Constructor class in Reflection API
Introduction
- Constructor class in Java is part of the Reflection API and represents a single constructor of a class (either public or private).
-
It is present in the
java.lang.reflectpackage. -
The
Constructorclass allows you to get information about, and instantiate objects using, constructors dynamically at runtime. -
It provides methods to get metadata about a constructor, such as its name, parameter types, modifiers, and also allows creating a new object using
newInstance(). -
Constructorclass inherits theExecutableclass and implements theGenericDeclarationandMemberinterfaces.
Important Methods of Constructor Class
-
Below are some important methods of
Constructorclass:- -
S.No Method Use 1 getName()Returns the name of the constructor (same as class name). 2 getParameterTypes()Returns an array of Classobjects representing the parameter types.3 getModifiers()Returns the Java language modifiers (public, private, etc.) as an int. 4 newInstance(Object... initargs)Creates a new instance of the class represented by this constructor with specified arguments. 5 isVarArgs()Checks if the constructor accepts a variable number of arguments. -
Program:
import java.lang.reflect.*; class Student { public String name; private int age; // Parameterized constructor public Student(String name, int age) { this.name = name; this.age = age; } // Default constructor private Student() { this.name = "Default"; this.age = 0; } public void display() { System.out.println("Name: " + name + ", Age: " + age); } } public class MainApp { public static void main(String[] args) throws Exception { Class<?> c = Student.class; // Get declared constructors for (Constructor<?> constructor : c.getDeclaredConstructors()) { System.out.println("\nConstructor Name: " + constructor.getName()); System.out.println("Modifiers: " + Modifier.toString(constructor.getModifiers())); // Print parameter types Class<?>[] params = constructor.getParameterTypes(); System.out.print("Parameter Types: "); for (Class<?> p : params) System.out.print(p.getSimpleName() + " "); System.out.println(); // Access private constructors constructor.setAccessible(true); // Dynamically create object Object obj; if (params.length == 0) obj = constructor.newInstance(); else obj = constructor.newInstance("John", 25); // Invoke display() method Method displayMethod = c.getMethod("display"); displayMethod.invoke(obj); } } }Output:
Constructor Name: Student Modifiers: private Parameter Types: Name: Default, Age: 0 Constructor Name: Student Modifiers: public Parameter Types: String int Name: John, Age: 25
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