Develop Reference

Calling a boolean method from a class

i just made a problem which should return if a number "isHappy" or not. A number is happy if it meets some criteria:
A happy number is a number defined by the following process:
-Starting with any positive integer, replace the number by the sum of the squares of its digits.
-Repeat the process until the number equals 1 (where it will stay), or it loops endlessly in a cycle which does not include 1.
-Those numbers for which this process ends in 1 are happy.
`
import java.util.HashSet;
import java.util.Set;
class Solution{
public int getNext(int n){
int totalSum = 0;
while(n > 0){
int last = n % 10;
n = n / 10;
totalSum += last * last;
}
return totalSum;
}
public boolean isHappy(int n){
Set<Integer> seen = new HashSet<>();
while( n!=1 && !seen.contains(n)){
seen.add(n);
n = getNext(n);
}
return n==1;
}
}
class Main {
public static void main(String[] args){
isHappy(23); //this doesnt work.
}
}
`
I don't know how to call this function, tried different methods like int number = 23 for ex and then isHappy(number) sout(isHappy(number)); , number.isHappy() although this makes no sense, intellij is telling me to make a method inside main but i dont think i must do this, think there s another way.

There are a couple of problems here. When you run a Java application it looks for the main method in the class you run. Your class has no main method, instead it has an inner class that has a main method.
class Solution {
// existing methods
// no 'Main' class wrapping method
public static void main(String[] argv) {
}
}
The second problem is that main is a 'static' method but isHappy is an 'instance' method. To call it you need an instance of the class
// inside the `main` function
var sol = new Solution();
if (sol.isHappy(23)) {
System.out.println("23 is happy");
} else {
System.out.println("23 is not happy");
}

You have completely solved the problem. You only have to decide what to do with the result of your calculations. You can simply print a string depending on the result.
Please note, that the methods in your Solution class are not static. So, you need to create an instance of the class and call the methods on that instance.
import java.util.HashSet;
import java.util.Set;
class Solution {
public int sumOfDigitsSquares(int n) {
int totalSum = 0;
while (n > 0) {
int last = n % 10;
n = n / 10;
totalSum += last * last;
}
return totalSum;
}
public boolean isHappy(int n) {
Set<Integer> seen = new HashSet<>();
while (n != 1 && !seen.contains(n)) {
seen.add(n);
n = sumOfDigitsSquares(n);
}
return n == 1;
}
}
class Main {
public static void main(String[] args) {
var solution = new Solution();
String answer = solution.isHappy(23) ? "Is happy" : "Not happy at all";
System.out.println(answer);
}
}
Here is one possible way to check all numbers from 1 to 100 for happiness and print the result for each number.
IntStream.rangeClosed(1, 100)
.mapToObj(i -> "" + i + " " + (solution.isHappy(i) ? "is happy" : "not happy"))
.forEach(System.out::println);

To call non-static methods of a class, you need an instance:
public static void main (String [] args) {
Solution foo = new Solution ();
boolean bar = foo.isHappy (49);
// do something with bar
}
To use a static method, you don't use an instance:
class Solution{
public static int getNext(int n){ ... etc ...
}
public static boolean isHappy(int n){ ... etc ...
}
public static void main (String [] args) {
boolean foo = isHappy (1024);
// do something with foo
}
Another problem is you are throwing away the result of isHappy (23).
System.out.println (isHappy(23));
System.out.println ("is 23 happy?" + (isHappy(23) ? "Yes" : "No");
are two possibilities.

