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Is it possible to modify the value of an attribute sent in argument of a function in Java?

I'm working on a calculator and I search how I can optimize my code.
The thing is that I have much code duplication due to if I'm working on the first number of the calculation or the second. So I'm searching if it is possible to modify the value of an attribute sent in argument of a function ? (I think not because I saw nowhere the answer).
Maybe I'm expressing myself badly so here is a code below to explain what I'm talking about:
public class MyClass
{
private static int number1 = 1;
private static int number2 = 2;
public MyClass()
{
changeValueOf(number1, 3);
}
private static void changeValueOf(int number, int value)
{
//Change here the value of the correct field
}
}

First of all, you can modify static variables inside the method:
private static void changeValueOf(int value)
{
number1 = value;
}
But I guess that is not what you a looking for :)
In Java (and in most other languages) primitive data type (int, short, long, etc) passed by value, e.g. the copy of value passes to the method (function).
And reference types (objects, e.g. created with new operator) passed by reference. So, when you modigy the value of reference type (object) you can see the changes in the outer scopes (for example, in method caller).
So, the answer is no - you cannot change the value of int so that the outer scope would see the updated value.
Howewer, you could wrap your int values with some object - and it change the value inside of it:
public class Example {
public static void main(String[] args) {
Example app = new Example();
// Could be static as well
Holder val1 = new Holder(1);
Holder val2 = new Holder(2);
app.changeValue(val1, 7);
System.out.println(val1.value); // 7
}
public void changeValue(Holder holder, int newValue) {
holder.value = newValue;
}
static class Holder {
int value;
Holder(int value) {
this.value = value;
}
}
}
Also, you could create an array with 2 values and update them inside the method, but it's not very good approach IMO
And finally, you could just return updated value and assign it to your variables:
public class Example {
private static int number1 = 2;
private static int number2 = 3;
public static void main(String[] args) {
Example app = new Example();
number1 = app.mul(number1, 7);
number2 = app.mul(number2, 7);
System.out.println(number1); // 14
System.out.println(number2); // 21
}
public int mul(int a, int b) {
return a * b;
}
}

One possibility is to use an array to store your variables, instead of separate variables with numbers affixed. Then you would write number[1] instead of number1 for example. You can pass the array index number around to indicate which variable you are referring to.
public class MyClass
{
private static int[] variables = {1, 2};
public MyClass()
{
// change value of first variable
changeValueOf(0, 3);
// now variable[0] = 3
}
private static void changeValueOf(int number, int value)
{
variables[number] = value;
}
}

Passing two variables to one method to get two results

I am trying to pass two variables along to a method and have the method give me back two independent results.
int numX = 5;
int numY = 3;
System.out.println(displayTwiceTheNumber(numX, numY));
}
public static int displayTwiceTheNumber(int numX, int numY) {
int numW, numZ;
numW, numZ = 2 * (numX, numY);
return numW numZ;
}
Java takes it that at numW, numZ = 2 * (numX, numY); that I am trying to redefine numX and numY. How do I phrase the last block to take two variables and give two results?

A single int function can only return 1 int at a time.
If you want to return 2 values, consider calling the function twice or creating a custom object to be used.

You need to change the return type of the function. Currently, the return type is int, so you have to return one integer.
To return two integer, you should consider returning an array or a list or something similar.
public static int[] displayTwiceTheNumber(int numX, int numY){
//your code that do something
int[] ret = {numW, numZ};
return ret;
}
Or knowing that this function would change the value of numW and numZ, you could declare those as global variable. Now, when you call this function, those variable will be changed. Then, you can use numW and numZ subsequently.
public int numW;
public int numZ;
public static void displayTwiceTheNumber(int numX, int numY){
//your code that do something and modifies numW and numZ
}
public static void anotherfunction(){
//after calling displayTwiceTheNumber, numW and numZ would have the appropriate value
//you can now just use numW and numZ directly
}

