Q25. Is it compulsory for a Try Block to be followed by a Catch Block in Java for Exception handling?

Ans: Try block needs to be followed by either Catch block or Finally block or both. Any exception thrown from try block needs to be either caught in the catch block or else any specific tasks to be performed before code abortion are put in the Finally block.

 

Q26. Is there any way to skip Finally block of exception even if some exception occurs in the exception block?

Ans: If an exception is raised in Try block, control passes to catch block if it exists otherwise to finally block. Finally block is always executed when an exception occurs and the only way to avoid execution of any statements in Finally block is by aborting the code forcibly by writing following line of code at the end of try block:

System.exit(0);

 

Q27. When the constructor of a class is invoked?

Ans: The constructor of a class is invoked every time an object is created with new keyword.

For example, in the following class two objects are created using new keyword and hence, constructor is invoked two times.

public class const_example {

    const_example() {

        system.out.println("Inside constructor");
    }
    public static void main(String args[]) {

        const_example c1 = new const_example();

        const_example c2 = new const_example();
    }
}

 

Q28. Can a class have multiple constructors?

Ans: Yes, a class can have multiple constructors with different parameters. Which constructor gets used for object creation depends on the arguments passed while creating the objects.

 

 

Q29. Can we override static methods of a class?

Ans: We cannot override static methods. Static methods belong to a class and not to individual objects and are resolved at the time of compilation (not at runtime).Even if we try to override static method,we will not get an complitaion error,nor the impact of overriding when running the code.

 

 

Q30. In the below example, what will be the output?

public class superclass {

    public void displayResult() {

        system.out.println("Printing from superclass");

    }

}

public class subclass extends superclass {

    public void displayResult() {

        system.out.println("Displaying from subClass");

        super.displayResult();

    }

    public static void main(String args[]) {

        subclass obj = new subclass();

        obj.displayResult();

    }

}

Ans: Output will be:

Displaying from subclass

Displaying from superclass

 

 

Q31. Is String a data type in java?

Ans: String is not a primitive data type in java. When a string is created in java, it's actually an object of Java.Lang.String class that gets created. After creation of this string object, all built-in methods of String class can be used on the string object.

 

 

Q32. In the below example, how many String Objects are created?

String s1="I am Java Expert";

String s2="I am C Expert";

String s3="I am Java Expert";

Ans: In the above example, two objects of Java.Lang.String class are created. s1 and s3 are references to same object.

 

 

Q33. Why Strings in Java are called as Immutable?

Ans: In java, string objects are called immutable as once value has been assigned to a string, it can't be changed and if changed, a new object is created.

In below example, reference str refers to a string object having value "Value one".

String str="Value One";

When a new value is assigned to it, a new String object gets created and the reference is moved to the new object.

str="New Value";

Q34. What's the difference between an array and Vector?

Ans: An array groups data of same primitive type and is static in nature while vectors are dynamic in nature and can hold data of different data types.

 

Q35. What is multi-threading?

Ans: Multi threading is a programming concept to run multiple tasks in a concurrent manner within a single program. Threads share same process stack and running in parallel. It helps in performance improvement of any program.

 

Q36. Why Runnable Interface is used in Java?

Ans: Runnable interface is used in java for implementing multi threaded applications. Java.Lang.Runnable interface is implemented by a class to support multi threading.

 

Q37. What are the two ways of implementing multi-threading in Java?

Ans: Multi threaded applications can be developed in Java by using any of the following two methodologies:

1. By using Java.Lang.Runnable Interface. Classes implement this interface to enable multi threading. There is a Run() method in this interface which is implemented.

2. By writing a class that extend Java.Lang.Thread class.

 

Q38. When a lot of changes are required in data, which one should be a preference to be used? String or StringBuffer?

Ans: Since StringBuffers are dynamic in nature and we can change the values of StringBuffer objects unlike String which is immutable, it's always a good choice to use StringBuffer when data is being changed too much. If we use String in such a case, for every data change a new String object will be created which will be an extra overhead.

 

Q39. What's the purpose of using Break in each case of Switch Statement?

Ans: Break is used after each case (except the last one) in a switch so that code breaks after the valid case and doesn't flow in the proceeding cases too.

If break isn't used after each case, all cases after the valid case also get executed resulting in wrong results.

 

Q40. How garbage collection is done in Java?

Ans: In java, when an object is not referenced any more, garbage collection takes place and the object is destroyed automatically. For automatic garbage collection java calls either System.gc() method or Runtime.gc() method.

 

Q41. How we can execute any code even before main method?

Ans: If we want to execute any statements before even creation of objects at load time of class, we can use a static block of code in the class. Any statements inside this static block of code will get executed once at the time of loading the class even before creation of objects in the main method.

 

Q42. Can a class be a super class and a sub-class at the same time? Give example.

Ans: If there is a hierarchy of inheritance used, a class can be a super class for another class and a sub-class for another one at the same time.

In the example below, continent class is sub-class of world class and it's super class of country class.

public class world {

..........

}
public class continenet extends world {

............

}
public class country extends continent {

......................

}

 

Q43. How objects of a class are created if no constructor is defined in the class?

Ans: Even if no explicit constructor is defined in a java class, objects get created successfully as a default constructor is implicitly used for object creation. This constructor has no parameters.

 

 

Q44. In multi-threading how can we ensure that a resource isn't used by multiple threads simultaneously?

Ans: In multi-threading, access to the resources which are shared among multiple threads can be controlled by using the concept of synchronization. Using synchronized keyword, we can ensure that only one thread can use shared resource at a time and others can get control of the resource only once it has become free from the other one using it.

 

Q45. Can we call the constructor of a class more than once for an object?

Ans: Constructor is called automatically when we create an object using new keyword. It's called only once for an object at the time of object creation and hence, we can't invoke the constructor again for an object after its creation.

 

Q46. There are two classes named classA and classB. Both classes are in the same package. Can a private member of classA can be accessed by an object of classB?

Ans: Private members of a class aren't accessible outside the scope of that class and any other class even in the same package can't access them.

 

Q47. Can we have two methods in a class with the same name?

Ans: We can define two methods in a class with the same name but with different number/type of parameters. Which method is to get invoked will depend upon the parameters passed.

For example in the class below we have two print methods with same name but different parameters. Depending upon the parameters, appropriate one will be called:

public class methodExample {

    public void print() {

        system.out.println("Print method without parameters.");

    }

    public void print(String name) {

        system.out.println("Print method with parameter");

    }

    public static void main(String args[]) {

        methodExample obj1 = new methodExample();

        obj1.print();

        obj1.print("xx");

    }

}

 

Q48. How can we make copy of a java object?

Ans: We can use the concept of cloning to create copy of an object. Using clone, we create copies with the actual state of an object.

Clone() is a method of Cloneable interface and hence, Cloneable interface needs to be implemented for making object copies.

 

Q49. What's the benefit of using inheritance?

Ans: Key benefit of using inheritance is reusability of code as inheritance enables sub-classes to reuse the code of its super class. Polymorphism (Extensibility ) is another great benefit which allow new functionality to be introduced without effecting existing derived classes.