How check if the array is full and only add to it if it's not? [duplicate]

I've got array. I've got an isFull method, which checks if the array is full, but I don't know how to use this to check if it's full, then if it's not full add to the array, otherwise disregard the add call.
The array should take 10 elements and then not accept any more. After 10 elements, it should 'be full' and disregard any addSpy calls.
How would you implement this?
public class ConcreteSubject extends AbstractSubject {
public int arySize;
private int i = 0;
private static AbstractSpy[] spies;
public ConcreteSubject(int a) {
arySize = a;
spies = new AbstractSpy[a];
}
#Override
public void addSpy(AbstractSpy spy) {
if (spies.length < 10) {
spies[i] = spy;
System.out.println("spy added at index " + i);
i++;
}
}
public void isFull() {
//1
boolean b = false;
for (int i = 0; i < spies.length; i++) {
if (spies[i] == null) {
b = true;
}
}
if (!b) {
System.out.println("Array is full");
} else {
System.out.println("Array not full");
}
}
public class TestSpies {
public static void main(String[] args) {
ConcreteSubject cs = new ConcreteSubject(10);
AbstractSpy spy = new ConcreteSpy();
AbstractSpy[] spies = new AbstractSpy[10];
cs.addSpy(spy);
cs.addSpy(spy);
cs.addSpy(spy);
cs.isFull();
}
}

spies.length < 10 isn't correct. It should be spies.length > 0 && i < spies.length to make sure that the following assignment spies[i] = spy; is always valid.
void isFull() should be boolean isFull(). Your implementation looks OK, just return b. full is a tricky word because technically an array is always "full". A better adjective would be populated, filled.
Since addSpy isn't filling null gaps but simply adds a spy to the end, isFull could be rewritten to return spies.length == i;.

The simplest way of doing it would be like that:
#Override
public void addSpy(AbstractSpy spy) {
if (!isFull())
{
spies[i] = spy;
System.out.println("spy added at index " + i);
i++;
}
}
To use that, you should change your isFull method to:
public boolean isFull() {
for (int i = 0; i < spies.length; i++) {
if (spies[i] == null) {
return false;
}
}
return true;
}

Keep a track of the number of filled cells of the array using a variable. And before inserting anything into it, check if the filled cells count strictly less than the size of the array (obviously you want to keep track of the array total size as well).

How to decide the state of an object before starting to code?

I have the following code for displaying the sum of two consecutive element of ArrayList until the element left is one.for example:-
if i entered
1 2 3 4 5
output
3 7 5 //adding the two consecutive last one is as it is
10 5//doing the same thing
15
code
import java.util.*;
import java.lang.Integer;
class Substan{
ArrayList <Integer> list = new ArrayList <Integer> ();
ArrayList <Integer> newList = new ArrayList <Integer> ();// this will be the list containing the next sequence.
int index=0;
int sum=0;
Substan(){
Scanner read = new Scanner(System.in);
String choice;
System.out.println("Enter the elements of the array");
do{
int element = read.nextInt();
list.add(element);
System.out.println("More?");
choice = read.next();
}while(choice.equals("y") || choice.equals("Y"));
}
/* precondition- we have the raw list that user has enterd.
postcondition - we have displayed all the sublists,by adding two consecutives numbers and the last one is having one element.
*/
void sublist(){
while(noofElementsIsNotOneInList()){
index =0;
while(newListIsNotComplete()){
if(nextElementIsThere()){
sum = addTheConsecutive();
}
else{
sum = getLastNumber();
}
storeSumInNewList();
}
displayTheNewList();
System.out.println("");
updateTheLists();
}
displayTheNewList(); //as we have danger of Off By One Bug (OBOB)
System.out.println("");
}
private boolean noofElementsIsNotOneInList(){
boolean isnotone = true;
int size = list.size();
if ( size == 1){
isnotone = false;
}
return isnotone;
}
private boolean newListIsNotComplete(){
boolean isNotComplete = true;
int listSize = list.size();
int newListSize = newList.size();
if (listSizeIsEven()){
if ( newListSize == listSize/2){
isNotComplete = false;
}
}
else{
if( newListSize == (listSize/2) +1){
isNotComplete = false;
}
}
return isNotComplete;
}
private boolean listSizeIsEven(){
if ( list.size()%2 == 0 ){
return true;
}
else{
return false;
}
}
/*
we are at some index.
returns true if we have an element at (index+1) index.
*/
private boolean nextElementIsThere(){
if ( list.size() == index+1 ){
return false;
}
else{
return true;
}
}
/* precondition-we are at index i
postcondition - we will be at index i+2 and we return sum of elements at index i and i+1.
*/
private int addTheConsecutive(){
int sum = list.get(index)+list.get(index+1);
index += 2;
return sum;
}
/* we are at last element and we have to return that element.
*/
private int getLastNumber(){
return list.get(index);
}
private void storeSumInNewList(){
newList.add(sum);
}
private void displayTheNewList(){
int size = newList.size();
for ( int i=0;i<size;i++){
System.out.print(newList.get(i)+" ");
}
}
/*precondition - we have processed all the elements in the list and added the result in newList.
postcondition - Now my list will be the newList,as we are processing in terms of list and newList reference will have a new object.
*/
private void updateTheLists(){
list = newList;
newList = new ArrayList <Integer>();// changing the newList
}
public static void main(String[] args) {
Substan s = new Substan();
s.sublist();
}
}
So i have done a lot of refinement of my code but having a problem of sharing the local variables with the other methods.for example i have used index instance for storing the index and initially i thought that i will put this as not an instance but a local variable in method sublist() but as it cannot be viewed from other methods which needed to use the index like addTheConsecutive().So considering that i put the index at class level.So is it wright approach that put the variables that are shared at class level rather than looking at only the state of the object initially before coding and stick to that and never change it?

