Java default constructor1/6/2024 What is a default constructor A default constructor is a constructor created by the compiler if we do not define any constructor (s) for a class. You will also notice that the constructor access level matches that of the class for which it was generated. Now that you know what a constructor is in Java and how to use it, let's now look into default constructors. Which demonstrates that it is the compiler which adds the default constructor, since it was not in your source code but it is indeed in your compiled class file. Almost all Java developers know that compiler adds a default constructor or better known as a no-argument constructor in every Java class, but many of them. public class Demo Īnd you compile it using the command javac Demo.java, and then you run the decompiler using javap -c Demo, the output should say something like: Compiled from "Demo.java"ġ: invokespecial #1 // Method java/lang/Object."":()V If you had a simple file like Demo.java containing just one class, e.g. If a class does not contain any constructor then during runtime JVM generates a default constructor which is known as system define default constructor. Default constructor can be either user defined or provided by JVM. You should see an executable named javap under $JDK_HOME/bin/ In Java, a constructor is said to be default constructor if it does not have any parameter. The Java Virtual Machine Specification, section 2.12 Constructors.Īlternatively, if you're using Oracle JDK or OpenJDK, you could easily demonstrate this behavior to verify that it is the compiler the one that does all the magic.Īll you need to do is to use the Java decompiler tool that comes with your JDK to see what bytecodes are being generated in your class files.The Java Language Specification (JLS), section 8.8.9 Default Constructor. ![]() ![]() For a formal reference, the nature of the default constructor is explained in both:
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