What do we mean when we talk about a class in Java? It\u2019s certainly not a class like you\u2019d have in school, with students and teachers. A class is a blueprint for an object in your code.
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Java is an object-oriented programming language. This means that it includes a number of features that help you write reusable code. Object-oriented programming is useful for programs of all sizes because it helps reduce repetition, which in turn makes your code more readable.
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In this guide, we\u2019re going to talk about two important concepts in Java: the class and the object. We\u2019ll discuss how they work, and how you can implement them, with reference to examples.<\/p>\n\n\n\n
A class is like a blueprint for an object. Whereas a blueprint for a bedroom may say what types of doors and windows are being installed, a class tells your program what methods and values an object should be able to access and store.
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Classes are useful because they allow you to declare a structure for a particular type of data once which you can reference multiple times by declaring objects.
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Let\u2019s define a class called Employee which has one variable and two methods:
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public class Employee {\n\tboolean isOnVacation;\n\tpublic void startVacation() {\n\t\tisOnVacation = true;\n\t\tSystem.out.println(\"You are now on vacation.\");\n\t}\n\tpublic void endVacation() {\n\t\tisOnVacation = false;\n\t\tSystem.out.println(\"You are not on vacation anymore.\");\n\t}\n}<\/pre><\/div>\n\n\n\nIn our code, we\u2019ve defined two methods: one to start an employee\u2019s vacation, and the other to end an employee\u2019s vacation. We have declared a boolean instance variable called isOnVacation to track this information.
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We haven\u2019t yet created any objects of our employee class. We\u2019ve only outlined the blueprint which will later define how those objects take shape.<\/p>\n\n\n\n
What is a Java Object?<\/h2>\n\n\n\n
An object is an instance of a particular class. Let\u2019s create an object using our Employee blueprint from earlier. We\u2019ll create an object called john:
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Employee john = new Employee();<\/pre><\/div>\n\n\n\nThis code creates an instance of the class Employee. Notice that we\u2019ve added two parenthesis after the name of our class, which tells our code to reference the Employee class. The new<\/code> keyword tells our code to create a new object of that class.
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Employee john = new Employee();\njohn.startVacation();\njohn.endVacation();<\/pre><\/div>\n\n\n\nLet\u2019s run our program and see what happens:<\/p>\n\n\n\n
You are now on vacation.\nYou are not on vacation anymore.<\/pre><\/div>\n\n\n\nOur object calls the two methods we defined in our Employee class. This causes the code within those methods to run. When we execute john.startVacation()<\/code>, for example, the value of the isOnVacation<\/code> boolean is set to true, and the message \u201cYou are now on vacation.\u201d is printed to the Java console.
<\/p>\n\n\n\nAlthough we\u2019ve defined the variable isOnVacation<\/code> inside our class, we can access the value it has been assigned in a particular object. Consider this code:
<\/p>\n\n\n\nEmployee john = new Employee();\njohn.startVacation();\njohn.endVacation();\nSystem.out.println(john.isOnVacation);\n<\/pre><\/div>\n\n\n\nThis code will start and end John\u2019s vacation \u2013 like in our last example \u2013 and then it will print out the value of the isOnVacation<\/code> variable to the console:<\/p>\n\n\n\nYou are now on vacation.\nYou are not on vacation anymore.\nfalse<\/pre><\/div>\n\n\n\nThe value of our isOnVacation<\/code> method is false<\/code>.<\/p>\n\n\n\nJava Constructors<\/h2>\n\n\n\n
The constructor method allows you to initialize data in a class. In our example above, we declared a variable in the main body of our class but we didn\u2019t initialize it with a default value.
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Consider the following code:
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class Employee {\n\tboolean isOnVacation;\n\tpublic Employee() {\n\t\tisOnVacation = false;\n\t\tSystem.out.println(\"Initialized values.\");\n\t}\n\t...\n}<\/pre><\/div>\n\n\n\nNow, let\u2019s create an object of our Employee class:
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Employee john = new Employee();\njohn.startVacation();<\/pre><\/div>\n\n\n\nWhen we create our john<\/code> object, our constructor will automatically run. This method allows you to set the default values that you want your class to hold. Our method returns:<\/p>\n\n\n\nInitialized values.\nYou are now on vacation.<\/pre><\/div>\n\n\n\nUpon execution, our constructor sets the value of isOnVacation<\/code> to false. It then prints out Initialized values<\/code> to our console.<\/p>\n\n\n\nCreating Multiple Objects<\/h2>\n\n\n\n
What if we have two employees? Can you still use the same class? Yes! That\u2019s the benefit of using classes: you can declare many objects using the same class.
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Consider the following main program:
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Employee john = new Employee();\njohn.startVacation();\njohn.endVacation();\nSystem.out.println(john.isOnVacation);\nEmployee ian = new Employee();\nian.startVacation();\nSystem.out.println(ian.isOnVacation);<\/pre><\/div>\n\n\n\nLet\u2019s run our code:<\/p>\n\n\n\n
Initialized values.\nYou are now on vacation.\nYou are not on vacation anymore.\nfalse\n \nInitialized values.\nYou are now on vacation.\ntrue<\/pre><\/div>\n\n\n\nWe\u2019ve created two objects of our Employee class: john and ian. In our code, we have started and ended John\u2019s vacation. Then, we created an object for Ian and started his vacation.
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When we print out the value of john.isOnVacation<\/code>, false is returned; when we do the same for Ian\u2019s object, true is returned. That\u2019s because each object of our class is separate. They use the same blueprint, but they can store their own values.<\/p>\n\n\n\n