Embedded Files
OBJECTIVES
I can:
call Object class methods through inheritance
utilize the Object class through inheritance
The Object class is the superclass of all other classes in Java and a part of the built-in java.lang package.
If a parent class isn't specified using the extends keyword, the class will inherit from the Object class.
What does a class inherit from the Object class?
The two main methods used most commonly on the exam are toString() and equals(Object), from the Object class at the bottom of the Java Quick Reference:
String toString()
boolean equals(Object other)
TOSTRING() METHOD
One commonly overridden method is toString(), which is often used to print out the attributes of an object.
It is a good idea to write your own toString method in every class.
In a subclass, toString() can call the superclass toString() method using super.toString() and then add on its own attributes.
In the following code, the Person class overrides the Object toString() method and the Student class overrides the Person toString() method.
The each add their own attributes!
Notice that the subclass called APStudent extents Student with a new attribute called APScore and overrides the toString() method to call the supeclass method and adds on the APscore.
EQUALS METHOD
One of the important things that gets inherited from the Object superclass is the equals(Object obj) method.
This method is used to test if the current object and the passed object called obj are equal.
But what does it mean?
In the code below, the equals method that is inherited from the Object class only returns true if the two object references refer to the same object:
The equals method inherited from the Object class only returns true when the two references point to the same object as shown in the code above and figure 1 below.
Notice above that only p3 and p4 refer to the same object.
OVERRIDING THE EQUALS METHOD
If you want to change how the inherited equals method works you can override it so that the new method is called instead of the inherited one.
The String class overrides the inherited equals method to return true when the two objects have the same characters in the same order as shown in the code below:
Any class can override the inherited equals method by providing a method with the same method signature (method name and parameter list) and return type.
The provided method will be called instead of the inherited one, which is why we say that the new method overrides the inherited method.
The Person class below overrides the inherited equals method.
This picture from the Java Visualizer shows the object references and objects.
You can step through this code in the Java Visualizer by clicking on this link: OverrideEquals
To write your own equals method, you must:
1) Use the public boolean equals(Object other) method signature
2) Type cast other to your Classname
3) Return whether this object's attribute(s) equals the other object's attribute(s) with == for primitive types like int and double, or equals for reference types like String or another class.
If you need to check multiple attributes, for example a name and an address for Person objects, you can use && to combine tests.
If you are writing an equals method for a subclass, you can call the superclass equals using the super keyword to check the attributes in the superclass and then check the attributes in the subclass.
SUMMARY
The Object class is the superclass of all other classes in Java and a part of the built-in java.lang package.
The following Object class methods and constructors, including what they do and when they are used, are part of the Jave Quick Reference:
String toString( )
boolean equals(Object other)
Subclasses of Object often override the equals and toString methods with class-specific implementations.
EVIDENCE
1) Complete the following Google Form. This form must be 100% correct and includes released AP practice questions. To stop working and return later, hit submit! You can "edit your response" and continue where you left off.
I.7 Evidence
2) In the following code, a bank account class contains the account holder's name and the money balance in the account. As usual, you are encouraged to work in pairs on this challenge. Write the following code and test each part before moving on to the next step:
a. Write a subclass called SavingsAccount that extends Account and adds an interest rate variable.
b. Write a constructor with 3 arguments (name, balance, interest rate) for the SavingsAccount class that uses the super constructor.
c. Write a toString() method for SavingsAccount that returns a call to the super toString() method and the interest rate.
d. Write an equals method for SavingsAccount that calls the superclass equals method and checks that the interest rates are equal.
public class Account
{
private String name;
private double balance;
public Account(String name, double balance)
{
this.name = name;
this.balance = balance;
}
public String toString()
{
return name + ", " + balance;
}
public boolean equals(Object other)
{
Account otherAccount = (Account) other;
return (this.balance == otherAccount.balance) && this.name.equals(otherAccount.name);
}
}
public class Main
{
public static void main(String[] args)
{
Account a1 = new Account("Smith", 1500);
System.out.println(a1);
// Uncomment this code to test SavingsAccount
/*
SavingsAccount a2 = new SavingsAccount("Jones", 1500, 4.5);
SavingsAccount a3 = new SavingsAccount("Jones", 1500, 4.5);
System.out.println(a2);
System.out.println(a2.equals(a3));
*/
}
}
Report abuse