A Single Java Program Can Contain How many Classe...

Ways Of Implementing Multiple Classes In A Single Java Program

In Java, multiple classes can be present in either a nested or non-nested manner. Let's understand how a Java program is compiled with nested and non-nested classes.

Nested class

• In Java, a class within another class is called a nested class. Nested classes are used to group related classes, thus increasing the encapsulation (i.e. grouping similar data under a single unit).
• The nested class can be declared public, private, protected, or default.

Let's create a nested class in Java where ClassA is the outer class and ClassB is the inner class.

ClassA.java :

When the above class is compiled with javac ClassA.java, it produces two class files, ClassA.class and ClassA$ClassB.class, one for the outer ClassA and another for the inner ClassB.

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Multiple non-nested classes

• We can have multiple non-nested classes in a single Java program, but there should be one public top-level class so that the compiler is aware of the starting point.
• The name of the Java file should be the same as the name of the public class in it.

Let's create a Java program with two non-nested classes, ClassA and ClassB, where ClassA is the public class.

ClassA.java :

When the above Java program is compiled with javac ClassA.java, it produces two class files ClassA.class and ClassB.class.

For ClassB in the previous nested classes example, ClassA$ClassB.class file is generated because ClassB is nested inside ClassA. But in the example, the compiler generated ClassB.class because ClassB is a non-nested class.

Example Of Using Two Classes In A Java Program

Nested class

We created a nested class with OuterClass and InnerClass each with a method print().

OuterClass.java :

Main.java :

Output :

Explanation : When we run the Main class, the compiler creates an OuterClass instance and uses it to create the nested InnerClass instance.

Non-nested class

Lets create two non-nested classes ClassA and ClassB each with a method print()

ClassA.java :

Main.java :

Output :

Explanation : When the compiler executes the Main class, it creates ClassA and ClassB instances and calls their print() method.