There is some rule of assignments while handling variables
  • Implicit casting or conversion
    • higher-type ht = lower type;  // implicit casting allowed
      • double d ;
      • int i ;
      • d = i ; // implicit casting allowed
    • lower-type  lt  = (lower-type) ht; // explicit  casting allowed
    • one-type ot = (one-type)anoather-type;//explicit  conversion
  • you are not allowed to assign
  • lower- type lt = higher type type // implicit casting allowed
The conversions (from a smaller class or data type to a larger) can be implicit, but it's good practice to convert explicitly. The conversions from  larger to smaller data types must be explicitly converted, or cast.
import java.io.*;
//this won't compile
// javac double_or_int.java
public class double_or_int
{
public static void main(String args[])throws IOException
{
/*this is a comment
compile as
javac multi_comment.java
call as java simple_comment */
BufferedReader br = new BufferedReader(new InputStreamReader(System.in));

System.out.print("Type 1st double number only: ");
String str = "";
str = br.readLine();
System.out.println(str);
double d1 = Double.parseDouble(str);
//System.out.println();
System.out.print("Type 1st integer number only: ");
str = br.readLine();
System.out.println(str);
int n1 = Integer.parseInt(str);
System.out.println("Added up n1 + n1 = " + (d1+n1));
System.out.println("Divide up d1 / n1 = " + (d1 /n1));
double d2 = n1 /n1 + d1;
System.out.println("Divide up double d2= n1 /n1 + d1 = " + d2);
int n3 = n1 /n1 + d1;
System.out.println("Divide up int n3 = n1 /n1 + d1 = " + n3);
}
}
The following code will compile, but result will surprise you.

import java.io.*;
//this won't compile
// javac double_or_int_b.java
public class double_or_int_b
{
public static void main(String args[])throws IOException
{
/*this is a comment
compile as
javac multi_comment.java
call as java simple_comment */
BufferedReader br = new BufferedReader(new InputStreamReader(System.in));

System.out.print("Type 1st double number only: ");
String str = "";
str = br.readLine();
System.out.println(str);
double d1 = Double.parseDouble(str);
//System.out.println();
System.out.print("Type 1st integer number only: ");
str = br.readLine();
System.out.println(str);
int n1 = Integer.parseInt(str);
System.out.println("Added up n1 + n1 = " + (d1+n1));
System.out.println("Divide up d1 / n1 = " + (d1 /n1));
double d2 = n1 /n1 + d1;
System.out.println("Divide up double d2= n1 /n1 + d1 = " + d2);
//now this inludes an inplicit casting
//higher type = accepting lower type or eqaual
double n3 = n1 /n1 + d1;
System.out.println("Divide up double n3 = n1 /n1 + d1 = " + n3);
}
}
If you wanted to have n1 should be divide by n1 + d1; the grammar precedence rule was incorrect.

import java.io.*;
//this won't compile
// javac double_or_int_c.java
public class double_or_int_c
{
public static void main(String args[])throws IOException
{
/*this is a comment
compile as
javac multi_comment.java
call as java simple_comment */
BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
System.out.print("Type 1st double number only: ");
String str = "";
str = br.readLine();
System.out.println(str);
double d1 = Double.parseDouble(str);
//System.out.println();
System.out.print("Type 1st integer number only: ");
str = br.readLine();
System.out.println(str);
int n1 = Integer.parseInt(str);
System.out.println("Added up n1 + n1 = " + (d1+n1));
System.out.println("Divide up d1 / n1 = " + (d1 /n1));
double d2 = n1 /n1 + d1;
System.out.println("Divide up double d2= n1 /n1 + d1 = " + d2);
//now this inludes an inplicit casting
//higher type = accepting lower type
double n3 = n1 /(n1 + d1);
System.out.println("Divide up double n3 = n1 /(n1 + d1) = " + n3);
}
}

import java.io.*;
// javac double_or_int_d.java
public class double_or_int_d
{
public static void main(String args[])throws IOException
{
BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
String str = "";
System.out.print("Type 1st double number only: ");
try
{
str = br.readLine();
if(str!=""){
System.out.println(str);
double d1 = Double.parseDouble(str);
//implicit conversion is not allowed
//int n1 = d1;
int n1 = (int)d1;
System.out.println("\texplicit conversion \nfrom double to integer : " + n1 + " is an integer type");
}
}
catch(NumberFormatException e) { System.out.println("data was blank");}
//implicit conversion from integer to double
System.out.print("Type an integer number only: ");
try
{
str = br.readLine();
if(str!=""){
double d1 = Double.parseDouble(str);
int n2 = Integer.parseInt(str);
d1 = n2;
System.out.println("\timplicit conversion from \ninteger to double : " + d1 + " is an double type ");
}
}
catch(NumberFormatException e) { System.out.println("data was blank");}
}
}