Java Interview test questions

1.Describe your understanding of Object-oriented

what is a class  What is an object class is a mold object is an instance of a mold

All things are objects A class is a set of objects that have the same characteristics and similar behavior

Relationship of classes and objects

in reality There are objects first . Post-Extraction production class

in the program First Design class after instantiating an object

Packaging

hide the specific implementation provide an external user interface to call the user

Safety Multiplexing

Inherited

A class inherits from another class all properties of the inherited class are obtained and The process of all methods except the construction method is called inheritance

Polymorphic

multiple subclasses inherit from the same parent class different implementations of the same name method for the parent class declaration known as polymorphic

2. A class is final modified to represent the class.

The final modified class becomes an end-state class that cannot be inherited by other classes. The class cannot contain abstract methods

3. What is the difference between string s= "Hello" and string b=new string ("Hello") ?
use the new keyword to represent a force in memory to open up memory space for storing data

4 . The difference between an interface and an abstract class

the interface is a special class

in the interface only the methods that are not implemented No attributes The method also does not need to be abstract to modify methods and abstract methods in an abstract class that can contain property implementations

a class only one abstract class can be inherited but multiple interfaces can be implemented

5,Math.Round (11.5) How much ? Math.Round ( -11.5) How much ?

12-11

6. How do iterators work in Java?

Traversing a collection element

7, scope public,private,protected, and the difference when not written

public on behalf all other classes are accessible

private representatives can only be accessed in this class.

Protected protected delegates can only be accessed in this class and in subclasses of this class

default the default can be accessed in the same package

8 . What are the differences between method overloads and overrides?

Overload

for a class method Name is the same   different parameters multiple overloaded methods can coexist

Override

for parent-child classes that form inheritance  method names and parameters are the same methods for parent-child class overrides Child class overrides parent class

9. What is the difference between String and stringbuffer and StringBuilder ?

String is an underlying data type provided by the system

StringBuffer and StringBuilder are both string manipulation classes

StringBuffer thread Safety but low access efficiency

StringBuilder Thread is unsafe but access efficiency is high

10, say arraylist,vector, LinkedList storage performance and characteristics ?

The storage mechanism looks at:

ArrayList and vectors are the same LinkedList are linked list structure

Characteristic view

ArrayList for full query expansion 50%

Vector compared to ArrayList thread safety efficiency and the expansion rate is doubled

LinkedList suitable for frequently inserted and deleted operations

11, the difference between HashMap and Hashtable ?

A) HashMap implement Map interface HashTable inherit from Dictionary class

b) thread insecure but high efficiency thread safety efficiency Low

c) Keys and values can be null key values cannot be null

When doyou use an abstract class and when do you use an interface?

what is the difference between a. Super and this?

Super accesses the parent class of the class super () to access the parent construction

this access to members of this class

Java Interview test questions