getDeclaredConstructors
- Java course
- Advanced
- Lesson 29/82
- 5 min
Learn constructor listing with getDeclaredConstructors.
Constructors with getDeclaredConstructors
Class.getDeclaredConstructors() returns all constructors of a class, including private ones. getConstructors returns only the public constructors.
Example: Constructor<?>[] cs = Book.class.getDeclaredConstructors(). In the returned array, c.getParameterCount() gives the parameter count. You fetch a single constructor not by name but by parameter types: getDeclaredConstructor(String.class).
Creating instances with newInstance
To create a new instance from a Constructor object you use newInstance(args). This is the reflection equivalent of a regular new call.
Example: Book b = (Book) c.newInstance("Java"). Here c is a Constructor<Book>. The parameter count and types must match the constructor; otherwise an exception is thrown. This pattern is the basis of frameworks creating instances from class names.
Practice Create a Book with Constructor.newInstance
Define a Book class with a public String title field and a public Book(String title) constructor. Get the constructor with Book.class.getDeclaredConstructor(String.class). Create an instance with newInstance("Java") and print the title field with System.out.println. Add throws Exception to main.
Starter code
import java.lang.reflect.Constructor;
// Define Book (title field and constructor)
// Get the constructor with getDeclaredConstructor
// Create an instance with newInstance, print titleSample solution
import java.lang.reflect.Constructor;
class Book {
public String title;
public Book(String title) {
this.title = title;
}
}
public class NewInstanceExample {
public static void main(String[] args) throws Exception {
Constructor<Book> c = Book.class.getDeclaredConstructor(String.class);
Book b = c.newInstance("Java");
System.out.println(b.title);
}
}Expected output
Java
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Which method returns all constructors, including private ones?
- getDeclaredConstructors Correct answer
- getConstructors
- getMethods
- getFields
Explanation: getDeclaredConstructors returns all constructors.
2 Match the Constructor methods
| Term | Meaning |
|---|---|
getDeclaredConstructors |
All constructors |
getConstructors |
Public constructors |
newInstance |
Create an instance |
getParameters |
Get parameters |
3 Complete the newInstance call
Object obj = constructor.____(args);Object obj = constructor.newInstance(args);Explanation: newInstance creates a new instance.