when you extend a private class. Are the public and protected members of class become private. if not any explanation.
if you extend a nested private class, it wont change public/protected modifiers of the members. Here is an example :
public class Clazz {
private static class NestedClazz {
public int value = 123;
}
public static class NestedClazzExt extends NestedClazz {
}
}
you can now access the inherited member: value from outside
public static void main(String[] args) {
NestedClazzExt nestedClazz = new Clazz.NestedClazzExt();
System.out.println(nestedClazz.value);
}
you can create private class in side a class . We call it as Nested classe. Means a class inside a class. The Concept itself is saying that you can create private class in side another class. The private class will act like as data member to the outer class.
So, You can't extend the private class.
Based on your query I tried to prepare a simple class.
public class pvtClass {
private class As {
public String abc = "private attribute";
public void print(){
System.out.println("privateClass");
}
}
class Ab extends As{
public String ab = "extended attribute";
public void printAb(){
System.out.println("extended class");
print();
System.out.println(abc);
}
}
public static void main(String as[]){
Ab ab1 = (new pvtClass()).new Ab();
As as1 = (new pvtClass()).new As();
ab1.printAb();
as1.print();
System.out.println(as1.abc);
}
}
If you have a look at this class, I have a private class named "As" which has public attribute and public methods. I have another class named "Ab" which extends "As". I have written a main method to invoke the private attribute and methods.
below is the output for the code snippet:
extended class
privateClass
private attribute
privateClass
private attribute
There is a difference between the access of the members of a class and the access to the type itself.
public class C {
private class InnerP1 {
public void m() {
System.out.println("InnerP1.m()");
}
}
private class InnerP2 extends InnerP1 {
public void p() {
this.m();
System.out.println("InnerP2.p()");
}
}
public InnerP1 strange() {
return new InnerP2();
}
}
In this example, the interface I is visible from outside class C. The classes InnerP1 and InnerP2 are not visible from outside C. Jave itself makes not restrictions to the visibility of types you use in your public interface. The method strange() of class C returns a result of class InnerP1. Since outside of C we do not know anything about the class InnerP1 other than it is subtype of Object, the only thing we can do is use the result of strange() as an Object.
public class D {
public static void main(String[] args) {
C c = new C();
Object o = c.strange();
if(o.equals(c.strange())) {
System.out.println("Strange things are going on here!");
}
}
}
As #KnusperPudding pointed out already, the visiblity of public members is not changed, we might just not have enough knowledge of the type itself to access them.
Access to members cannot be restricted by sub-classing. When you mark a class as private then access via the class name is restricted i.e. to the same .java file, however once you have an instance of this class it can be accessed at least as easily as the super class.
Is it possible for the nested inner classes ABar and BBar to access main class's variables? For example:
public class Foo {
public ABar abar = new ABar();
public BBar bbar = new BBar();
public int someCounter = 0;
public class ABar {
public int i = 0;
public void someMethod(){
i++;
someCounter++;
}
}
public class BBar {
public void anotherMethod(){
bbar.someMethod();
someCounter++;
}
}
}
// then called using: //
Foo myFoo = new Foo();
myFoo.bbar.anotherMethod();
Edit
Seems the code I typed would have worked if i'd have tried it first; was trying to get help without being too specific. The code I'm actually having trouble with
Fails because of the error 'cannot make static reference to the non-static field stage'
public class Engine {
public Stage stage = new Stage();
// ...
public class Renderer implements GLSurfaceView.Renderer {
// ...
#Override
public void onDrawFrame(GL10 gl) {
stage.alpha++;
}
}
public class Stage extends MovieClip {
public float alpha = 0f;
}
In your code, yes, it is possible.
Non-static nested classes (inner classes) have access to other members
of the enclosing class, even if they are declared private. Static
nested classes do not have access to other members of the enclosing
class.
See: Nested Classes
If your inner class extends the outer class, then it will have access to the outer class public and protected members. I just tired it and it worked. The construct is a bit odd, because it implies a sort of infinite loop in the class definition, but it seems to do the job.
