💻AP Computer Science A
2 min read•Last Updated on June 18, 2024
Avanish Gupta
Milo Chang
Avanish Gupta
Milo Chang
Now that we have learned about inheritance, what even allows our classes and objects that we have created to work the way they do? Where do the general characteristics of all objects come from? The answer lies in the Object class.
The Object class is the superclass of all other classes as well as arrays and other data types. The Object class is part of the java.lang package.
When we call a constructor to a "top-level class" that the coder hasn't declared a superclass for, the Object constructor is implicitly called. In other words, the Object constructor is implicitly called when we call a constructor in a class that doesn't explicitly extend another class. This will give the object some properties and methods that are common to all classes.
In the Object class, there are also a few methods that are provided by default. The three that we will look at are as follows:
The hashCode() method returns what is known as a hash code, an integer that is used to represent the memory location of the object. This is usually based on the properties of an object and any two objects with the same properties should have the same hash code.
This goes along with the equals() method that we first learned about in Unit 3. This method determines whether two objects are equal to each other based on whether they have the same properties or not. If two objects are determined to be equal, then they must have the same hash code.
Finally, we have the toString() method from Unit 5. This method returns a string representation of the object that the method is called on. This usually prints the properties of the object, but by default, the method returns objectName + "@" + hashCode() , which in most situations, does not give any useful information about an object.
For the three methods above to achieve their intended results, we usually override these methods and write our own class-specific implementation of these methods to make sure that the methods work the way we want them to.
The equals() method is used to compare two objects for equality. It checks if the values of the objects are the same, rather than comparing their memory addresses.
Term 1 of 8
The equals() method is used to compare two objects for equality. It checks if the values of the objects are the same, rather than comparing their memory addresses.
Term 1 of 8
The equals() method is used to compare two objects for equality. It checks if the values of the objects are the same, rather than comparing their memory addresses.
Term 1 of 8
The java.lang package is a built-in package in Java that provides fundamental classes and interfaces for the Java language. It includes classes such as String, Integer, and Math.
Object class: The Object class is the root class of all classes in Java. It provides common methods that are inherited by all other classes.
System class: The System class contains methods and properties related to the system environment, such as input/output streams and system properties.
Math class: The Math class provides mathematical functions and constants, allowing you to perform calculations in your Java programs.
Properties are characteristics or attributes of an object that define its state. They store data and can be accessed and modified by the object's methods.
Encapsulation: Encapsulation is the concept of bundling data (properties) and methods together within an object to hide implementation details.
Getters and Setters: Getters are methods used to retrieve the value of a property, while setters are methods used to modify or update the value of a property.
Inheritance: Inheritance allows objects to inherit properties from other objects, creating a hierarchy where child objects inherit properties from parent objects.
Methods are functions defined within a class that perform specific tasks or actions when called upon by an object. They can manipulate data, interact with other objects, or return values.
Parameters: Parameters are variables used in method declarations to receive input values when the method is called. They allow methods to accept and work with different data.
Return Type: The return type of a method specifies the type of value that the method will return after its execution. It can be void (no return value) or any other data type.
Overloading: Method overloading is a feature that allows multiple methods within a class to have the same name but different parameters, enabling flexibility and code reusability.
The hashCode() method is used in Java to generate a unique integer value representing an object's identity. It is often overridden in classes when custom equality comparisons need to be made.
equals(): The equals() method is used to compare two objects for equality based on their content or attributes.
Object class: The Object class is the root class in Java from which all other classes are derived. It provides basic methods such as hashCode(), equals(), and toString().
Hash table: A hash table is a data structure that uses hash codes to store and retrieve values efficiently. It is often used for fast lookup operations.
The equals() method is used to compare two objects for equality. It checks if the values of the objects are the same, rather than comparing their memory addresses.
== (double equal sign): The double equal sign is another way to compare objects for equality, but it compares their memory addresses instead of their values.
String: A String is a sequence of characters. In Java, you can use equals() to compare two String objects and check if they have the same content.
compareTo(): The compareTo() method is used to compare two objects based on their natural ordering. It returns an integer value that indicates whether one object is less than, equal to, or greater than another object.
The toString() method is used to convert an object into a string representation. It returns a string that represents the state of the object.
getClass(): This method returns the runtime class of an object. It can be useful when you want to determine the type of an object at runtime.
format(): The format() method is used to create formatted strings by replacing placeholders with actual values. It allows you to control how data is displayed in strings.
StringBuilder: StringBuilder is a mutable sequence of characters that can be modified without creating new instances. It provides efficient ways to build strings dynamically.
Overriding refers to providing a different implementation for a method inherited from a superclass or interface in its subclass or implementing class respectively. It allows the subclass to customize the behavior of inherited methods.
super: The super keyword is used to refer to the superclass or parent class. It can be used to call overridden methods or access superclass members from within a subclass.
final: When applied to a method, the final keyword prevents it from being overridden in any subclasses. It ensures that the method's implementation remains unchanged.
abstract: Abstract methods are declared without an implementation in an abstract class or interface. Subclasses must provide their own implementation for these methods.