getDeclaredConstructors

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.

Exercise 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.

Show example 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);
    }
}

Test yourself

Which method returns all constructors, including private ones?
- getDeclaredConstructors
- getConstructors
- getMethods
- getFields
Show answer

getDeclaredConstructors — getDeclaredConstructors returns all constructors.
Match the Constructor methods

getDeclaredConstructors

All constructors
getConstructors

Public constructors
newInstance

Create an instance
getParameters

Get parameters

Complete the newInstance call

Object obj = constructor.____(args);

Show answer

Object obj = constructor.newInstance(args);

Next lesson: Summary - Reflection

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