Consider this:
An object can communicate with other(s) only by sharing its attributes. So, if you need an object to read the state of another, the only way it can be done is by giving it "permission" to read the other object attributes.
You have two ways to do that:
Declaring the object attributes public, or
Creating getXXX() methods (makes sense for private attributes)
I personally prefer option two, because the getXXX() method returns the value ("state") of a particular attribute without the risk of being modified. Of course, if you need to modify a private attribute, you should also write a setXXX() method.
Example:
public class MyClass {
private int foo;
private String bar;
/*
* Code
*/
public int getFoo() {
return foo;
}
public String getBar() {
return bar;
}
public void setFoo(int foo) {
this.foo = foo;
}
public void setBar(String bar) {
this.bar = bar;
}
/*
* More code
*/
}
This way all the object attributes are encapsulated, and:
they cannot be read by any other object, unless you specifically call the appropriate getXXX() function, and
cannot be altered by other objects, unless you specifically call the appropriate setXXX() function.

Compare it with the non-abstracted version.
for (int index = 0; index < list.size(); index += 2) {
int sum = list.get(index);
if (index + 1 < list.size() {
sum += list.get(index + 1);
}
newList.add(sum);
}
Now, top-down refining the algorithm using names is a sound methodology, which helps in further creative programming.
As can seen, when abstracting the above again:
while (stillNumbersToProcess()) {
int sum = sumUpto2Numbers();
storeSumInNewList(sum);
}
One may keep many variables like sum as local variables, simplifying state.
One kind of helpful abstraction is the usage of conditions, in a more immediate form:
private boolean listSizeIsEven() {
return list.size() % 2 == 0;
}
private boolean nextElementIsThere() {
return index + 1 < list.size();
}

There's no point in declaring index at Class level since you dont want it to be a member or an instance of that class. Instead make it local to the method and pass it to other methods as argument where you want to access it.

