Most important

Question 1

What is a constructor

Answer 1

A block of code that is executed when a class is instantiated.

Question 2

By default if you create a class and don’t specify a constructor the compiler will generate one for you. The one generated is called the default ________

Answer 2

no-arg constructor

Question 3

What is a method in Java?

Answer 3

A method is a block of code which only runs when it is called. You can pass data, known as parameters, into a method. Methods are used to perform certain actions, and they are also known as functions.

Question 4

In a Java method, what is the difference between a parameter & an argument?

Answer 4

Parameters refers to the list of variables in a method declaration. Arguments are the actual values that are passed in when the method is invoked. When you invoke a method, the arguments used must match the declaration’s parameters in type & order.

Question 5

What is Method Overloading in Java?

Answer 5

If a class has multiple methods having the same name but different in parameters, it is known as Method overloading.

Question 6

What are the different ways to overload a method in Java?

Answer 6

1. By changing number of arguments

2. By changing the data type

Question 7

What is State in Java?

Answer 7

State is a behavioral design pattern that allows an object to change the behavior when its internal state changes.

Question 8

What is the Object Class in Java?

Answer 8

The object class is the parent class of all classes defined in java even if you do not explicitly extend it.

Question 9

What is Polymorphism?

What is it’s most common use in OOP?

Answer 9

The ability of an object to take on many forms. The most common use of polymorphism in OOP occurs when a parent class reference is used to refer to a child class object. Any Java object that can pass more than one IS-A test is considered to be polymorphic. In Java, all Java objects are polymorphic since any object will pass the IS-A test for their own type and for the class Object.

Question 10

What is a reference variable in Java?

Answer 10

A link to an instance of an object in memory

Question 11

What is an API?

Answer 11

Short for “Application Programming Interface”, it is a library of functions you can use to interact with a program or component.

Question 12

What is the java.lang package?

Answer 12

The default package, it is implicitly imported into all java classes, so it requires no declaration. Many useful classes come from this package including system and string.

Question 13

The string class in Java is an “immutable” class, what does this mean?

Answer 13

It can not be changed

Question 14

What is special about the string class in Java?

Answer 14

1. It can be instantiated in two different ways unlike all other classes, (with or without the new keyword)
2. String objects are often recycled by the JVM instead of creating new ones in what is called the “String Pool” Which saves memory.

Question 15

What is the difference between equals() & ==?

Answer 15

The operator == is used to compare the value of primitives ex.
int x = 6;
int y = 5;
if (x ==6) { .. }
If you compare two reference variables using the == operator will compare the reference address of the objects each variable points to. It may not return true even if the contents of those two objects are the same.

This is why we use the equals() method with objects. It is expected to that it will help us compare the values of objects with one another ex.
String s = "Hello";
String s2 = new String("Hello");
if (s.equals(s2)) {..}
This is because the equals() method of the String class has been overridden to compare the contents of the current String and one passed in as an argument.

Question 16

If you want to create a mutable, i.e. changeable string what would you use?

Answer 16

A StringBuilder or StringBuffer object.

Question 17

When Should you use a String builder Object?

Answer 17

You use a StringBuilder when you are not using threads, or are performing thread control safely somewhere else.

Question 18

When should you use a String buffer object?

Answer 18

You can use a StringBuffer whenever you need a mutable string object, and also need it to be thread-safe. A StringBuffer object will guarantee that only one thread can access it at a time, preventing confusion. This can slow performance

Question 19

What are “keywords” in Java?

Answer 19

Reserved words in Java that give the compiler specific instructions (so they can’t be used to name fields, methods, or classes)

Question 20

What is the “static” keyword in Java?

Answer 20

This modifier defines “class” variable & methods, as opposed to “instance” members

Question 21

What does the “final” keyword do in Java?

Answer 21

1. Makes things immutable (unchangeable)
2. Primitive vars can’t modify values
3. Reference vars can’t switch references
4. Methods can’t be overridden
5. Classes can’t be extended
   * Your application will not compile if you try to break these rules

Question 22

What does the keyword “Abstract” do in Java?

Answer 22

It defines an incomplete entity, each concrete subclass will be forced to define implementation details for itself.
An abstract class cannot be instantiated, it can only ever be extended. Probably research more on this

Question 23

What can the keyword “final” be applied to in Java?

Answer 23

A class, method, or variable. In all cases it means the structure cannot be changed.

Question 24

When applied to a variable what does the keyword “final” mean?

Answer 24

That you cannot reassign the variable to another value. You must initialize the final variable when declaring it.

Question 25

When applied to a method what does the keyword “final” mean?

Answer 25

That you cannot override the method

Question 26

When applied to a class what does the keyword “final” mean?

Answer 26

That you cannot create a subclass of this class

Question 27

What is a common example of a “final” class in Java?

Answer 27

The string class