Overview: Use a tuple. In this example I use an tuple to return more than one result. Tuple means to return more than one result type. In this example I return a tuple of two integer types. My class TupleCustom contains one method function1 which receives two parameters of type integer: x and y. I create a tuple of type integer and return the tuple as a variable. Internally, the precomiler converts the tuple json than back to a tuple with variable Item1, Item2...ItemN in the unit test method.
public class TupleCustom
{
public async Task<Tuple<int, int>> Function1(int x, int y)
{
Tuple<int, int> retTuple = new Tuple<int, int>(x, y);
await Task.Yield();
return retTuple;
}
}
public class TestSuite
{
private readonly ITestOutputHelper output;
public TestSuite(ITestOutputHelper output)
{
this.output = output;
}
[Fact]
public async Task TestTuple()
{
TupleCustom custom = new TupleCustom();
Tuple<int, int> mytuple = await custom.Function1(1,2);
output.WriteLine($" Item1={mytuple.Item1} Item2={mytuple.Item2} ");
}

When I have this problem I create a private utility class for handling the return values. By doing it this way, you can pass various types in the argument list. Aspects of the class can be tailored to your requirements.
public static void main(String [] args) {
int numX = 5;
double numY = 3.0;
Nums n = displayTwiceTheNumber(numX, numY);
System.out.println(n.numX);
System.out.println(n.numY);
}
public static Nums displayTwiceTheNumber(int numX, double numY) {
int numW;
double numZ;
// do something with arguments.
// in this case just double them and return.
return new Nums(2*numX, 2*numY);
}
private static class Nums {
int numX;
double numY;
public Nums(int nx, double ny) {
this.numX = nx;
this.numY = ny;
}
public String toString() {
return "(" + numX + ", " + numY +")";
}
}
Prints
10
6.0

get the value of variable from another method

How do i print the value of variable which is defined inside another method?
This might be a dumb question but please help me out as i am just a beginner in programming
public class XVariable {
int c = 10; //instance variable
void read() {
int b = 5;
//System.out.println(b);
}
public static void main(String[] args) {
XVariable d = new XVariable();
System.out.println(d.c);
System.out.println("How to print value of b here? ");
//d.read();
}
}

You can't. b is a local variable. It only exists while read is executing, and if read executes multiple times (e.g. in multiple threads, or via recursive calls) each execution of read has its own separate variable.
You might want to consider returning the value from the method, or potentially using a field instead - it depends on what your real-world use case is.
The Java tutorial section on variables has more information on the various kinds of variables.

You need to return value from your read() methods.
public class XVariable {
int c = 10; //instance variable
int read() {
int b = 5;
return b;
}
public static void main(String[] args) {
XVariable d = new XVariable();
System.out.println(d.c);
System.out.println(read());
//d.read();
}
}

Return b from the read method and print it
public class XVariable {
int c = 10; //instance variable
int read() {
int b = 5;
return b;
}
public static void main(String[] args) {
XVariable d = new XVariable();
System.out.println(d.c);
System.out.println(d.read());
}
}

Setting variable values to be based on other variables

Say we have variables int a = 0; and int c;.
Is it possible to make it so that c is always equal to something like a + 1 without having to redundantly retype c = a + 1 over and over again
Thanks!

No, it is not possible to make one variable track another variable. Usually, this is not desirable either: when a value of one variable is tied to the value of another variable, you should store only one of them, and make the other one a computed property:
int getC() { return a+1; }
A less abstract example is a connected pair of age and date of birth. Rather than storing both of them, one should store date of birth alone, and make a getter method for computing the current age dynamically.

Since you have 2 variables tied in a specific way, consider using custom object to wrap a and c values. Then you can control the object state inside the class logic. You can do something like this:
public class ValuePair {
private final int a;
private final int c;
public ValuePair(int a) {
this.a = a;
this.c = a + 1;
}
public int getA() {
return a;
}
public int getC() {
return c;
}
}

Firstly, The answer is no, you can't do it directly in Java, but you can redesign your int class, There is an example:
public class Test {
public static void main(String[] args) throws IOException {
MyInt myInt1 = new MyInt(1);
KeepIncrementOneInt myInt2 = new KeepIncrementOneInt(myInt1);
System.out.println(myInt2.getI());
myInt1.setI(2);
System.out.println(myInt1.getI());
System.out.println(myInt2.getI());
}
}
class MyInt { //your own int class for keep track of the newest value
private int i = 0;
MyInt(int i) {
this.i = i;
}
public int getI() {
return this.i;
}
public void setI(int i) {
this.i = i;
}
}
class KeepIncrementOneInt { //with MyInt Class to get the newest value
private final MyInt myInt;
KeepIncrementOneInt(MyInt myInt) {
this.myInt = myInt;
}
public int getI() {
return this.myInt.getI() + 1; //get the newest value and increment one.
}
}
Create your own Int class, because we need a reference type to keep track of the newest the value a. like the MutableInt in apache commons.
Create a always increment 1 class with your own Int class as a member.
In getI method, it's always from the reference Int class get the newest value a.