I think you are asking the wrong question.
Your class variables make very little sense, as do many of the methods. This is mostly because:
Your class is doing too much
Your algorithm is a little odd
The class variables that you do have make much more sense passed as method parameters. Some methods need to see them, and some don't.
Your class is also a little odd, in that calling subList twice on the same class will not produce the same answer.
The code is littered with methods I don't quite see the point in, such as:
private boolean noofElementsIsNotOneInList(){
boolean isnotone = true;
int size = list.size();
if ( size == 1){
isnotone = false;
}
return isnotone;
}
Shouldn't this be:
private boolean noofElementsIsNotOneInList(){
return list.size() == 1;
}
And it makes no sense for it to use some arbitrary List, pass one in so that you know which List you are checking:
private boolean noofElementsIsNotOneInList(final Collection<?> toCheck){
return toCheck.size() == 1;
}
The same logic can be applied to almost all of your methods.
This will remove the instance variables and make your code much more readable.
TL;DR: Using lots of short appropriately named methods: good. Having those methods do things that one wouldn't expect: bad. Having lots of redundant code that makes things very hard to read: bad.
In fact, just to prove a point, the whole class (apart from the logic to read from stdin, which shouldn't be there anyway) can transformed into one short, recursive, method that requires no instance variables at all:
public static int sumPairs(final List<Integer> list) {
if (list.size() == 1)
return list.get(0);
final List<Integer> compacted = new LinkedList<>();
final Iterator<Integer> iter = list.iterator();
while (iter.hasNext()) {
final int first = iter.next();
if (iter.hasNext()) compacted.add(first + iter.next());
else compacted.add(first);
}
return sumPairs(compacted);
}
Now you could break this method apart into several appropriately named shorter methods, and that would make sense. It's sometimes more helpful to start from the other end. Sketch out the logic of your code and what it's trying to do, then find meaningful fragments to split it into. Possibly after adding unit tests to verify behaviour.

what about doing by Recursion:
public int calculateSum(List<Integer> nums) {
displayList(nums);
if (nums.size() == 1) {
return nums.get(0);
}
List<Integer> interim = new ArrayList<Integer>();
for (int i = 0; i < nums.size(); i = i + 2) {
if (i + 1 < nums.size()) {
interim.add(nums.get(i) + nums.get(i + 1));
} else {
interim.add(nums.get(i));
}
}
return calculateSum(interim);
}
public static void displayList(List<Integer> nums){
System.out.println(nums);
}
Steps:
Run calculate sum until list has 1 element
if list has more than 1 element:
iterate the list by step +2 and sum the element and put into a new List
again call calculate sum

Check duplicates of numbers input by user

I'm doing a program where user input five numbers and in the end the numbers are printed out which is working fine. What I can't get to work is a boolean function to check for duplicates. It should check for duplicates as the user write them in, so e.g. if number one is 5 and the second numbers is also 5, you should get an error until you write in a different number. Meaning if the user input a duplicate it should NOT be saved in the array. This is obviously an assignment, so I'm just asking for a hint or two.
This program is written based on pseudo-code given to me, and therefore I have to use a boolean to check for duplicates with the public boolean duplicate( int number ) class.
I've tried getting my head around it and tried something by myself, but obviously I'm doing a stupid mistake. E.g.:
if(int i != myNumbers[i])
checkDuplicates = false
else
checkDuplicates = true;
return checkDuplicates;
DuplicatesTest class:
public class DuplicatesTest {
public final static int AMOUNT = 5;
public static void main(String[] args) {
Duplicates d = new Duplicates(AMOUNT);
d.inputNumber();
d.duplicate(AMOUNT);
d.printInputNumbers();
}
}
Duplicates class:
public class Duplicates {
private int amount;
private int[] myNumbers;
private boolean checkDuplicates;
public Duplicates(int a) {
amount = a;
myNumbers = new int[amount];
}
public void inputNumber() {
for(int i = 0; i < amount; i++ ) {
int input = Integer.parseInt(JOptionPane.showInputDialog("Input 5 numbers"));
myNumbers[i] = input;
}
}
public boolean duplicate( int number ) {
<BOOLEAN TO CHECK FOR DUPLICATES, RETURN FALSE OR TRUE>
}
public void printInputNumbers() {
JTextArea output = new JTextArea();
output.setText("Your numbers are:" + "\n");
for(int i = 0; i < myNumbers.length; i++) {
if (i % 5 == 0) {
output.append("\n");
}
output.append(myNumbers[i] + "\t");
}
JOptionPane.showMessageDialog(null, output, "Numbers", JOptionPane.PLAIN_MESSAGE);
}
}
Sorry if the code tag is messy, I had some trouble with white fields in between and such. I'm new here.

