Java面试题每日10问(2)

蔚落 2023-09-23 21:36 34阅读 0赞

> Core Java - OOPs Concepts: Constructor Interview Questions

# 1.What is difference between constructor and method in Java? #

<table> 
 <thead> 
  <tr> 
   <th>Java Constructor</th> 
   <th>Java Method</th> 
  </tr> 
 </thead> 
 <tbody> 
  <tr> 
   <td>A constructor is used to initialize the state of an object.</td> 
   <td>A method is used to expose the behavior of an object.</td> 
  </tr> 
  <tr> 
   <td>A constructor must not have a return type.</td> 
   <td>A method must have a return type.</td> 
  </tr> 
  <tr> 
   <td>The constructor is invoked implicitly.</td> 
   <td>The method is invoked explicitly.</td> 
  </tr> 
  <tr> 
   <td>The Java compiler provides a default constructor if you don’t have any constructor in a class.</td> 
   <td>The method is not provided by the compiler in any case.</td> 
  </tr> 
  <tr> 
   <td>The constructor name must be same as the class name.</td> 
   <td>The method name may or may not be same as the class name.</td> 
  </tr> 
 </tbody> 
</table>

# 2.Why use the parameterized constructor? #

**parameterized constructor:**  
A constructor which has a specific number of parameters is called a parameterized constructor.

*  The parameterized constructor is used to provide different values to distinct objects.
 *  However, you can provide the same values also.

# 3.How many types of constructors are used in Java? #

There are two types of constructors in Java.

*  Default Constructor  
    – default constructor does not accept any value.  
    – default constructor is mainly used to initialize the instance variable with the default values.  
    – if the class have no constructor defined, it is invoked implicitly by the compiler
 *  Parameterized Constructor  
    – the parameterized constructor is a constructor that can initialize instance variables with given values.  
    – the constructors can accept the arguments called parameterized constructors.  
    ![在这里插入图片描述][6a14aeae473c4ae79669b562f10c1b11.png]  
    Java Copy Constructor

# 4.Does constructor return any value? #

yes, The constructor implicitly returns the current instance of the class

# 5. What do you understand by copy constructor in Java? #

There is no copy constructor in java.  
here are many ways to copy the values of one object into another in java.

*  By constructor
 *  By assigning the values of one object into another
 *  By clone() method of Object class

> Core Java - OOPs Concepts: static keyword Interview Questions

# 6.What is the static variable? #

*  The static variable is used to refer to the common property of all objects (that is not unique for each object)
 *  Static variable gets memory only once in the class area at the time of class loading.
 *  Using a static variable saves memory
 *  Static variable belongs to the class rather than the object.
 *  the staic can be:
    
     *  Variable (also known as a class variable)
     *  Method (also known as a class method)
     *  Block
     *  Nested class  
        ![在这里插入图片描述][b0284691097749ecb4b1b89d4424e94e.png]

public class Student1 {
        
        int rollno;
        String name;
        static String college ="ITS";
    
        Student1(int r, String n){
        
            rollno = r;
            name = n;
        }
        void display (){
        System.out.println(rollno+" "+name+" "+college);}
    
        public static void main(String args[]){
        
            Student1 s1 = new Student1(111,"Karan");
            Student1 s2 = new Student1(222,"Aryan");
    
            s1.display();
            s2.display();
        }
    }

111 Karan ITS
    222 Aryan ITS

# 7.What is the static method? #

*  A static method belongs to the class rather than the object.
 *  There is no need to create the object to call the static methods.
 *  A static method can access and change the value of the static variable.

# 8.What are the restrictions that are applied to the Java static methods? #

Two main restrictions are applied to the static methods.

*  The static method can not use non-static data member or call the non-static method directly.
 *  this and super cannot be used in static context as they are non-static.

# 9.Why is the Java main method static? #

*  It is because the object is not required to call a static method.
 *  If it were a non-static method, JVM creates an object first then call main() method that will lead the problem of extra memory allocation.

# 10. What is Java static block? #

*  It is used to initialize the static data member.
 *  It is executed before the main method at the time of classloading.

class A2{
          
      static{
        System.out.println("static block is invoked");}  
      public static void main(String args[]){
          
       System.out.println("Hello main");  
      }  
    }

static block is invoked
    Hello main

[6a14aeae473c4ae79669b562f10c1b11.png]: https://img-blog.csdnimg.cn/6a14aeae473c4ae79669b562f10c1b11.png
[b0284691097749ecb4b1b89d4424e94e.png]: https://img-blog.csdnimg.cn/b0284691097749ecb4b1b89d4424e94e.png