swap two numbers using call by reference

Can we swap two numbers in Java using pass by reference or call by reference?
Recently when I came across swapping two numbers in Java I wrote
class Swap{
int a,b;
void getNos(){
System.out.println("input nos");
a = scan.nextInt();
b = scan.nextInt(); // where scan is object of scanner class
}
void swap(){
int temp;
temp = this.a;
this.a = thisb;
this.b = this.a;
}
}
In the main method I call the above mentioned methods and take two integers a,b and then using the second method I swap the two numbers, but relative to the object itself....
Does this program or logic come under pass by reference?
And is this correct solution?

Yes and no. Java never passes by reference, and your way is one workaround. But yet you create a class just to swap two integers. Instead, you can create an int wrapper and use pass it, this way the integer may be separated when not needed:
public class IntWrapper {
public int value;
}
// Somewhere else
public void swap(IntWrapper a, IntWrapper b) {
int temp = a.value;
a.value = b.value;
b.value = temp;
}
As the comments show, I might not have been clear enough, so let me elaborate a little bit.
What does passing by reference mean? It means that when you pass an argument to the method, you can change the original argument itself inside this method.
For example, if Java was pass-by-reference, the following code will print out x = 1:
public class Example {
private static void bar(int y) {
y = 10;
}
public static void main(String[] args) {
int x = 1;
bar(x);
System.out.println("x = " + x);
}
}
But as we know, it prints 0, since the argument passed to the bar method is a copy of the original x, and any assignment to it will not affect x.
The same goes with the following C program:
static void bar(int y) {
y = 1;
}
int main(int argc, char * argc[]) {
int x = 0;
bar(x);
printf("x = %d\n", x);
}
If we want to change the value of x, we will have to pass its reference (address), as in the following example, but even in this case, we will not pass the actual reference, but a copy of the reference, and by dereferencing it we will be able to modify the actual value of x. Yet, direct assignment to the reference will no change the reference itself, as it is passed by value:
static void bar(int &y) {
*y = 1;
y = NULL;
}
int main(int argc, char * argc[]) {
int x = 0;
int * px = &x;
bar(px);
printf("x = %d\n", x); // now it will print 1
printf("px = %p\n", px); // this will still print the original address of x, not 0
}
So passing the address of the variable instead of the variable itself solves the problem in C. But in Java, since we don't have addresses, we need to wrap the variable when we want to assign to it. In case of only modifying the object, we don't have that problem, but again, if we want to assign to it, we have to wrap it, as in the first example. This apply not only for primitive, but also for objects, including those wrapper objects I've just mentioned. I will show it in one (longer) example:
public class Wrapper {
int value;
private static changeValue(Wrapper w) {
w.value = 1;
}
private static assignWrapper(Wrapper w) {
w = new Wrapper();
w.value = 2;
}
public static void main(String[] args) {
Wrapper wrapper = new Wrapper();
wrapper.value = 0;
changeValue(wrapper);
System.out.println("wrapper.value = " + wrapper.value);
// will print wrapper.value = 1
assignWrapper(w);
System.out.println("wrapper.value = " + wrapper.value);
// will still print wrapper.value = 1
}
}
Well, that's it, I hope I made it clear (and didn't make too much mistakes)

import java.util.*;
public class Main
{
int a,b;
void swap(Main ob)
{
int tmp=ob.a;
ob.a=ob.b;
ob.b=tmp;
}
void get()
{
Scanner sc=new Scanner(System.in);
System.out.println("Enter a and b: ");
a=sc.nextInt();
b=sc.nextInt();
}
public static void main(String[] args) {
Main ob=new Main();
ob.get();
ob.swap(ob);
System.out.println(ob.a+" "+ob.b);
}}