I am trying to use static blocks like this:
I have a base class called Base.java
public class Base {
static public int myVar;
}
And a derived class Derived.java:
public class Derived extends Base {
static
{
Base.myVar = 10;
}
}
My main function is like this:
public static void main(String[] args) {
System.out.println(Derived.myVar);
System.out.println(Base.myVar);
}
This prints the out put as 0 0 where as I expected 10 0. Can somebody explain this behavior? Also, if I want my derived classes to set the values for a static variable how can I achieve that?
As I understand. You don't call any Derived properties (myVar belongs to Base, not to Derived). And java is not running static block from Derived. If you add some static field to Derived and access it, then java executes all static blocks.
class Base {
static public int myVar;
}
class Derived extends Base {
static public int myVar2;
static
{
Base.myVar = 10;
}
}
public class Main {
public static void main( String[] args ) throws Exception {
System.out.println(Derived.myVar2);
System.out.println(Base.myVar);
}
}
From java specification, when class is initialized (and static block got executed):
12.4.1 When Initialization Occurs A class or interface type T will be initialized immediately before the first occurrence of any one of the following:
• T is a class and an instance of T is created.
• T is a class and a static method declared by T is invoked.
• A static field declared by T is assigned.
• A static field declared by T is used and the field is not a constant variable (§4.12.4).
• T is a top level class (§7.6), and an assert statement (§14.10) lexically nested within T (§8.1.3) is executed.
Static initializer-blocks aren't run until the class is initialized. See Java Language Specification paragraphs 8.7 (Static initializers) and 12.4.1 (When initialization occurs):
A static initializer declared in a class is executed when the class is
initialized (§12.4.2). Together with any field initializers for class
variables (§8.3.2), static initializers may be used to initialize the
class variables of the class.
Here's a similar example straight out of JLS 12.4.1:
class Super {
static int taxi = 1729;
}
class Sub extends Super {
static { System.out.print("Sub "); }
}
class Test {
public static void main(String[] args) {
System.out.println(Sub.taxi);
}
}
This program prints only:
1729
because the class Sub is never initialized; the reference to Sub.taxi
is a reference to a field actually declared in class Super and does
not trigger initialization of the class Sub.
There is a single copy of myVar and both parent and child class will share the same.
Untill and unless child class get initilized.
When we do
class Base {
public static int myVar = 0;
static {
System.out.println("Base");
}
}
class Derived extends Base {
static {
System.out.println("Derived");
Base.myVar = 9;
}
}
public class StaticBlock {
public static void main(String[] args) {
System.out.println(Base.myVar);
System.out.println(Derived.myVar);
}
}
The Output will be
Base
0
0
That means derived class's static block not executing..!!
Here is the link to the Java Specification - section 8.7 talks about static initializers. It gives good details about how they should function and the order in which they get called. http://docs.oracle.com/javase/specs/jls/se7/html/jls-8.html#jls-8.7
I am trying to understand my way around polymorphism in Java. I created a parent class that has too many common methods that all children will use in the same manner.
Each of the subclasses' children all share static information, These variables or information will be used in the methods declared only in the parent.
The problem wish accessing static variables from Parent methods seems not really possible,
Its a solution to declare the common information per instance but since there will be 1000s of instances its such a waste of memory.
A simple elaboration of what i mean is the following code :
class testParent {
static int k;
public void print()
{
System.out.println(k);
}
}
class testChild2 extends testParent
{
static
{
testChild2.k =2;
}
}
public class testChild1 extends testParent{
static
{
testChild1.k = 1;
}
public static void main(String[] args)
{
new testChild1().print();
new testChild2().print();
new testChild1().print();
}
}
the output i expect was
1
2
1.
but what happens is :
1
2
2
One might think that on the initiation of each subclass the static variables of this subclass is set and then all methods referring to this subclass has access to the corresponding 'k' value.
But what actually happens is that all subclasses edit in the same static variable that is shared along all subclasses and hence destroys my whole point of using static variables for each subclass and its instances and using commmon methods in the Parent accessing these variables.
Any idea how can this be done ?
An option is to access the subclasses' static data through an abstract (non-static) method:
abstract public class Parent {
protected abstract int getK();
public void print() {
System.out.println(getK());
}
}
public class Child1 extends Parent {
private static final int CHILD1_K = 1;
protected int getK() { return CHILD1_K; }
}
public class Child2 extends Parent {
private static final int CHILD2_K = 2;
protected int getK() { return CHILD2_K; }
}
When you make new testChild2().print(); the static block on testChield2 was executed and change the value to 2.
static blocks only execute once when loaded by the ClassLoader.
