How to access the private variables of a class in

问题:

This is a question I was asked in an interview: I have class A with private members and Class B extends A. I know private members of a class cannot be accessed, but the question is: I need to access private members of class A from class B, rather than create variables with the same value in class B.

回答1:

The interviewer was either testing your knowledge of access modifiers, or your approach to changing existing classes, or both.

I would have listed them (public, private, protected, package private) with an explanation of each. Then gone on to say that class A would need to be modified to allow access to those members from class B, either by adding setters and getters, or by changing the access modifiers of the members. Or class B could use reflection. Finally, talk about the pros and cons of each approach.

回答2:

Reflection? Omitting imports, this should work:

public class A {

    private int ii = 23;

}

public class B extends A {

    private void readPrivateSuperClassField() throws Exception {
        Class<?> clazz = getClass().getSuperclass();
        Field field = clazz.getDeclaredField("ii");
        field.setAccessible(true);
        System.out.println(field.getInt(this));
    }

    public static void main(String[] args) throws Exception {
        new B().readPrivateSuperClassField();
    }

}

It'll not work if you do something like that before the of invocation readPrivateSuperClassField();:

System.setSecurityManager(new SecurityManager() {
        @Override
        public void checkMemberAccess(Class<?> clazz, int which) {
            if (clazz.equals(A.class)) {
                throw new SecurityException();
            } else {
                super.checkMemberAccess(clazz, which);    
            }
        }
    });

And there are other conditions under which the Reflection approach won't work. See the API docs for SecurityManager and AccessibleObject for more info. Thanks to CPerkins for pointing that out.

I hope they were just testing your knowledge, not looking for a real application of this stuff ;-) Although I think an ugly hack like this above can be legit in certain edge cases.

回答3:

The architecture is broken. Private members are private because you do not want them accessed outside the class and friends.

You can use friend hacks, accessors, promote the member, or #define private public (heh). But these are all short term solutions - you will probably have to revisit the broken architecture at some stage.

回答4:

By using public accessors (getters & setters) of A's privates members ...

回答5:

You cannot access private members from the parent class. You have make it protected or have protected/public method that has access to them.

EDIT : It is true you can use reflection. But that is not usual and not good idea to break encapsulation.

回答6:

A nested class can access to all the private members of its enclosing class—both fields and methods. Therefore, a public or protected nested class inherited by a subclass has indirect access to all of the private members of the superclass.

public class SuperClass
{
    private int a = 10;
    public void makeInner()
    {
        SubClass in = new SubClass();
        in.inner();
    }
    class SubClass
    {
        public void inner()
        {
            System.out.println("Super a is " + a);
        }
    }
    public static void main(String[] args)
    {
        SuperClass.SubClass s = new SuperClass().new SubClass();
        s.inner();
    }
}

回答7:

If I'm understanding the question correctly, you could change private to protected. Protected variables are accessible to subclasses but behave like private variables otherwise.

回答8:

By using setters and getters u can access it

回答9:

From JLS §8.3. Field Declarations:

A private field of a superclass might be accessible to a subclass - for example, if both classes are members of the same class. Nevertheless, a private field is never inherited by a subclass.

I write the example code:

public class Outer
{
    class InnerA
    {
        private String text;
    }
    class InnerB extends InnerA
    {
        public void setText(String text)
        {
            InnerA innerA = this;
            innerA.text = text;
        }
        public String getText()
        {
            return ((InnerA) this).text;
        }
    }
    public static void main(String[] args)
    {
        final InnerB innerB = new Outer().new InnerB();
        innerB.setText("hello world");
        System.out.println(innerB.getText());
    }
}

The explanation of the accessibility of InnerA.text is here JLS §6.6.1. Determining Accessibility:

Otherwise, the member or constructor is declared private, and access is permitted if and only if it occurs within the body of the top level class (§7.6) that encloses the declaration of the member or constructor.

回答10:

You can use the setters and getters of class A. Which gives same feeling as if You are using a class A's object.

回答11:

Have you thought about making them protected ? Just to be sure you are aware of this option, if you are then pardon me for bringing up this trivia ;)

回答12:

• Private members cant be accessed in derived class
• If you want to access means you can use getter and setter methods.

class A
{
 private int a;
 void setA(int a)
 {
    this.a=a;
 }
 int getA()
 {
   return a;
 }
}

Class B extends A
{
 public static void main(String[] arg)
  {
     B obj= new B();
     obj.setA(10);
     System.out.println("The value of A is:"+obj.getA());
  } 
}

回答13:

Private will be hidden until you have been given the right access to it. For instance Getters or setters by the programmer who wrote the Parent. If they are not visible by that either then accept the fact that they are just private and not accessible to you. Why exactly you want to do that??

回答14:

I don't know about Java, but in some languages nested types can do this:

    class A {
        private string someField;
        class B : A {
            void Foo() {
                someField = "abc";
            }
        }
    }

Otherwise, use an accessor method or a protected field (although they are often abused).

回答15:

A private member is accessible in subclass in a way that you cannot change the variable, but you are able to access the variable as read only.

回答16:

Obviously, making them protected, or adding setters/getters is the preferred technique. Reflection is a desperation option.

Just to show off to the interviewer, IF "access" means read access, and IF Class A generates XML or JSON etc., you could serialize A and parse the interesting fields.

