Java Interview Questions – 基础篇

对于Entry level 或 Junior level的职位,技术面试中,80%的题目是非常基础的,能不能抓住这80% 是很重要的,如果基础的题目都不能回答正确,面试基本上就失败了。

1. Explain JDK, JRE and JVM?

JDK vs JRE vs JVM

JDK JRE JVM
It stands for Java Development Kit. It stands for Java Runtime Environment. It stands for Java Virtual Machine.
It is the tool necessary to compile, document and package Java programs. JRE refers to a runtime environment in which Java bytecode can be executed. It is an abstract machine. It is a specification that provides a run-time environment in which Java bytecode can be executed.
It contains JRE + development tools. It’s an implementation of the JVM which physically exists. JVM follows three notations: Specification, Implementation, and Runtime Instance.

2. Explain public static void main(String args[]) in Java.  (划重点)

main() in Java is the entry point for any Java program. It is always written as public static void main(String[] args).

  • public: Public is an access modifier,  specify who can access this method. Public means that this Method will be accessible by any Class.
  • static: It is a keyword in java which identifies it is class-based. main() is made static in Java so that it can be accessed without creating the instance of a Class. In case, main is not made static then the compiler will throw an error as main() is called by the JVM before any objects are made and only static methods can be directly invoked via the class.
  • void: It is the return type of the method. Void defines the method which will not return any value.
  • main: It is the name of the method which is searched by JVM as a starting point for an application with a particular signature only. It is the method where the main execution occurs.
  • String args[]: It is the parameter passed to the main method.

Q3. Why Java is platform independent?

Java is called platform independent because of its byte codes which can run on any system irrespective of its underlying operating system.

Q4. Why Java is not 100% Object-oriented?

Java is not 100% Object-oriented because it makes use of eight primitive data types such as boolean, byte, char, int, float, double, long, short which are not objects.

Q5. What are wrapper classes in Java?

  • Wrapper classes convert the Java primitives into the reference types (objects).
  • Every primitive data type has a class dedicated to it.  Wrapper classes  wrap the primitive data type into an object of that class.

Q6. What are constructors in Java?  (划重点)

  • a block of code which is used to initialize an object.
  • It must have the same name as that of the class.
  • It has no return type and it is automatically called when an object is created.

There are two types of constructors:

  1. Default Constructor:  a default constructor is the one which does not take any inputs.  Default constructors are the no argument constructors which will be created by default in case it isn’t defined by the user. Its main purpose is to initialize the instance variables with the default values for object creation.
  2. Parameterized Constructor: The parameterized constructor  is capable of initializing the instance variables with the provided values.

Q7. What is singleton class in Java and how can we make a class singleton?  (senior 必考题)

Singleton class is one instance can be created at any given time, it can be made singleton by making its constructor private.

Q8. What is the difference between Array list and vector in Java?

ArrayList Vector
Array List is not synchronized.  Vector is synchronized.
Array List is fast as it’s non-synchronized. Vector is slow as it is thread safe.
If an element is inserted into the Array List, it increases its Array size by 50%. Vector defaults to doubling size of its array.
Array List does not define the increment size. Vector defines the increment size.
Array List can only use Iterator for traversing an Array List. Vector can use both Enumeration and Iterator for traversing.

Q9. What is the difference between equals() and == in Java?  (划重点)

  • Equals() method is defined in Object class in Java and used for checking equality of two objects defined by business logic.
  • “==” or equality operator in Java is a binary operator  used to compare primitives and objects.
  •  public boolean equals(Object o) is the method provided by the Object class. The default implementation uses == operator to compare two objects. For example: method can be overridden like String class. equals() method is used to compare the values of two objects.

Q10. What are the differences between Heap and Stack Memory in Java?

The major difference between Heap and Stack memory are:

Features Stack Heap
Memory Stack memory is used only by one thread of execution. Heap memory is used by all the parts of the application.
Access Stack memory can’t be accessed by other threads. Objects stored in the heap are globally accessible.
Memory Management Follows LIFO manner to free memory. Memory management is based on the generation associated with each object.
Lifetime Exists until the end of execution of the thread. Heap memory lives from the start till the end of application execution.
Usage Stack memory only contains local primitive and reference variables to objects in heap space. Whenever an object is created, it’s always stored in the Heap space.

Q11. What is a package in Java? List down various advantages of packages.

