What is Variable Shadowing and Hiding in Java

Java allows us to declare a variable whenever we need it, We can categorize all our variables into 3 categories which have different-different scopes
  1. Instance Variables - are defined inside a class and have object level scope.
  2. Class Variables - are defined inside a class with static keyword, these variables have a class level scope and are common to all objects of the same class
  3. Local Variables - are defined inside a method or in any conditional block, have the block-level scope and only accessible in the block where it defined.
what-is-variable-hiding-shadowing

What is Variable Shadowing

Variable shadowing happens when we define a variable in a closure scope with a variable name and we have already defined a variable in outer scope with the same name.

In other words, when a local variable has the same name as one of the instance variable, the local variable shadows the instance variable inside the method block.

In the following example, there is an instance variable named x and inside method printLocalVariable(), we are shadowing it by the local variable x.

class Parent {

    // Declaring instance variable by name `x`
    String x = "Parent`s Instance Variable";

    public void printInstanceVariable() {
        System.out.println(x);
    }

    public void printLocalVariable() {
        // Shadowing instance variable `x` by a local variable with same name
        String x = "Local Variable";
        System.out.println(x);

        // If we still want to access instance variable, we do that by using `this.x`
        System.out.println(this.x);
    }
}

What is variable Hiding

Variable Hiding happens when we define a variable in child class with a variable name which we have already used to define a variable in the parent class. A child class can declare a variable with the same name as an inherited variable from its parent class, thus hiding the inherited variable.

In other words, when the child and parent class both have a variable with same name child class's variable hides parent class's variable.

In the below example, we are hiding the variable named x in the child class while it is already defined by its parent class.

class Child extends Parent {

    // Hiding Parent class's variable `x` by defining a variable in child class with same name.
    String x = "Child`s Instance Variable";

    @Override
    public void printInstanceVariable() {
        System.out.print(x);

        // If we still want to access variable from super class, we do that by using `super.x`
        System.out.print(", " + super.x + "\n");
    }
}

Variable Hiding is not the same as Method Overriding

While variable hiding looks like overriding a variable similar to method overriding but it is not, Overriding is applicable only to methods while hiding is applicable variables.

In the case of method overriding, overridden methods completely replaces the inherited methods so when we try to access the method from parent's reference by holding child's object, the method from child class gets called. You can read more about overriding on Everything About Method Overloading Vs Method Overriding, Why We Should Follow Method Overriding Rules, How Does JVM Handle Method Overloading and Overriding Internally.

But in variable hiding child class hides the inherited variables instead of replacing, so when we try to access the variable from parent's reference by holding child's object, it will be accessed from the parent class.

When an instance variable in a subclass has the same name as an instance variable in a super class, then the instance variable is chosen from the reference type.

public static void main(String[] args) throws Exception {

    Parent parent = new Parent();
    parent.printInstanceVariable(); // Output - "Parent`s Instance Variable"
    System.out.println(parent.x); // Output - "Parent`s Instance Variable"

    Child child = new Child();
    child.printInstanceVariable();// Output - "Child`s Instance Variable, Parent`s Instance Variable"
    System.out.println(child.x);// Output - "Child`s Instance Variable"

    parent = child; // Or parent = new Child();
    parent.printInstanceVariable();// Output - "Child`s Instance Variable, Parent`s Instance Variable"
    System.out.println(parent.x);// Output - Parent`s Instance Variable

    // Accessing child's variable from parent's reference by type casting
    System.out.println(((Child) parent).x);// Output - "Child`s Instance Variable"
}

In above example when we call overridden method printInstanceVariable() on parent while holding Child's object in it we can see the output is Child`s Instance Variable, Parent`s Instance Variable because in child class method is printing Child class's x variable and super.x.

But when we call System.out.println(parent.variable); on same parent reference which is holding child's object, it prints Parents Instance Variable because new Child() object keeps parent's x as well as child's x and hides parent's x. So, in this case, x is chosen from the class that is the reference type.

But if we wanted to access child's variable even if we are using parent reference we can do that by using (Child) parent).variable.

When our variables are private or is in another package and has default access, such variables are not visible outside that class and child class cannot access them. So there no confusion and that is why we should always stick to General Guidelines to create POJOs and declare our variables with private access and also provide proper get/set methods to access them.

Do you want to know more about variable hiding?, In the article Why Instance Variable Of Super Class Is Not Overridden In Sub Class,  I have discussed why variables do not follow overriding, why variable hiding is not designed same as method overriding and why instance variable is chosen from reference type instead of the object? Please go ahead and read it.

You can find complete code on this Github Repository and please feel free to provide your valuable feedback.

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6 comments :

  1. You overlooked the most important instruction regarding variable hiding...
    The general rule is: Don't do it!!!

    It just leads to bugs and confusion. Yes, it may be necessary sometimes, when you don't have control over the parent class' code, or something like that. But when you do it, you should always feel disappointment and a sense of failure. If you can avoid doing it, you probably should.

    ReplyDelete
    Replies
    1. Thanks for your comment!, I totally agree to you and that is why I have mentioned "When our variables are private or is in another package and has default access, such variables are not visible outside that class and child class cannot access them. So there no confusion and that is why we should always stick to General Guidelines to create POJOs and declare our variables with private access and also provide proper get/set methods to access them."

      I know it creates confusion and that's reason we should be aware of it in the first place.

      Delete
  2. This article covers obscuring too: http://programming.guide/java/overloading-overriding-shadowing-hiding-obscuring.html

    ReplyDelete
  3. It is a great job, I like your posts and wish you all the best. and I hope you continue this job well.
    NutraT line

    ReplyDelete