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Java Questionnaire I

What will happen if main method is declared as private?

If the main function is declared as private then the method will accessible only within the package level and JVM will not be able to access the Main method. So in that case code will compile but you will get run time error - Main method not public

What is the base class of all classes in Java?

java.lang.Object

What is the difference between interface and abstract class in Java?

      1. A class can implement multiple interfaces, but can inherit maximum one abstract class.

      2. 'implements' keyword is used to inherit an interface whereas 'extends' keyword is used to inherit an abstract class.

      3. If a class is inheriting an interface, then all the methods in the interface needs to be implemented, whereas in abstract class only abstract method needs to be implemented by the class.

      4. Interface does not have any constructors while Abstract classes can have constructors.

      5. Methods in Interface can’t be static while non abstract methods can be static.

What is a package in Java?

A package is a collection of related entities(like classes, interface, enums etc). Packages are used for

      1. Resolving class name conflicts - Two classes with same name can exists in different packages.

      2. Access control - If default access is provided to the class then that class is not accessible from other classes outside the package.

      3. Reusability - If a class contains some logic which can be reused, then other classes can use the class by importing the package.

What is difference between Path and Classpath in Java?

Path variable is for storing the location of executable files and batch files. Operating system looks into path variables for executables.

Classpath variable is for storing the location of class libraries and user defined classes. JVM system looks into path variables for class libraries and user defined classes.

What is meant by pass by reference and pass by value in Java?

In call by value, copy of parameters are passed as input parameters in a function. So if the value of the parameter is changed in the called method then the original value of the parameters remains intact. See the code below

class CallByValue
{
	void change(int i)
	{
		i=i*20;
		System.out.println("inside method i= "+i);
	}
}

class MainClass
{
	public static void main(String args[])
	{
		CallByValue v=new CallByValue();
		int x=20;
		System.out.println("Before call x =" + x);
		v.change(x);
		System.out.println("After call x =" + x);
	}
}

The output of the code will be

Before call x =20

inside method i= 400

After call x =20

As we can see from the output, even if the value of parameter in the called method changes then original value remains intact.

In call by reference, original parameters are passed as input parameters in the method. So if the value of the parameter is changed in the called method then the original value of the parameters also gets changed. See the code below

class CallByReference
{
	int i;
	CallByReference(int j)
	{
		i=j;
	}

	void change(CallByReference r)
	{
		r.i=r.i*20;
		System.out.println("inside method i ="+r.i);
	}
}

class MainClass
{
	public static void main(String args[])
	{
		CallByReference r1=new CallByReference(20);
		System.out.println("Before call i ="+r1.i);
		r1.change(r1);
		System.out.println("After call i ="+r1.i);
	}
}

The output of the code will be

Before call i =20

inside method i =400

After call i =400

As we can see from the output, even if the value in the called method changes then original value also changes.

How to convert String to a number in Java?

parseInt() method is used to convert string to number. See the code below

class ConvertStringToNumberDemo
{
	public static void main(String[] args) 
        {
        	String str = "1234";
                int num = Integer.parseInt(str);	
		System.out.println(num);		
	}
}

Output - 1234

In System.out.println(), what is System, out and println?

System is a package which belongs to java.lang package. out is a class which belongs to System package and println is a method of out class which prints the argument passes to the standard console in a new line.

What is JVM?

Java Virtual Machine is a Java component which converts compiled Java byte code to machine code.

What is the difference between the boolean & operator and && operator?

Boolean & operator is a bitwise and operator. Bitwise and operator returns true if and only if both of the bits in its operands are true else returns false. Consider an example below

class BitwiseAndDemo
{
  public static void main(String args[])
  {
        boolean  operandOne, operandTwo, result;
        operandOne = true;
        operandTwo = false;
        result = operandOne & operandTwo;
        System.out.printf("result is = " + result);	
  }
}

This program will return false because second operand is false.

Boolean && operator is conditional AND operator. Conditional and operator first evaluates first expression i.e. left hand side expression. If the first expression returns true then only second expression is evaluated.

class ConditionalAndDemo
{
  public static void main(String args[])
  {
      boolean  operandOne, operandTwo, result;
      operandOne = true;
      operandTwo = false;
      result = operandOne & operandTwo;
      System.out.printf("result is = " + result);	
  }
}

This program will return false because second expression is false.

So the difference between the 2 operators is - bitwise operator evaluates both the expressions whereas conditional operator evaluates first expression, if first expression returns true then second expression is evaluated.

Posted By -

Karan Gupta

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