Difference between Advanced JAVA and Core JAVA?

Java is categorized into 3 types:

• J2SE ( Java2 Standard Edition)  
• J2EE ( Java2 Enterprise Edition)  
• J2ME ( Java2 Micro/Mobile Edition)

Core Java:-

1. Core Java is the basic of Java programming technology concept.
2. This basic concepts in Java is normally referred as 'Core Java' programming.
3. The Core Java comprises the Single Tier Architecture.

Read More

call to super must be first statement in constructor

call to super must be first statement in constructor

The error message is very clear actually:

i) We can call super in the constructor of a subclass
ii) If we call super in the constructor of a subclass, it must be the first statement in that constructor; i.e, before writing any other statement we call to super must be made.

It is mentionable that, super(parameters) calls the constructor of the super class.

See the code below:

The super class...

public class Employee
{
private int id;
private String name;
public Employee()
{
}
public Employee(int id,String name)
{
this.id = id;
this.name = name;
}
}

The subclass...

public class SalariedEmployee extends Employee
{
private double salary;
public SalariedEmployee()
{
}
public SalariedEmployee(int id,String name,double salary)
{
this.salary = salary; //this is the first statement now
super(id,name); //error : call to super must be first statement in constructor
}
}

The correct subclass will be...

public class SalariedEmployee extends Employee
{
private double salary;
public SalariedEmployee()
{
}
public SalariedEmployee(int id,String name,double salary)
{
super(id,name); // now call to super is the first statement in the constructor
this.salary = salary; // it is now the second statement
}
}

Read More

recursive constructor invocation

recursive constructor invocation

If a constructor calls itself, then the error message "recursive constructor invocation" is shown. It may happen when we overload constructors and call the wrong constructor (itself) accidentally.

See the wrong code below:

public class Employee
{
  private int id;
  private String name;
  public Employee(int id,String name)
  {
    this.id = id;
    this.name = name;
  }
  public Employee(int id)
  {
    this(id); 
    /* this(id) calls the constructor having one parameter of int type. Thus this is calling itself.*/
  }
  public Employee()
  {
  }
}

In the above code, there are three constructors:
i) Employee(int id,String name)
ii) Employee(int id)
iii) Employee()

Within the second constructor, we have written this(id) which means it calls a constructor with one int parameter and see that it is the constructor itself. It means we have not invoked the right constructor. If we want to invoke the first constructor Employee(int id,String name) within the second constructor we should write this(id,null). We are providing null for the String name as no value is available for the name variable in the second constructor but now it calls the first constructor Employee(int id,String name) as we have provided  two parameters now.

See the corrected code below:

public class Employee
{
  private int id;
  private String name;
  public Employee(int id,String name)
  {
    this.id = id;
    this.name = name;
  }
  public Employee(int id)
  {
    this(id,null); // this(id,null) calls another constructor having two parameters
  }
  public Employee()
  {
  }
}

Summary:

If you see the error message "recursive constructor invocation", check that you have provided correct no and type of parameters while calling one constructor in another one.

Read More

invalid method declaration; return type required

invalid method declaration; return type required

The Java error "invalid method declaration; return type required" can occur for any of the following reasons:

1. You are trying to define a constructor (of course without any return type; not even void)  but the constructor name is not given same as the class name.

See the wrong code below:

public class Employee
{
 private int id;
 private String name;
 public employee(int id,String name) // constructor name is not same as the class name.
 {
  this.id = id;
  this.name = name;
 }
}

So check the spelling of the constructor name and also note that Java is a case sensitive language.

2. If you are really defining a method (not a constructor) you have forgotten to declare the return type as the wrong code given below:

public class Test
{
public aMethod() // no return type given
{
System.out.println("This is a method");
}
}

The correct code is:

public class Test
{
public void aMethod() // return type void is given now
{
System.out.println("This is a method");
}
}

Read More

modifier static not allowed here

modifier static not allowed here

Java error "modifier static not allowed here" can occur for any of the following reasons:

1. Constructor is declared as static(!)

A constructor cannot be static. In the wrong code below:

public class Person
{
private String firstName;
private String lastName;
public static Person()
{
//...
}
}

Here the constructor Person is declared as static. As the constructor must be non-static, the correct code is:

public class Person
{
private String firstName;
private String lastName;
public Person()
{
//...
}
}

2. Interface contains static method(!)

All methods of any interface is abstract and hence they must be non static. See the code below:

public interface MyInterface
{
public static void myMetod();
}

The above code is wrong as the interface method is declared as static. Corrected code is:

public interface MyInterface
{
public void myMetod();
}

Read More

identifier expected

identifier expected

This error message is shown when the statements are not written in proper place. Most often, mistakenly, we may write the processing task (for example assigning value to a variable; writing a loop etc.) outside of any method. In this situation we may see this type of error message.

