The constructor ... is not visible

When you invoke the private constructor of a class in another class, you’ll get the error message “The constructor ... is not visible”. And when the private constructor of a super class is invoked from the subclass implicitly or explicitly, you’ll get the following:

Implicit super constructor ... is not visible for default constructor. Must define an explicit constructor.

Implicit super constructor ... is not visible. Must explicitly invoke another constructor.

See an example below:

public class SuperClass

{

       private SuperClass()

       {

              System.out.println("Super class private constructor");

       }

      

       public SuperClass(int n)

       {

              System.out.println("Super class protected constructor");

       }

}

In the above code, the class has two constructors. Access modifier for one is private and another is public. So if you create an instance of this class using the private constructor in another class, you’ll encounter the error.

You cannot write SuperClass  superClassObject = new SuperClass();

You should call other constructors that you can access as per access modifier rule or the other way of instantiating the class (if available in that class). Sometimes such class has some getInstance() like methods.

Again, when you inherit a class that has private default constructor, you have to write your own constructor and invoke a constructor (that is accessible) within your constructor. In such case, if you do not write any constructor in the subclass you’ll get “Implicit super constructor ... is not visible for default constructor. Must define an explicit constructor.”

This happens because when you do not define any constructor, a default constructor is automatically created by the Java compiler and the default constructor invokes the default constructor of the super class. This is how the constructor is invoked implicitly.

See the code below:

//Code1

public class Subclass extends SuperClass

{

      

}

Or

//Code2

public class Subclass extends SuperClass

{

       public Subclass()

       {

             

       }

}

I have not defined any constructor in code1. In code2, I have defined a constructor but have not invoked any constructor explicitly. So the super class default constructor will implicitly be called.

In the code below I have defined a constructor. But still I called the super class default constructor explicitly through “super()”. I’ll encounter the error.

public class Subclass extends SuperClass

{

       public Subclass()

       {

              super();

       }

}

Now the solutions are:

1.      

      We cannot call the private constructor of class from the subclass or other class. You should look the other constructors or the other way of instantiating class.

2.       If the default constructor of a super class is private, you have to write your own constructor in the subclass and explicitly call visible constructors.

For the above scenario, correct coding would be:

public class Subclass extends SuperClass

{

       public Subclass()

       {

              super(10);

       }

}

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java.lang.UnsupportedOperationException at java.util.AbstractList.remove

When you create a java.util.List object from an array using Arrays.asList method and after that if you try to clear or remove the elements from the list you'll get this run time error. The reason is that Arrays.asList method returns a fixed-size list backed by the specified array.

See in the example code below:

int[] myArray = {5,10,15,20};

List intList = Arrays.asList(myArray);

intList.clear();

Or

int[] myArray = {5,10,15,20};

List intList = Arrays.asList(myArray);

for(int i=0;i

{

intList.remove(i);

}

You'll get error from intList.clear() or intList.remove(i) because you cannot remove from a fixed-size array. And also you cannot add any new element in this list.

If you require to copy array elements in a list object and later need to add/delete elements from the list, you can do it in the following way.

  int[] myArray = {5,10,15,20};

List intList = new ArrayList();

System.out.println(intList); // print empty list

//copy the array elements into the list

for(int i=0;i

{

intList.add(myArray[i]);

}

System.out.println(intList); // print list after copying array elements

//You can now add

intList.add(25);

System.out.println(intList); // print list after adding new element

//You can remove

intList.remove(0);

System.out.println(intList); // print list after removing an element

//You can remove all the elements

intList.clear();

System.out.println(intList); // print list after removing all the elements

Below is the output of this sample code:

[]

[5, 10, 15, 20]

[5, 10, 15, 20, 25]

[10, 15, 20, 25]

[]

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Resource leak: 'scanner' is never closed

If you do not close the java.util.Scanner object after reading data, you can get an warning saying "Resource leak: 'scanner' is never closed".

In the code below see that the Scanner object is closed in a finally block after reading all inputs using the Scanner object.

int number1, number2;

Scanner scanner = new Scanner(System.in);

try
{
System.out.println("Ener the first number");
number1 = scanner.nextInt();
System.out.println("Ener the second number");
number2 = scanner.nextInt();
}
finally
{
scanner.close();
}

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Dimensions expected after this token / '[' expected

Java error "Dimensions expected after this token" or " '[' expected " can occur while creating array elements or allocating memory using the keyword new. If the square brackets [] are not used properly, you can get this error message.

See the code below:

int myArray[]=new int{10,20,30,40};

Here this is a syntax error. You have to put square brackets [] after new int. To initialize an array the correct code would be:

int myArray[]=new int[]{10,20,30,40};

or just

int myArray[]={10,20,30,40};

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Cannot invoke length() on the array type

The error message "Cannot invoke length() on the array type" or "cannot find symbol ... method length()" is related with the use of array.

Java array has a built in property length which is a variable, not method. If you put parenthesis () after the array property length, you'll get this error.

See the example below:

int[] number = {5,10,15};

for(int i=0; i<number.length(); i++)
{
    System.out.println(number[i]);
}

Here number.length() is wrong. To resolve the error, remove the parenthesis ().

The correct code:

for(int i=0; i<number.length; i++)
{
    System.out.println(number[i]);
}

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