{"id":6560,"date":"2024-04-21T23:56:41","date_gmt":"2024-04-22T06:56:41","guid":{"rendered":"https:\/\/zentut.com\/java-tutorial\/java-object\/"},"modified":"2024-04-21T23:57:11","modified_gmt":"2024-04-22T06:57:11","slug":"java-object","status":"publish","type":"page","link":"https:\/\/www.zentut.com\/java-tutorial\/java-object\/","title":{"rendered":"Java Object"},"content":{"rendered":"\n

Summary<\/strong>: in this tutorial, you will learn about Java Object<\/code> which serves as the base class for all classes in Java.<\/p>\n\n\n\n

Introduction to Java Object<\/h2>\n\n\n\n

The java.lang.Object<\/code> class is the base class of all classes<\/a> in Java.<\/p>\n\n\n\n

It means that when you define a class, it is implicitly a subclass of the Object<\/code> class. Also, the class automatically inherits a set of methods from the Object<\/code> class.<\/p>\n\n\n\n

Suppose that you have a Person<\/code> class with two fields ssn<\/code> and name<\/code>, and a constructor<\/a>:<\/p>\n\n\n

class<\/span> Person<\/span> <\/span>{\n    private<\/span> String ssn;\n    private<\/span> String name;\n\n    \/\/ Getters and setters (omitted for brevity)<\/span>\n\n    public<\/span> Person<\/span>(String ssn, String name)<\/span><\/span>{\n        this<\/span>.ssn = ssn;\n        this<\/span>.name = name;\n    }\n}<\/code><\/span>Code language:<\/span> Java<\/span> (<\/span>java<\/span>)<\/span><\/small><\/pre>\n\n\n
It is like defining the Person<\/code> class that extends the Object<\/code> class:<\/p>\n\n\n
class<\/span> Person<\/span> extends<\/span> Object<\/span> <\/span>{\n   \/\/ ...<\/span>\n}<\/code><\/span>Code language:<\/span> Java<\/span> (<\/span>java<\/span>)<\/span><\/small><\/pre>\n\n\n
However, it is not necessary since Java does it implicitly for us.<\/p>\n\n\n\n
We’ll explore the methods provided by the Object<\/code> class and demonstrate how to override them to enhance the utility of your class.<\/p>\n\n\n\n
Java Object methods<\/h2>\n\n\n\n
1) getClass()<\/h3>\n\n\n\n
The getClass()<\/code> method returns the runtime class of an object:<\/p>\n\n\n
var<\/span> cls = obj.getClass();<\/code><\/span>Code language:<\/span> Java<\/span> (<\/span>java<\/span>)<\/span><\/small><\/pre>\n\n\n
In this syntax, the cls<\/code> is an instance of the java.lang.Class<\/code> class.<\/p>\n\n\n\n
Typically, you use the getClass()<\/code> method to check if two objects are instances of the same class.<\/p>\n\n\n\n
If the getClass()<\/code> method of two objects returns the same value, they are instances of the same class.<\/p>\n\n\n\n
For example, the following shows the return value of the getClass()<\/code> method of a Person<\/code> object:<\/p>\n\n\n
public<\/span> class<\/span> App<\/span> <\/span>{\n    public<\/span> static<\/span> void<\/span> main<\/span>(String[] args)<\/span> <\/span>{\n        var<\/span> person = new<\/span> Person(\n                \"212-15-8397\"<\/span>,\n                \"John Doe\"<\/span>\n        );\n        System.out.println(person.getClass());\n    }\n}<\/code><\/span>Code language:<\/span> Java<\/span> (<\/span>java<\/span>)<\/span><\/small><\/pre>\n\n\n
Output:<\/p>\n\n\n
class<\/span> Person<\/span><\/span><\/code><\/span>Code language:<\/span> Java<\/span> (<\/span>java<\/span>)<\/span><\/small><\/pre>\n\n\n
2) equals(Object obj)<\/h3>\n\n\n\n
By default, the equals()<\/code> method returns true if two variables reference the same object. <\/p>\n\n\n\n
Typically, you override the equals()<\/code> method to compare two objects by their contents.<\/p>\n\n\n\n
The following example creates two Person objects with the same ssn<\/code> and name<\/code>, and uses the equals()<\/code> method to compare them:<\/p>\n\n\n