This one give the output you want:
class testParent {
static int k;
public void print()
{
System.out.println(k);
}
}
class testChild2 extends testParent
{
{
testChild2.k =2;
}
}
public class testChild1 extends testParent{
{
testChild1.k = 1;
}
public static void main(String[] args)
{
new testChild1().print();
new testChild2().print();
new testChild1().print();
}
}
Non static code blocks execute everytime the class is instanciated.
Premature optimization is the root of all evil. I don't think you'll run into any memory issues with thousands of instances, each with their own data, unless you're working on a tiny embedded system of some kind. Static variables are not intended to do what you're trying to do with them.
Static variables are specific to the class itself. If you want the same field in different instances of a class to have different values, then that field cannot be static.
The solution: don't make k static.
class testParent {
int k;
public void print()
{
System.out.println(k);
}
}
class testChild2 extends testParent
{
{
this.k =2;
}
}
class testChild1 extends testParent{
{
this.k = 1;
}
public static void main(String[] args){
new testChild1().print();
new testChild2().print();
new testChild1().print();
}
}
Demo
(ignore the static class business - that's just to make it work in ideone).
I am a Java developer. In an interview I was asked a question about private constructors:
Can you access a private constructor of a class and instantiate it?
I answered 'No' but was wrong.
Can you explain why I was wrong and give an example of instantiating an object with a private constructor?
One way to bypass the restriction is to use reflections:
import java.lang.reflect.Constructor;
public class Example {
public static void main(final String[] args) throws Exception {
Constructor<Foo> constructor = Foo.class.getDeclaredConstructor();
constructor.setAccessible(true);
Foo foo = constructor.newInstance();
System.out.println(foo);
}
}
class Foo {
private Foo() {
// private!
}
#Override
public String toString() {
return "I'm a Foo and I'm alright!";
}
}
You can access it within the class itself (e.g. in a public static factory method)
If it's a nested class, you can access it from the enclosing class
Subject to appropriate permissions, you can access it with reflection
It's not really clear if any of these apply though - can you give more information?
This can be achieved using reflection.
Consider for a class Test, with a private constructor:
Constructor<?> constructor = Test.class.getDeclaredConstructor(Context.class, String[].class);
Assert.assertTrue(Modifier.isPrivate(constructor.getModifiers()));
constructor.setAccessible(true);
Object instance = constructor.newInstance(context, (Object)new String[0]);
The very first question that is asked regarding Private Constructors in Interviews is,
Can we have Private constructor in a Class?
And sometimes the answer given by the candidate is, No we cannot have private constructors.
So I would like to say, Yes you can have private Constructors in a class.
It is no special thing, try to think it this way,
Private: anything private can be accessed from within the class only.
Constructor: a method which has same name as that of class and it is implicitly called when object of the class is created.
or you can say, to create an object you need to call its constructor, if constructor is not called then object cannot be instantiated.
It means, if we have a private constructor in a class then its objects can be instantiated within the class only. So in simpler words you can say, if the constructor is private then you will not be able to create its objects outside the class.
What's the benefit
This concept can be implemented to achieve singleton object (it means only one object of the class can be created).
See the following code,
class MyClass{
private static MyClass obj = new MyClass();
private MyClass(){
}
public static MyClass getObject(){
return obj;
}
}
class Main{
public static void main(String args[]){
MyClass o = MyClass.getObject();
//The above statement will return you the one and only object of MyClass
//MyClass o = new MyClass();
//Above statement (if compiled) will throw an error that you cannot access the constructor.
}
}
Now the tricky part, why you were wrong, as already explained in other answers, you can bypass the restriction using Reflection.
I like the answers above, but there are two more nifty ways of creating a new instance of a class which has private constructor. It all depends on what you want to achieve and under what circumstances.
1: Using Java instrumentation and ASM
Well in this case you have to start the JVM with a transformer. To do this you have to implement a new Java agent and then make this transformer change the constructor for you.
First create the class transformer. This class has a method called transform. Override this method and inside this method you can use the ASM class reader and other classes to manipulate the visibility of your constructor. After the transformer is done, your client code will have access to the constructor.
