How to create an Object with private constructor without using a static method in the class?

[chaitanya karthikk]

Hi all, I would like to know how to create an object or a class with private constructor.

I know one such way. Creating a static method in the class which will create an object and return the object of the class. The singleton approach.

I think I can use Class.forName(String). But I am not pretty sure.

I hope reflection api also will help but not sure.

Can anyone please tell me what are the different ways to create an object whose class has a private Constructor?

Thank you all in advance. Good day.

[John Jai]

If the constructor is marked private, you will receive IllegalAccessException while trying to create an instance using reflection. Read more on creating instances using reflection here.

[chaitanya karthikk]

John Jai wrote:If the constructor is marked private, you will receive IllegalAccessException while trying to create an instance using reflection. Read more on creating instances using reflection here.

What about Class.forName(String). Is there any way to create an instance using this? I wrote this code

package net.autodata.vwoa.web.controller.admin;


public class Test {

	public static void main(String kk[]) {
		try {
			Class<Sample> c = (Class<Sample>) Class.forName("net.autodata.vwoa.web.controller.admin.Sample");
			Sample sample = (Sample) c.newInstance();
			sample.callMe();
		} catch (ClassNotFoundException e) {
			e.printStackTrace();
		} catch (InstantiationException e) {
			e.printStackTrace();
		} catch (IllegalAccessException e) {
			e.printStackTrace();
		}
	}
}

class Sample {
	private Sample() {}
	
	public void callMe() {
		System.out.println(123);
	}
}

When I tried to execute it i got this error

java.lang.IllegalAccessException: Class net.autodata.vwoa.web.controller.admin.Test can not access a member
 of class net.autodata.vwoa.web.controller.admin.Sample with modifiers "private"
	at sun.reflect.Reflection.ensureMemberAccess(Reflection.java:95)
	at java.lang.Class.newInstance0(Class.java:366)
	at java.lang.Class.newInstance(Class.java:325)
	at net.autodata.vwoa.web.controller.admin.Test.main(Test.java:22)

So now I hope it is not possible with Class.forName(String).

Am I right?

[chaitanya karthikk]

Okay, the document says that Class.forName(String) needs a public zero argument constructor.

[Campbell Ritchie]

chaitanya karthikk wrote: . . . Class.forName(String) needs a public zero argument constructor.

Surely you mean the newInstance() method.

[Randall Twede]

is there a problem with using a static method? the API sure does it often enough. one i particularly like is BigInteger.valueOf(long)

[Rishi Shah]

Class<?> clazz = Class.forName("ClassMoose"); // Not necessarily the best approach, won't work if you use obfuscation
Constructor<?> constructor = clazz.getConstructors[0];
constructor.setAccessible(true);
Object o = constructor.newInstance();

[Campbell Ritchie]

Beware of reflection; you are losing performance and that approach may introduce errors; the original designer wanted that constructor private for a reason.

[Randall Twede]

amen on that!
of course all java programmers add extra(unneeded) modifiers just to show off.

[Campbell Ritchie]

Careful, RT. One of these days somebody will believe you

[chaitanya karthikk]

Rishi Shah wrote:

Class<?> clazz = Class.forName("ClassMoose"); // Not necessarily the best approach, won't work if you use obfuscation
Constructor<?> constructor = clazz.getConstructors[0];
constructor.setAccessible(true);
Object o = constructor.newInstance();

This is not working

package test;

import java.lang.reflect.Constructor;
import java.lang.reflect.InvocationTargetException;

public class PrivateConstructor {
public static void main(String[] args) throws ClassNotFoundException, 
InstantiationException, IllegalAccessException, 
IllegalArgumentException, InvocationTargetException {
	Class<?> clazz = Class.forName("test.Sample"); // Not necessarily the best approach, won't work if you use obfuscation
	Constructor<?>[] constructors = clazz.getConstructors();  
	Constructor constructor=null;
	System.out.println(constructors.length);
	if(constructors.length>0){
		constructor=constructors[0];
		constructor.setAccessible(true);  
		Object o = constructor.newInstance(); 
		Sample s = (Sample) o;
		System.out.println(s);
		s.callMe();
	}
}
}

class Sample {
	private Sample() {}
	
	public void callMe() {
		System.out.println(123);
	}
}

It prints 0. When I make it public the control is entering if condition.

[Darryl Burke]

Rishi and chaitanya, both of you might do well to read the API for Class#getConstructors()

Returns an array containing Constructor objects reflecting all the public constructors of the class represented by this Class object.

[chaitanya karthikk]

Darryl Burke wrote:Rishi and chaitanya, both of you might do well to read the API for Class#getConstructors()

Returns an array containing Constructor objects reflecting all the public constructors of the class represented by this Class object.

I was trying to tell Rishi that it is not possible.

[Rishi Shah]

Darryl Burke wrote:Rishi and chaitanya, both of you might do well to read the API for Class#getConstructors()

Returns an array containing Constructor objects reflecting all the public constructors of the class represented by this Class object.

Yeah, here's the right answer:

final Constructor<CodeRanch> constructor;
constructor = CodeRanch.class.getDeclaredConstructor(String.class); // Parameter is the type of params in the constructor (e.g. String)
constructor.setAccessible(true);
CodeRanch codeRanch = constructor.newInstance("Stringsss");

[ktelagar krish]

Hi Chaitanya,

This code works now. I made a few changes.

 
import java.lang.reflect.Constructor;  
import java.lang.reflect.InvocationTargetException;  
  
public class PrivateConstructor {  
public static void main(String[] args) throws ClassNotFoundException,   
InstantiationException, IllegalAccessException,   
IllegalArgumentException, InvocationTargetException {  
    Class clazz = Class.forName("Sample"); // Not necessarily the best approach, won't work if you use obfuscation  
    Constructor[] constructors = clazz.getDeclaredConstructors();    
    Constructor constructor=null;    
    System.out.println(constructors[0].getName());
   
        constructor=constructors[0];  
        constructor.setAccessible(true);    
        Object o = constructor.newInstance(); 
        System.out.println(o);
        ((Sample) o).callMe();
}  
}  
  
class Sample {  
    private Sample() {}  
      
    public void callMe() {  
        System.out.println(123);  
    }  
} 

[Campbell Ritchie]

Yes, it works, but as previously suggested, setAccessible(true) is cheating. You can set a SecurityManager object which will prohibit that, but I am not sure how to do it.

And welcome to the Ranch

[ktelagar krish]

Hi Campbell,

Yes, Security manager doesnt allow this.and Thank you...

[Campbell Ritchie]

You’re welcome