• When there are no more references to an object, the object is finalized and when the Garbage Collections starts these finalized objects gets collected this will done automatically by jvm.
• But if we want to call  Garbage collection explicitly, There are methods
  1.System.gc();
  2.Runtime.gc(); 

• The java.lang.Runtime.freeMemory() method returns the amount of free memory in the Java Virtual Machine. Calling the gc method may result in increasing the value returned by freeMemory

Shallow copy:

• Shallow copy is a bit-wise copy of an object. A new object is created that has an exact copy of the values in the original object. If any of the fields of the object are references to other objects, just the reference addresses are copied i.e., only the memory address is copied.

Example Program:
take a sample class
Step 1:
public class Sample {

int a;
int b;

Sample(int a, int b){
    this.a=a;
    this.b=b;
}

}
Step 2:
package com.oops;

public class empclone implements Cloneable {

    Sample s;
    int a;
   
    empclone(int a, Sample s){
        this.a=a;
        this.s=s;
      
    }
   
    public Object clone()throws CloneNotSupportedException{
      
        return super.clone();
        }
          
   
    public static void main(String[] args) {
    
        empclone a= new empclone(2, new Sample(3,3));
        empclone b=null;
   
        try {
             b=(empclone)a.clone();
          
        } catch (CloneNotSupportedException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
        System.out.println(a.s.a);
        System.out.println(b.s.a);
      
        a.s.a=12;
        System.out.println(a.s.a);
        System.out.println(b.s.a);
    }

}

Output:
3
3
12
12
whenever we implements cloneable interface and ovverides clone() method
public Object clone()throws CloneNotSupportedException{         
        return super.clone();
        } 
By default it will give shallow copy means if any objects presents in orninal class it simply gives reference so if we change anything in original object changes will made to copied/cloned object
because both are pointing to same object

Deep copy:

•  A deep copy copies all fields, and makes copies of dynamically allocated memory pointed to by the fields. A deep copy occurs when an object is copied along with the objects to which it refers. 

Class:

• Class is a structure.
• Binding the data with its related and corresponding functions.
• Class is the base for encapsulation.
• Class is a user defined data type in java.
• Class will act as the base for encapsulation and implement the concept of encapsulation through objects.
• Any java applications looks like collection of classes but where as c- application looks like collection of functions

Object:

• Object is nothing but instance (dynamic memory allocation) of a class.
• The dynamic memory allocated at run time for the members [non-static variables] of the class is known as object.

For more points click here

• Oops concepts are the rules which are supposed to be satisfied by a programming language in order to call that programming language as an Object Oriented Programming Language
• The three oops concepts are
          i.Encapsulation
          ii. Polymorphism
          iii.Inheritance
• Encapsulation:
  The process of binding the data with corresponding functions.
• Polymorphism:
  Defining multiple methods with same name.
• Inheritance:
  Getting the properties from one class object to another class object 

• Compilation Error -This is because you can pass an Object or a String or char[]. Since null can fit in both, the compiler doesn't know which method to use, leading to compile error.


• Method Overloading:
      1.public void prinltln(String str) { }
      2.public void prinltln(char[] ch){ }
      3.public void prinltln(Object ch){ 
•  It seems the call System.out.print(null) is ambiguous to compiler because print(null) here will find the two best specific matches i.e. print(String) and print(char[]) . So compiler is unable to determine which method to call here .
• Compilation Error:
     System.out.println(null)

• Compile Fine:
       System.out.println((String)null);//null
       System.out.println((char[])null);
       System.out.println((Object)null);//null
• It's the compiler type-checking the parameters of the method call.
• But here we need to know one more thing  System.out.println((char[])null); will compile fine but at run time will throw runtime exception.

• System is a class Which is present in java.lang package.
  Out is a static final field (variable) of  Printstream class
  Println(): method of Printstream Class
• Class System{
  public static final Printstream Out;
  }
• Class Printstream{
  public void println(){}}