  • Packages are the collection of related classes and interfaces which are bundled together.
  • By using packages, developers can easily modularize the code and optimize its reuse.
  • Also, the code within the packages can be imported by other classes and reused.
  • Packages help in avoiding name clashes
  • They provide easier access control on the code
  • Packages can also contain hidden classes which are not visible to the outer classes and only used within the package
  • Creates a proper hierarchical structure which makes it easier to locate the related classes

Q12. What are access modifiers in Java?  (划重点)

In Java, access modifiers are special keywords which are used to restrict the access of a class, constructor, data member and method in another class. Java supports four types of access modifiers:

  1. Default
  2. Private
  3. Protected
  4. Public
Modifier Default Private Protected Public
Same class YES YES YES YES
Same Package subclass YES NO YES YES
Same Package non-subclass YES NO YES YES
Different package subclass NO NO YES YES
Different package non-subclass NO NO NO YES

Q13. Define a Java Class.

A class in Java is a blueprint which includes all your data.  A class contains fields (variables) and methods to describe the behavior of an object. Let’s have a look at the syntax of a class.

1
2
3
class Abc {
member variables // class body
methods}

Q14. What is an object in Java and how is it created?

An object is a real-world entity that has a state and behavior. An object has three characteristics:

  1. State
  2. Behavior
  3. Identity

An object is created using the ‘new’ keyword.   ClassName objA = new ClassName();

Q15. What is Object Oriented Programming? 

Object-oriented programming (OOPs) is a programming model or approach where the programs are organized around objects rather than logic and functions.

OOP  focuses on the objects that are required to be manipulated instead of logic.

OOP is ideal for the programs large and complex codes and needs to be actively updated or maintained.

Q16. What are the main concepts of OOPs in Java? (划重点)

Object-Oriented Programming or OOPs is a programming style that is associated with concepts like:

  1. InheritanceInheritance is a process where one class acquires the properties of another.
  2. EncapsulationEncapsulation in Java is a mechanism of wrapping up the data and code together as a single unit.
  3. AbstractionAbstraction is the methodology of hiding the implementation details from the user and only providing the functionality to the users.
  4. PolymorphismPolymorphism is the ability of a variable, function or object to take multiple forms.

Q17. What is the difference between a local variable and an instance variable?

local variable

  • it is typically used inside a method, constructor, or a block and has only local scope.
  • it can be used only within the scope of a block.
if(x > 100)
{
String test = "test1";
}

instance variable 

  • it is a variable which is bounded to its object itself. These variables are declared within a class, but outside a method.
  • Every object  will create it’s own copy of the variable while using it. Thus, any changes made to the variable won’t reflect in other instances of that class and will be bound to that particular instance only.
class Test{
public String empName;
public int empAge;
}

Q18. Differentiate between the constructors and methods in Java?  (划重点)

Methods Constructors
1. Used to represent the behavior of an object 1. Used to initialize the state of an object
2. Must have a return type 2. Do not have any return type
3. Needs to be invoked explicitly 3. Is invoked implicitly
4. No default method is provided by the compiler 4. A default constructor is provided by the compiler if the class has none
5. Method name may or may not be same as class name 5. Constructor name must always be the same as the class name

Q19. What is final keyword in Java?  (划重点)

final is a special keyword in Java that is used as a non-access modifier.

  • final variable

final variable value can’t be changed once assigned. In case the no value has been assigned to the final variable then using only the class constructor a value can be assigned to it.

  • final method –  final method can’t be overridden by the inheriting class.

  • final class –  can’t be extended by any subclass class but it can extend other non-final class.  

    • To avoid derived class from overriding a particular base class functionality.
    • This is for security purpose, where base class is giving some important core functionality of the framework where derived class is not supposed to change it.
    • java made all wrapper classes as final classes to avoid be changed.

Q20. What is the difference between break and continue statements?

break continue
1. Can be used in switch and loop (for, while, do while) statements 1. Can be only used with loop statements
2. It causes the switch or loop statements to terminate the moment it is executed 2. It doesn’t terminate the loop but causes the loop to jump to the next iteration
3. It terminates the innermost enclosing loop or switch immediately 3. A continue within a loop nested with a switch will cause the next loop iteration to execute
Example break:
for (int i = 0; i < 5; i++)
{
if (i == 3)
{
break;
}
System.out.println(i);
}
Example continue:
for (int i = 0; i < 5; i++)
{
if(i == 2)
{
continue;
}
System.out.println(i);
}
Q21.What is an infinite loop in Java? Explain with an example.