Don't store the numbers in an array. Use a Set<Integer> instead. And then do a Set#contains() operation. It's O(1) operation which is actually far better than iterating over the array to search for duplicates.
Ok, if it's a compulsion to use an array, then you should modify your current approach, to return true as soon as you find a duplicate, instead of iterating over the array again. In your current approach, since you are setting the boolean variable to false in the else block, your method will return false if the last element of the array is not the same as what you are checking. So, just modify your approach to:
// loop over the array
if (number == myNumbers[i])
return true;
// outside the loop, if you reach, return false
return false;
Note that your current if statement will not compile. You are declaring an int variable there, which you can't do.
if (int i == myNumbers[i]) // this is not a valid Java code.

int nums[] = new int[5];
int count = 0;
public boolean duplicate(int number)
{
boolean isDup = false;
for (int i = 0; i <= count; i++)
{
if (number == nums[i])
{
isDup = true;
break;
}
}
if (!isDup)
{
count++;
nums[count] = number;
}
return isDup;
}

Correct way to check if a method is invoked recursively

IMPORTANT : THE EXAMPLE IS WRONG, I EXPLAIN WHY AT THE BOTTOM
As the title stated the question is about to define a way to determine when the current executing method is invoked in a recursively way.
I have think about having a "query method" that return a boolean indicating if the invoker method (this is, the method that invokes the "query method") has been already invoked before.
How to check that : just peeking at the stack trace and see if the method we want to check figures two or more times in the stack trace.
Having explaining that, here is the implementation of a method and the respective use of it.
This is not correct...
public class Test
{
public static boolean isRecusivelyInvoqued () {
StackTraceElement[] traces = Thread.currentThread().getStackTrace();
boolean res = false;
// the first belong to "getStackTrace" and the second to "isRecusivelyInvoqued" (this method)
if (traces.length > 2) {
String invokedMethodName = traces[2].getMethodName(); // the third is the method we want to check
for (int i = 3; i < traces.length && !res; i++)
{
res = invokedMethodName.equals(traces[i].getMethodName());
i++;
}
}
return res;
}
// this is a recursive method, used to verify the correct functioning
public static int factorial (int n) {
System.out.println(isRecusivelyInvoqued());
if (n == 0) {
return 1;
}
else {
return n * factorial(n-1);
}
}
public static void main(String[] args)
{
System.out.println(factorial(4));
}
}
I realize if a method in differents namespaces (Class or instance) has the same name, it will return that is invoquedRecursively. I thing that one solution we got this far is that is correct ;) jeje.
This is working for me... is there a better way to archive my goal? How to determine when the current executing method is invoked recursively?

How about this: Your method passes a boolean to the next invocation of the recursive method that tells it that it has been invoked recursively:
public static int factorial (int n) {
return privateFactorial(n, false);
}
private static int privatefactorial(int n, boolean calledRecursively) {
System.out.println(calledRecursively);
if (n == 0) {
return 1;
}
else {
return n * privateFactorial(n-1, true); // tell next invocation here!
}
}

Another option is to add a "is_recursively_invoked" parameter to your recursive function:
public static int factorial (int n, boolean isInvokedRecursively) {
System.out.println(isInvokedRecursively);
if (n == 0) {
return 1;
}
else {
return n * factorial(n-1, true); // these function calls are recursive
}
}
and in your main:
System.out.println(factorial(4, false)); // this function call isn't recursive

If your only goal is to determine if a given method calls itself, then introspect the byte code using any byte code analysis framework and see if there's a call to the method inside the method body.
If you need data about recursion depth then I would use AspectJ (or equivalent) to instrument the method with around advice that can increment a counter. This also eliminates the need for the method itself to do additional work to support your requirement.
That said, I don't understand the need for the requirement; if the method produces the correct answer, and it relies on recursion, then it's using recursion.

You can achieve this using an static boolean variable
Here is a sample:
private static boolean isRecursiveCall = false;
private static int factorial (int n) {
if (n == 0) {
return 1;
}
else {
isRecursiveCall = true;
return n * factorial(n-1);
}
}
public static int findFactorial(int n){
isRecursiveCall = false;
factorial(n);
}
public static void main(String[] args){
findFactorial(2);
}