You can read more about this here: Changing a private Java constructor with ASM
2: Rewrite the constructor code
Well, this is not really accessing the constructor, but still you can create an instance. Let's assume that you use a third-party library (let's say Guava) and you have access to the code but you don't want to change that code in the jar which is loaded by the JVM for some reason (I know, this is not very lifelike but let's suppose the code is in a shared container like Jetty and you can't change the shared code, but you have separate class loading context) then you can make a copy of the 3rd party code with the private constructor, change the private constructor to protected or public in your code and then put your class at the beginning of the classpath. From that point your client can use the modified constructor and create instances.
This latter change is called a link seam, which is a kind of seam where the enabling point is the classpath.
Using java Reflection as follows :
import java.lang.reflect.Constructor;
import java.lang.reflect.InvocationTargetException;
class Test
{
private Test() //private constructor
{
}
}
public class Sample{
public static void main(String args[]) throws ClassNotFoundException, InstantiationException, IllegalAccessException, NoSuchMethodException, SecurityException, IllegalArgumentException, InvocationTargetException
{
Class c=Class.forName("Test"); //specify class name in quotes
//----Accessing private constructor
Constructor con=c.getDeclaredConstructor();
con.setAccessible(true);
Object obj=con.newInstance();
}
}
Yes you could, as mentioned by #Jon Steet.
Another way of accessing a private constructor is by creating a public static method within this class and have its return type as its object.
public class ClassToAccess
{
public static void main(String[] args)
{
{
ClassWithPrivateConstructor obj = ClassWithPrivateConstructor.getObj();
obj.printsomething();
}
}
}
class ClassWithPrivateConstructor
{
private ClassWithPrivateConstructor()
{
}
public void printsomething()
{
System.out.println("HelloWorld");
}
public static ClassWithPrivateConstructor getObj()
{
return new ClassWithPrivateConstructor();
}
}
You can of course access the private constructor from other methods or constructors in the same class and its inner classes. Using reflection, you can also use the private constructor elsewhere, provided that the SecurityManager is not preventing you from doing so.
Yes, we can access the private constructor or instantiate a class with private constructor. The java reflection API and the singleton design pattern has heavily utilized concept to access to private constructor.
Also, spring framework containers can access the private constructor of beans and this framework has used java reflection API.
The following code demonstrate the way of accessing the private constructor.
class Demo{
private Demo(){
System.out.println("private constructor invocation");
}
}
class Main{
public static void main(String[] args){
try{
Class class = Class.forName("Demo");
Constructor<?> con = string.getDeclaredConstructor();
con.setAccessible(true);
con.newInstance(null);
}catch(Exception e){}
}
}
output:
private constructor invocation
I hope you got it.
I hope This Example may help you :
package MyPackage;
import java.lang.reflect.Constructor;
/**
* #author Niravdas
*/
class ClassWithPrivateConstructor {
private ClassWithPrivateConstructor() {
System.out.println("private Constructor Called");
}
}
public class InvokePrivateConstructor
{
public static void main(String[] args) {
try
{
Class ref = Class.forName("MyPackage.ClassWithPrivateConstructor");
Constructor<?> con = ref.getDeclaredConstructor();
con.setAccessible(true);
ClassWithPrivateConstructor obj = (ClassWithPrivateConstructor) con.newInstance(null);
}catch(Exception e){
e.printStackTrace();
}
}
}
Output:
private Constructor Called
Reflection is an API in java which we can use to invoke methods at runtime irrespective of access specifier used with them.
To access a private constructor of a class:
My utility class
public final class Example{
private Example(){
throw new UnsupportedOperationException("It is a utility call");
}
public static int twice(int i)
{
int val = i*2;
return val;
}
}
My Test class which creates an object of the Utility class(Example)
import java.lang.reflect.Constructor;
import java.lang.reflect.Field;
import java.lang.reflect.InvocationTargetException;
class Test{
public static void main(String[] args) throws Exception {
int i =2;
final Constructor<?>[] constructors = Example.class.getDeclaredConstructors();
constructors[0].setAccessible(true);
constructors[0].newInstance();
}
}
When calling the constructor it will give the error
java.lang.UnsupportedOperationException: It is a utility call
But remember using reflection api cause overhead issues
Look at Singleton pattern. It uses private constructor.