An infinite loop is an instruction sequence in Java that loops endlessly until a functional exit met.

For example:

public class InfiniteForLoopDemo
{
public static void main(String[] arg) {
for(;;)
System.out.println("Welcome to Ottawa!");
// To terminate this program press ctrl + c in the console.
}
}

Q22. What is the difference between this() and super() in Java?  (划重点)

super() and this(), both are special keywords that are used to call the constructor.

this() super()
1. this() represents the current instance of a class 1. super() represents the current instance of a parent/base class
2. Used to call the default constructor of the same class 2. Used to call the default constructor of the parent/base class
3. Used to access methods of the current class 3. Used to access methods of the base class
4.  Used for pointing the current class instance 4. Used for pointing the superclass instance
5. Must be the first line of a block 5. Must be the first line of a block

Q23. What is Java String Pool?

Java String pool refers to a collection of Strings which are stored in heap memory.  whenever a new object is created, String pool first checks whether the object is already present in the pool or not. If it is present, then the same reference is returned to the variable else new object will be created in the String pool and the respective reference will be returned.

 

Q24. Differentiate between static and non-static methods in Java.

Static Method Non-Static Method
1. The static keyword must be used before the method name 1. No need to use the static keyword before the method name
2. It is called using the class (className.methodName) 2. It is can be called like any general method
3. They can’t access any non-static instance variables or methods 3. It can access any static method and any static variable without creating an instance of the class

Q25. What is constructor chaining in Java?

In Java, constructor chaining is the process of calling one constructor from another with respect to the current object. Constructor chaining is possible only through legacy where a subclass constructor is responsible for invoking the superclass’ constructor first. There could be any number of classes in the constructor chain. Constructor chaining can be achieved in two ways:

  1. Within the same class using this()
  2. From base class using super()

Q26. Difference between String, String Builder, and String Buffer.

Factor String String Builder String Buffer
Storage Area Constant String Pool Heap Area Heap Area
Mutability Immutable Mutable Mutable
Thread Safety Yes Yes No
Performance Fast Slow Fast

Q27. What is a classloader in Java?

The Java ClassLoader is a subset of JVM (Java Virtual Machine) that is responsible for loading the class files. Whenever a Java program is executed it is first loaded by the classloader. Java provides three built-in classloaders:

  1. Bootstrap ClassLoader
  2. Extension ClassLoader
  3. System/Application ClassLoader

Q28. Why Java Strings are immutable in nature?  (划重点)

In Java, string objects are immutable in nature which simply means once the String object is created its state cannot be modified. Whenever you try to update the value of that object instead of updating the values of that particular object, Java creates a new string object. Java String objects are immutable as String objects are generally cached in the String pool. Since String literals are usually shared between multiple clients, action from one client might affect the rest. It enhances security, caching, synchronization, and performance of the application.

Q29. What is the difference between an array and an array list?

Array ArrayList
Cannot contain values of different data types Can contain values of different data types.
Size must be defined at the time of declaration Size can be dynamically changed
Need to specify the index in order to add data No need to specify the index
Arrays are not type parameterized Arraylists are type
Arrays can contain primitive data types as well as objects Arraylists can contain only objects, no primitive data types are allowed

Q30. What is a Map in Java?

Map is an interface of Util package which maps unique keys to values. The Map interface is not a subset of the main Collection interface and thus it behaves little different from the other collection types. Below are a few of the characteristics of Map interface:

  1. Map doesn’t contain duplicate keys.
  2. Each key can map at max one value.

Q31. What is collection class in Java? List down its methods and interfaces.   (划重点)

The collection is a framework that acts as an architecture for storing and manipulating a group of objects. Using Collections you can perform various tasks like searching, sorting, insertion, manipulation, deletion, etc. Java collection framework includes the following:

  • Interfaces
  • Classes
  • Methods

 

In case you are facing any challenges with these java interview questions, please comment on your problems in the section below.

Q32. What is JIT compiler in Java?

JIT stands for Just-In-Time compiler in Java. It is a program that helps in converting the Java bytecode into instructions that are sent directly to the processor. By default, the JIT compiler is enabled in Java and is activated whenever a Java method is invoked. The JIT compiler then compiles the bytecode of the invoked method into native machine code, compiling it “just in time” to execute. Once the method has been compiled, the JVM summons the compiled code of that method directly rather than interpreting it. This is why it is often responsible for the performance optimization of Java applications at the run time.

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