public<\/span> class<\/span> App<\/span> <\/span>{\n    public<\/span> static<\/span> void<\/span> main<\/span>(String[] args)<\/span> <\/span>{\n        var<\/span> p1 = new<\/span> Person(\"212-15-8397\"<\/span>, \"John Doe\"<\/span>);\n        var<\/span> p2 = new<\/span> Person(\"212-15-8397\"<\/span>, \"John Doe\"<\/span>);\n        var<\/span> result = p1.equals(p2);\n        System.out.println(result);\n    }\n}<\/code><\/span>Code language:<\/span> Java<\/span> (<\/span>java<\/span>)<\/span><\/small><\/pre>\n\n\n
Output:<\/p>\n\n\n
false<\/span><\/code><\/span>Code language:<\/span> Java<\/span> (<\/span>java<\/span>)<\/span><\/small><\/pre>\n\n\n
Since the Person class inherits the equals()<\/code> method from the Object class, the equals()<\/code> method checks if two variables reference the same object:<\/p>\n\n\n\n
The program displays false because p1<\/code> and p2<\/code> variables reference different Person<\/code> objects even though they have the same ssn<\/code> and name<\/code> values.<\/p>\n\n\n
import<\/span> java.util.Objects;\n\nclass<\/span> Person<\/span> <\/span>{\n    private<\/span> String ssn;\n    private<\/span> String name;\n\n    \/\/ Getters and setters (omitted for brevity)<\/span>\n\n    public<\/span> Person<\/span>(String ssn, String name)<\/span> <\/span>{\n        this<\/span>.ssn = ssn;\n        this<\/span>.name = name;\n    }\n\n    @Override<\/span>\n    public<\/span> boolean<\/span> equals<\/span>(Object obj)<\/span> <\/span>{\n\n        if<\/span> (this<\/span> == obj) return<\/span> true<\/span>;\n\n        if<\/span> (obj == null<\/span> || obj.getClass() != getClass()) {\n            return<\/span> false<\/span>;\n        }\n\n        Person other = (Person) obj;\n\n        return<\/span> ssn.equals(other.ssn) && name.equals(other.name);\n\n    }\n}<\/code><\/span>Code language:<\/span> Java<\/span> (<\/span>java<\/span>)<\/span><\/small><\/pre>\n\n\n
In this Person<\/code> class, we override the equals()<\/code> method of the Object<\/code> class that returns true if all the fields of the of the Person<\/code> objects are equal.<\/p>\n\n\n\n
First, check if the object being compared is the same as the current instance and return true if they are the same reference:<\/p>\n\n\n
if<\/span> (this<\/span> == obj) return<\/span> true<\/span>;<\/code><\/span>Code language:<\/span> Java<\/span> (<\/span>java<\/span>)<\/span><\/small><\/pre>\n\n\n
Second, return false if the object being compared is null or if its class differs from the class of the current instance:<\/p>\n\n\n
if<\/span> (obj == null<\/span> || obj.getClass() != getClass()) {\n   return<\/span> false<\/span>;\n}<\/code><\/span>Code language:<\/span> Java<\/span> (<\/span>java<\/span>)<\/span><\/small><\/pre>\n\n\n
Third, compare the ssn<\/code> and name<\/code> fields of the object being compared with the current instance and return true if they are equal:<\/p>\n\n\n
return<\/span> ssn.equals(other.ssn) && name.equals(other.name);<\/code><\/span>Code language:<\/span> Java<\/span> (<\/span>java<\/span>)<\/span><\/small><\/pre>\n\n\n
Now, if two Person<\/code> objects have the same ssn<\/code> and name<\/code>, they should be equal when you compare them using the equals()<\/code> method:<\/p>\n\n\n
public<\/span> class<\/span> App<\/span> <\/span>{\n    public<\/span> static<\/span> void<\/span> main<\/span>(String[] args)<\/span> <\/span>{\n        var<\/span> p1 = new<\/span> Person(\"212-15-8397\"<\/span>, \"John Doe\"<\/span>);\n        var<\/span> p2 = new<\/span> Person(\"212-15-8397\"<\/span>, \"John Doe\"<\/span>);\n\n        var<\/span> result = p1.equals(p2);\n\n        System.out.println(result); \/\/ true<\/span>\n    }\n}<\/code><\/span>Code language:<\/span> Java<\/span> (<\/span>java<\/span>)<\/span><\/small><\/pre>\n\n\n
3) hashCode()<\/h3>\n\n\n\n
The hashCode()<\/code> method returns a unique integer that represents an object. If two objects are equal, they should have the same hash code. <\/p>\n\n\n\n
Therefore, if you override the equals()<\/code> method, you should also override the hashcode()<\/code> method.<\/p>\n\n\n\n