Yes you can instantiate an instance with a private constructor using Reflection, see the example I pasted below taken from java2s to understand how:
import java.lang.reflect.Constructor;
import java.lang.reflect.InvocationTargetException;
class Deny {
private Deny() {
System.out.format("Deny constructor%n");
}
}
public class ConstructorTroubleAccess {
public static void main(String... args) {
try {
Constructor c = Deny.class.getDeclaredConstructor();
// c.setAccessible(true); // solution
c.newInstance();
// production code should handle these exceptions more gracefully
} catch (InvocationTargetException x) {
x.printStackTrace();
} catch (NoSuchMethodException x) {
x.printStackTrace();
} catch (InstantiationException x) {
x.printStackTrace();
} catch (IllegalAccessException x) {
x.printStackTrace();
}
}
}
The basic premise for having a private constructor is that having a private constructor restricts the access of code other than own class' code from making objects of that class.
Yes we can have private constructors in a class and yes they can be made accessible by making some static methods which in turn create the new object for the class.
Class A{
private A(){
}
private static createObj(){
return new A();
}
Class B{
public static void main(String[]args){
A a=A.createObj();
}}
So to make an object of this class, the other class has to use the static methods.
What is the point of having a static method when we are making the constructor private?
Static methods are there so that in case there is a need to make the instance of that class then there can be some predefined checks that can be applied in the static methods before creation of the instance. For example in a Singleton class, the static method checks if the instance has already been created or not. If the instance is already created then it just simply returns that instance rather than creating a new one.
public static MySingleTon getInstance(){
if(myObj == null){
myObj = new MySingleTon();
}
return myObj;
}
We can not access private constructor outside the class but using Java Reflection API we can access private constructor. Please find below code:
public class Test{
private Test()
System.out.println("Private Constructor called");
}
}
public class PrivateConsTest{
public void accessPrivateCons(Test test){
Field[] fields = test.getClass().getDeclaredFields();
for (Field field : fields) {
if (Modifier.isPrivate(field.getModifiers())) {
field.setAccessible(true);
System.out.println(field.getName()+" : "+field.get(test));
}
}
}
}
If you are using Spring IoC, Spring container also creates and injects object of the class having private constructor.
I tried like this it is working. Give me some suggestion if i am wrong.
import java.lang.reflect.Constructor;
class TestCon {
private TestCon() {
System.out.println("default constructor....");
}
public void testMethod() {
System.out.println("method executed.");
}
}
class TestPrivateConstructor {
public static void main(String[] args) {
try {
Class testConClass = TestCon.class;
System.out.println(testConClass.getSimpleName());
Constructor[] constructors = testConClass.getDeclaredConstructors();
constructors[0].setAccessible(true);
TestCon testObj = (TestCon) constructors[0].newInstance();
//we can call method also..
testObj.testMethod();
} catch (Exception e) {
e.printStackTrace();
}
}
}
Simple answer is yes we can have private constructors in Java.
There are various scenarios where we can use private constructors. The major ones are
Internal Constructor chaining
Singleton class design pattern
Also have another option create the getInstance() where we can create instance of private constructor inside same class and return that object.
class SampleClass1{
private SampleClass1() {
System.out.println("sample class constructor");
}
public static SampleClass1 getInstance() {
SampleClass1 sc1 = new SampleClass1();
return sc1;
}
}
public class SingletonDemo {
public static void main(String[] args) {
SampleClass1 obj1 = SampleClass1.getInstance();
}
}
We can create instance of private class by creating createInstance() in the same class and simply call the same method by using class name in main():
class SampleClass1{
private SampleClass1() {
System.out.println("sampleclass cons");
}
public static void createInstance() {
SampleClass1 sc = new SampleClass1();
}
}
public class SingletonDemo {
public static void main(String[] args) {
//SampleClass1 sc1 = new SampleClass1();
SampleClass1.createInstance();
}
}
Well, you can also if there are any other public constructors. Just because the parameterless constructor is private doesn't mean you just can't instantiate the class.
you can access it outside of the class its very easy to access
just take an example of singaltan class we all does the same thing make the private constructor and access the instance by static method here is the code associated to your query
ClassWithPrivateConstructor.getObj().printsomething();
it will definately work because i have already tested