Then the solution is to write such statements inside the appropriate methods.

See the example code below:

public class IdentifierExptected
{
int number1,number1,sum;


number1=10; // identifier expected
number2=20; // identifier expected
sum=number1+number2; // identifier expected

/*
The assignment statement must be written inside a method. Variables can also be assigned/ initialized during the declaration.
*/


public void display()
{
System.out.println("Number 1 = "+number1);
System.out.println("Number 2 = "+number2);
System.out.println("Sum = "+sum);
}


}

See the solution now:

public class IdentifierExptectedSolved
{
int number1=10,number2=20,sum;


public void sum()
{
sum=number1+number2;
}


public void display()
{
System.out.println("Number 1 = "+number1);
System.out.println("Number 2 = "+number2);
System.out.println("Sum = "+sum);
}



}

Read More

cyclic inheritance involving ...

cyclic inheritance involving ...

public class Person extends Employee

{

private String firstName;

private String lastName;

public Person(String firstName,String lastName)

{

this.firstName=firstName;

this.lastName=lastName;

}

}

Person is a class which inherits Employee and see the code below where Employee also inherit the Class Person.

public class Employee extends Person { private double salary; public Employee(String firstName,String lastName,double salary) { super(firstName,lastName); this.salary=salary; } } In the above code we will see the error message "cyclic inheritance involving Person public class Person extends Employee" Actually Person should be the super class here and Employee is the subclass. But here the relationsship is such that Employee is the subclass of Person, again Person is the subclass of Employee. It's a cyclic inheritance which is not possible. The solution of this problem is identifying the proper relationship i.e, finding out the super class and the subclass. In our case Person is the super class, so it should not try to inherit Employee and Employee is the subclass which will inherit Person. Correction is ommiting "extends Employee" from the Person class signature as below: public class Person { private String firstName; private String lastName; public Person(String firstName,String lastName) { this.firstName=firstName; this.lastName=lastName; } } public class Employee extends Person { private double salary; public Employee(String firstName,String lastName,double salary) { super(firstName,lastName); this.salary=salary; } }

Read More

reached end of file while parsing

reached end of file while parsing

The error 'reached end of file while parsing' occurs because of curly braces } problem. You have given either more or less curly braces } than the required number of } braces.

See the code below:

public class ReachedEndOfFileWhileParsing
{
public static void main(String args[])
{
System.out.println("java-error-massages.blogspot.com");

}

Here two curly braces are started but only one is ended. We need to put another } after the display statement here like below:

public class ReachedEndOfFileWhileParsingSolution
{
public static void main(String args[])
{
System.out.println("java-error-massages.blogspot.com");
}// it is now given

}

Read More

cannot find symbol / cannot resolve symbol

cannot find symbol / cannot resolve symbol

“cannot find symbol” or “cannot resolve symbol” is the most common error message which the programmers get from the compiler. However this error message may mean that the variable/method/class/object used are not declared/defined properly. In most cases, this happens because:

· The variable/object is not declared or the spelling of variable/object declaration & later use of that variable/object is not same.

· The method is not defined or the spelling of the method definition and method calling is not same or the number and/or type of parameters are wrongly given when calling the method or the method where it is defined is not imported properly (in case of class library methods, the package where the method is defined is not imported)

· If the message is shown for a class, the package of that class might not have been imported or the spelling is wrong.

The solution of that problem is to check the spelling at first, Java programmers see this message most often just because of the wrong spelling( be careful about the case also, as Java is a case sensitive language). If the spelling is correct check whether the required package is imported.

Here is an example of this error :

public class CannotFindSymbol
{
public static void main(String args[])
{
int quantity;
Scanner input=new Scanner(System.in);
System.out.print(“Enter the quantity: ”);
quntity=imput.nextInteger();
System.out.println(“Your number is ”+quantity);
}
}

In the above code, there are many lines where we will get this error message.