To generate a hash code for a sequence of input values, you can use the static method hash()<\/code> of the Objects<\/code> class from java.util<\/code> package.<\/p>\n\n\n\n
Here’s the modified version of the Person<\/code> class that includes a hashCode<\/code> method:<\/p>\n\n\n
import<\/span> java.util.Objects;\n\nclass<\/span> Person<\/span> <\/span>{\n    private<\/span> String ssn;\n    private<\/span> String name;\n\n    \/\/ Getters and setters (omitted for brevity)<\/span>\n\n    public<\/span> Person<\/span>(String ssn, String name)<\/span> <\/span>{\n        this<\/span>.ssn = ssn;\n        this<\/span>.name = name;\n    }\n\n    @Override<\/span>\n    public<\/span> boolean<\/span> equals<\/span>(Object obj)<\/span> <\/span>{\n\n        if<\/span> (this<\/span> == obj) return<\/span> true<\/span>;\n\n        if<\/span> (obj == null<\/span> || obj.getClass() != getClass()) {\n            return<\/span> false<\/span>;\n        }\n\n        Person other = (Person) obj;\n\n        return<\/span> ssn.equals(other.ssn) && name.equals(other.name);\n\n    }\n\n    @Override<\/span>\n    public<\/span> int<\/span> hashCode<\/span>()<\/span> <\/span>{\n        return<\/span> Objects.hash(ssn, name);\n    }\n}<\/code><\/span>Code language:<\/span> Java<\/span> (<\/span>java<\/span>)<\/span><\/small><\/pre>\n\n\n
The following displays the hash codes of the p1<\/code> and p2<\/code> objects:<\/p>\n\n\n
public<\/span> class<\/span> App<\/span> <\/span>{\n    public<\/span> static<\/span> void<\/span> main<\/span>(String[] args)<\/span> <\/span>{\n        var<\/span> p1 = new<\/span> Person(\"212-15-8397\"<\/span>, \"John Doe\"<\/span>);\n        var<\/span> p2 = new<\/span> Person(\"212-15-8397\"<\/span>, \"John Doe\"<\/span>);\n\n        var<\/span> result = p1.equals(p2);\n\n        System.out.println(result); \/\/ true<\/span>\n\n        System.out.println(p1.hashCode());\n        System.out.println(p2.hashCode());\n\n    }\n}<\/code><\/span>Code language:<\/span> Java<\/span> (<\/span>java<\/span>)<\/span><\/small><\/pre>\n\n\n
Output:<\/p>\n\n\n
true<\/span>\n-1693213186<\/span>\n-1693213186<\/span><\/code><\/span>Code language:<\/span> Java<\/span> (<\/span>java<\/span>)<\/span><\/small><\/pre>\n\n\n
4) toString()<\/h3>\n\n\n\n
The toString()<\/code> method returns a string representation of an object. The following displays the default string representation of a Person<\/code> object:<\/p>\n\n\n
public<\/span> class<\/span> App<\/span> <\/span>{\n    public<\/span> static<\/span> void<\/span> main<\/span>(String[] args)<\/span> <\/span>{\n        var<\/span> person = new<\/span> Person(\n                \"212-15-8397\"<\/span>,\n                \"John Doe\"<\/span>\n        );\n        System.out.println(person);\n    }\n}<\/code><\/span>Code language:<\/span> Java<\/span> (<\/span>java<\/span>)<\/span><\/small><\/pre>\n\n\n
Output:<\/p>\n\n\n
Person@9<\/span>b139dfe<\/code><\/span>Code language:<\/span> Java<\/span> (<\/span>java<\/span>)<\/span><\/small><\/pre>\n\n\n
When we pass the Person<\/code> object to the println()<\/code> method, it invokes the toString()<\/code> method of the Person<\/code> object and displays the resulting string on the screen. <\/p>\n\n\n\n
As you can see clearly from the output, the default return value of the toString()<\/code> is not human-readable.<\/p>\n\n\n\n
Usually, you override the toString()<\/code> method to provide the human-readable string representation of the object. For example:<\/p>\n\n\n
