Object: The Cosmic Superclass

All classes defined without an explicit extends clause automatically extend Object

• All classes defined without an explicit extends clause automatically extend Object
• Most useful methods:
  • String toString()
  • boolean equals(Object otherObject)
  • Object clone()
• Good idea to override these methods in your classes

Overriding the toString Method

• Returns a string representation of the object

• Useful for debugging:

Rectangle box = new Rectangle(5, 10, 20, 30);
String s = box.toString();
// Sets s to java.awt.Rectangle[x=5,y=10,width=20,height=30]"

• toString is called whenever you concatenate a string with an object:

"box=" + box;
// Result: "box=java.awt.Rectangle[x=5,y=10,width=20,height=30]"

• Object.toString prints class name and the hash code of the object

• BankAccount momsSavings = new BankAccount(5000);
  String s = momsSavings.toString();
  // Sets s to something like "BankAccount@d24606bf"

• To provide a nicer representation of an object, override toString:

public String toString()
{
   return "BankAccount[balance=" + balance + "]";
}

• This works better:

BankAccount momsSavings = new BankAccount(5000);
String s = momsSavings.toString();
// Sets s to "BankAccount[balance=5000]"

Overriding the equals Method

• Equals tests for equal contents

• Define the equals method to test whether two objects have equal state

• When redefining equals method, you cannot change object signature; use a cast instead:

public class Coin
{
   . . .
   public boolean equals(Object otherObject)
   {
      Coin other = (Coin) otherObject;
      return name.equals(other.name) && value ==
         other.value;
   }
   . . .
}

• You should also override the hashCode method so that equal objects have the same hash code

Should the call x.equals(x) always return true?

   Answer: It certainly should – unless, of course, x is null.

Can you implement equals in terms of toString? Should you?

   Answer: If toString returns a string that describes all
   instance fields, you can simply call toString on the implicit
   and explicit parameters, and compare the results. However,
   comparing the fields is more efficient than converting them into
   strings.

Overriding the clone Method

• Copying an object reference gives two references to same object

• BankAccount account2 = account;

• Sometimes, need to make a copy of the object

• Define clone method to make new object (see Advanced Topic 10.6)

• Use clone:

BankAccount clonedAccount = (BankAccount)account.clone();

• Must cast return value because return type is Object

• Creates shallow copies

• Does not systematically clone all subobjects

• Must be used with caution

• It is declared as protected; prevents from accidentally calling x.clone() if the class to which x belongs hasn't redefined clone to be public

• You should override the clone method with care (see Advanced Topic 10.6)