At first it will say Scanner cannot be resolved/found because the package of this class java.util is not imported.

Also problem in ‘quntity’ as the variable is declared as ‘quantity’

Error in ‘imput’ as the object is created as ‘input’

If you correct all the errors it will show error message for nextIntger as the actual method name is nextInt in class Scanner

Now the correct code is:

import java.util.*;
public class CannotFindSymbolSolution
{
public static void main(String args[])
{
int quantity;
Scanner input=new Scanner(System.in);
System.out.print("Enter the quantity: ");
quantity=input.nextInt();
System.out.println("Your number is "+quantity);
}
}

Read More

missing method body, or declare abstract

missing method body, or declare abstract

If we put a ; (semicolon) after the method header (i.e., before starting the method body) when we are defining a concrete method (which method has body) then we will see “missing method body, or declare abstract”. Another reason is if we don’t write the keyword ‘abstract’ for an abstract method (a method without body), we’ll also see this message.
To resolve the problem we must remove the ; (semicolon) from the header of the concrete method or if it is an abstract method the keyword ‘abstract’ must be written in the method header.

Here is an example of this error:

public abstract class MissingMethodBodyOrDeclareAbstract
{
public void display(String message);
{
System.out.println(message);
}
public void display();
}

Here, a ; (semicolon) is given after the method header of the first display method (with parameter). And for the second display method, keyword ‘abstract’ is not stated in the header and it doesn’t have any body also. Here it is to be noted that if a class contains an abstract method, the class must also be abstract.

The correct code is:

public abstract class MissingMethodBodyOrDeclareAbstractSolution
{
public void display(String message)
{
System.out.println(message);
}
public abstract void display();
}

Read More

package system does not exist

package system does not exist

Very simple error ‘package system does not exist’- most probably you have used ‘err’ or ‘in’ or ‘out’ field of the ‘System’ class of ‘java.lang’ package but you have given small s instead of capital S in the class name System.

See the below code:

public class PackageSystemDoesNotExistError
{
public static void main(String args[])
{
system.out.println("package system does not exist error"); // here s is small
}
}

But in the following code:

public class PackageSystemDoesNotExistErrorSolved
{
public static void main(String args[])
{
System.out.println("package system does not exist error solved");//S is capital
}
}

Though System is not a package, as you have written system.out.println – three parts here (system, out and println) a package like statement, Java compiler considers it as package but as this package doesn’t exist, the error message is shown.

So the solution is just change the small s of system to capital S.

Read More

class, interface, or enum expected

class, interface, or enum expected

You see this error message because of any one of the followings:

1. You misspelled the keyword class (fully lower case) or you did not write it at all for writing your class.

public Class Test //actual keyword is class
{
public void myMethod()
{
System.out.println("class, interface, or enum expected?");
}
}

In the above example Class (C is in upper case) is written instead of class (fully lower case).

2. Remember you cannot declare variables outside of class body like below.

int variable; //variable must be declared inside the class body
public class Test
{
public void myMethod()
{
System.out.println("class, interface, or enum expected?");
}
}

To solve this declare variables inside the class body as below.

public class Test
{
int variable; //variable must be declared inside the class body
public void myMethod()
{
System.out.println("class, interface, or enum expected?");
}
}

3. All methods are also defined within the method body. If you define any method outside of the class body you will see this error message.

public class Test
{
public void myMethod()
{
System.out.println("method inside the class body");
}
} //end of class

public void anotherMethod()
{
System.out.println("method outside of the class body");
}

In the above code, the method anotherMethod is defined after the end of the class. So this error occurred. Most often it occurs just because of incorrect use of {} (curly braces). Check all { are closed with } in the proper place.

4. If you are defining an interface, check you spelled it correctly in fully lowercase.

public Interface Test //actual keyword is interface
{
public void myMethod();

}

In the above code, keyword interface is misspelled with upper I.

5. Same way if you define an enum spell the keyword enum correctly.

public Enum Day //actual keyword is enum
{
SUNDAY, MONDAY, TUESDAY, WEDNESDAY,
THURSDAY, FRIDAY, SATURDAY
}

In the above example Enum is incorrect as the E is in upper case, it will be lower case e.

Read More