import<\/span> java.util.Objects;\n\nclass<\/span> Person<\/span> <\/span>{\n    private<\/span> String ssn;\n    private<\/span> String name;\n\n    \/\/ Getters and setters (omitted for brevity)<\/span>\n\n    public<\/span> Person<\/span>(String ssn, String name)<\/span> <\/span>{\n        this<\/span>.ssn = ssn;\n        this<\/span>.name = name;\n    }\n\n    @Override<\/span>\n    public<\/span> boolean<\/span> equals<\/span>(Object obj)<\/span> <\/span>{\n\n        if<\/span> (this<\/span> == obj) return<\/span> true<\/span>;\n\n        if<\/span> (obj == null<\/span> || obj.getClass() != getClass()) {\n            return<\/span> false<\/span>;\n        }\n\n        Person other = (Person) obj;\n\n        return<\/span> ssn.equals(other.ssn) && name.equals(other.name);\n\n    }\n\n    @Override<\/span>\n    public<\/span> int<\/span> hashCode<\/span>()<\/span> <\/span>{\n        return<\/span> Objects.hash(ssn, name);\n    }\n\n    @Override<\/span>\n    public<\/span> String toString<\/span>()<\/span> <\/span>{\n        return<\/span> \"Person{\"<\/span> +\n                \"ssn='\"<\/span> + ssn + '\\''<\/span> +\n                \", name='\"<\/span> + name + '\\''<\/span> +\n                '}'<\/span>;\n    }\n}<\/code><\/span>Code language:<\/span> Java<\/span> (<\/span>java<\/span>)<\/span><\/small><\/pre>\n\n\n
In this example, we override the toString() method of the Person<\/code> class to display all the field names and values.<\/p>\n\n\n\n
If you run the program that displays a Person<\/code> object on the screen, you’ll see something like this:<\/p>\n\n\n
Person{ssn='212-15-8397'<\/span>, name='John Doe'<\/span>}<\/code><\/span>Code language:<\/span> Java<\/span> (<\/span>java<\/span>)<\/span><\/small><\/pre>\n\n\n
clone() & finalize()<\/h3>\n\n\n\n
The Object<\/code> class also has the following methods:<\/p>\n\n\n\n
    \n
  • clone()<\/code>: This method creates a shallow copy of an object <\/li>\n\n\n\n
  • finalize()<\/code>: This method performs the cleanup operations. <\/li>\n<\/ul>\n\n\n\n
    In practice, you’ll rarely use these methods so we don’t cover them in this tutorial. <\/p>\n\n\n\n
    Summary<\/h2>\n\n\n\n
      \n
    • In Java, every class is a subclass of the java.lang.Object<\/code> class.<\/li>\n\n\n\n
    • The Java Object<\/code> class provides a number of useful methods for overriding in the subclass including equals()<\/code>, hashCode()<\/code>, and toString()<\/code>.<\/li>\n\n\n\n
    • Override equals()<\/code> and hashCode()<\/code> to provide logic for comparing two objects by their contents.<\/li>\n\n\n\n
    • Override toString()<\/code> method to provide a meaningful string representation of an object.<\/li>\n\n\n\n
    • Use the getClass()<\/code> method to check the object’s type at runtime.<\/li>\n<\/ul>\n","protected":false},"excerpt":{"rendered":"
      Summary: in this tutorial, you will learn about Java Object which serves as the base class for all classes in Java. Introduction to Java Object The java.lang.Object class is the base class of all classes in Java. It means that when you define a class, it is implicitly a subclass of the Object class. Also, […]<\/p>\n","protected":false},"author":2,"featured_media":0,"parent":2190,"menu_order":39,"comment_status":"closed","ping_status":"closed","template":"","meta":{"footnotes":""},"class_list":["post-6560","page","type-page","status-publish","hentry"],"_links":{"self":[{"href":"https:\/\/www.zentut.com\/wp-json\/wp\/v2\/pages\/6560","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.zentut.com\/wp-json\/wp\/v2\/pages"}],"about":[{"href":"https:\/\/www.zentut.com\/wp-json\/wp\/v2\/types\/page"}],"author":[{"embeddable":true,"href":"https:\/\/www.zentut.com\/wp-json\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"https:\/\/www.zentut.com\/wp-json\/wp\/v2\/comments?post=6560"}],"version-history":[{"count":2,"href":"https:\/\/www.zentut.com\/wp-json\/wp\/v2\/pages\/6560\/revisions"}],"predecessor-version":[{"id":6562,"href":"https:\/\/www.zentut.com\/wp-json\/wp\/v2\/pages\/6560\/revisions\/6562"}],"up":[{"embeddable":true,"href":"https:\/\/www.zentut.com\/wp-json\/wp\/v2\/pages\/2190"}],"wp:attachment":[{"href":"https:\/\/www.zentut.com\/wp-json\/wp\/v2\/media?parent=6560"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}