回答17:

    Class A
{
  private int i;

  int getValue()
    {
      return i;
  }
}
class B extends A
{
   void getvalue2()
    {
      A a1= new A();
      sop(a1.getValue());
    }
}

回答18:

To access private variables of parent class in subclass you can use protected or add getters and setters to private variables in parent class..

回答19:

You can't access directly any private variables of a class from outside directly.

You can access private member's using getter and setter.

回答20:

Ways to access the superclass private members in subclass :

1. If you want package access just change the private fields to protected. It allows access to same package subclass.
2. If you have private fields then just provide some Accessor Methods(getters) and you can access them in your subclass.

3. You can also use inner class e.g

   public class PrivateInnerClassAccess {
   private int value=20;
   
   class InnerClass {
   
   public void accessPrivateFields() {
       System.out.println("Value of private field : " + value);
   }
   
   }
   
   public static void main(String arr[])
   {
       PrivateInnerClassAccess access = new PrivateInnerClassAccess();
       PrivateInnerClassAccess.InnerClass innerClass = access.new InnerClass();
       innerClass.accessPrivateFields();
   
   }
   
   }
   

   4 .You can also use Reflection e.g

    public class A {
   private int value;
   public A(int value)
   {
       this.value = value;
   }
   }
   
   public class B {
   public void accessPrivateA()throws Exception
   {
       A a = new A(10);
       Field privateFields = A.class.getDeclaredField("value");
       privateFields.setAccessible(true);
       Integer value = (Integer)privateFields.get(a);
       System.out.println("Value of private field is :"+value);
   }
   
   
   public static void main(String arr[]) throws Exception
   {
       B b = new B();
       b.accessPrivateA();
   }
   }
   

回答21:

Directly we can't access it. but Using Setter and Getter we can access,

Code is :

class AccessPrivate1 {    

    private int a=10; //private integer    
    private int b=15;    
    int getValueofA()    
    {    
        return this.a;    

    }    

    int getValueofB()    
    {    
        return this.b;    
    }    
}    


public class AccessPrivate{    
    public static void main(String args[])    
    {    
        AccessPrivate1 obj=new AccessPrivate1();    

        System.out.println(obj.getValueofA()); //getting the value of private integer of class AccessPrivate1    

        System.out.println(obj.getValueofB()); //getting the value of private integer of class AccessPrivate1    
    }    
}    

回答22:

You can use Accessors (getter and setter method) in your Code.

回答23:

By using setter method you can use else with the help of refection you can use private member of class by setting that member say a - take a from class and set a.setAccessible(true);

回答24:

You may want to change it to protected. Kindly refer this

https://docs.oracle.com/javase/tutorial/java/javaOO/accesscontrol.html

If this is something you have to do at any cost just for the heck of doing it you can use reflection. It will give you list of all the variables defined in the class- be it public, private or protected. This surely has its overhead but yes, it is something which will let you use private variables. With this, you can use it in any of the class. It does not have to be only a subclass Please refer to the example below. This may have some compilation issues but you can get the basic idea and it works

private void getPropertiesFromPrivateClass(){

    Field[] privateVariablesArray = PrivateClassName.getClass().getDeclaredFields();
    Set<String> propertySet = new HashSet<String>();
    Object propertyValue;
    if(privateVariablesArray.length >0){
        for(Field propertyVariable :privateVariablesArray){
            try {

                if (propertyVariable.getType() == String.class){
                    propertyVariable.setAccessible(true);
                    propertyValue = propertyVariable.get(envtHelper);
                    System.out.println("propertyValue");
                }
            } catch (IllegalArgumentException illegalArgumentException) {
                illegalArgumentException.printStackTrace();

            } catch (IllegalAccessException illegalAccessException) {
                illegalAccessException.printStackTrace();

            }
        }

Hope this be of some help. Happy Learning :)

回答25:

Below is the example for accessing the private members of superclass in the object of subclass.

I am using constructors to do the same.

Below is the superclass Fruit

public class Fruit {
    private String type;

    public Fruit() {
    }

    public Fruit(String type) {
        super();
        this.type = type;
    }

    public String getType() {
        return type;
    }

    public void setType(String type) {
        this.type = type;
    }

}

Below is subclass Guava which is inheriting from Fruit

public class Guava extends Fruit{
    private String name;

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public Guava(String name,String type) {
        super(type);
        this.name=name;
    }
}

Below is the main function where we are creating an object of subclass and also displaying the member of superclass.

public class Main {

    public static void main(String[] args) {

        Guava G1=new Guava("kanpuria", "red");
        System.out.println(G1.getName()+" "+G1.getType());

    }

}

回答26:

Note that a private field of a superclass might be accessible to a subclass (for example,if both classes are memebers of the same class),Nevertheless,a private field is never inherited by a subclass

回答27:

Simple!!!

public class A{
private String a;
private String b;
//getter and setter are here
}

---

public class B extends A{
public B(String a, String b){ //constructor
super(a,b)//from here you got access with private variable of class A
}
}

thanks

回答28:

Modifiers are keywords that you add to those definitions to change their meanings. The Java language has a wide variety of modifiers, including the following:

• Java Access Modifiers
• Non Access Modifiers

To use a modifier, you include its keyword in the definition of a class, method, or variable. The modifier precedes the rest of the statement.

There is more information here:

http://tutorialcorejava.blogspot.in/p/java-modifier-types.html