(long timeout)
• void wait(long timeout, int nanos)

toString()

• Provides string representatoin
• System.out.println() implicitly call this method

@Override
public String toString() {
  String str = "(";
  ...
  str += ")";
  return str;
}

StringBuilder

• String are inmutable, appending a string creates new copy (time consuming O(n^2))
• Use StringBuilder class to create mutable string object

@Override
public String toString() {
  StringBuilder returnSB = new StringBuilder("{");
  for (int i = 0; i < size - 1; i += 1) {
    returnSB.append(items[i].toString());
    returnSB.append(", ");
  }
  returnSB.append(items[size - 1]);
  returnSB.append("}");
  return returnSB.toString();
}

Simpler toString

@Override
public String toString() {
  List<String> listOfItems = new ArrayList<>();
  for (T x : this) {
    listOfItems.add(x.toString());
  }
  return "{" + String.join(', ', listOfItems) + "}";
}

Extra: ArraySet.of()

public static <T> ArraySet<T> of(T... stuff) {
  ArraySet<T> returnSet = new ArraySet<T>();
  for (T x : stuff) {
    returnSet.add(x);
  }
  return returnSet;
}

• T... stuff Vararg: variable number of arguments
• <T> ArraySet<T> static method do not know what is T (related to object), so we need to specify

equals()

• equals() and == have different behaviour
  • == check same object (memory)
  • equals() defined by implementation

Example: equals() for ArraySet

public boolean equals(Object other) {
  /* Same object */
  if (this == other) {
    return true;
  }
  if (other == null) {
    return false;
  }
  /* Remember to check the type (generic) */
  if (other.getClass() != this.getClass()) {
    return false;
  }
  ArraySet<T> o = (ArraySet<T>) other;
  if (o.size() != this.size()) {
    return false;
  }
  for (T item : this) {
    if (!o.contains(item)) {
      return false;
    }
  }
  return true;
}