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Value of Arraylist is unexpected

I have this method that adds temp to xy array.
public void printBoard(int temp[][],int type){
if(type == 1) {
xyArray.add(new Array(temp));
numberOfSolutionsXY++;
}
}
xyArray is global and declared as
public ArrayList<Array> xyArray = new ArrayList();
When I check its value before exiting the method printBoard, it prints the expected 2d array. But when I go to another method, the value of xyArray is different. It has the value of the other Arraylist.
I checked back and forth many times, I did not just mistype the parameter. It's not happening only on xyArray, the same happens to other arraylists: regArray, xArray and yArray
This is how I check the value
System.out.println("\n xy \n"+xyArray.toString());
I do solveBoard method and tries to add the expected value of arrays but different values were appended. They were all the same.
problemArray.add(new Array(board));
solveBoard(board,0,0,4);
if(numberOfSolutions>0)
solutionArray.add(new ArrayList(regularArray));
solveBoard(board,0,0,3);
if(numberOfSolutionsX>0)
solutionArray.add(new ArrayList(xArray));
if(dimension%2==1){
solveBoard(board,0,0,2);
if(numberOfSolutionsY>0)
solutionArray.add(new ArrayList(yArray));
solveBoard(board,0,0,1);
if(numberOfSolutionsXY>0)
solutionArray.add(new ArrayList(xyArray));
}
solutionsArray.add(new ArrayList(solutionArray));
System.out.println("\nsolutions\n"+regularArray.toString());
All of them have the value of problemArray. Even though i placed problemArray.add(new Array(board)); at the bottom, they still get its value.
This is the class Array i've made
public class Array {
int[][] array;
public Array(int[][] initialArray){
array = initialArray;
}
public String toString(){
String x = "";
for(int i = 0;i<array[0].length;i++){
for(int j = 0;j<array[0].length;j++)
x+=array[i][j];
x+="\n";
}
return x;
}
}
Full code here

Try to rename your "Array" class, for example to "MyArray"
And i suppose there are some troubles with Array class constructor.
Try to copy initialArray in to your local variable.
public class MyArray {
int[][] localArray;
public MyArray(int[][] initialArray) {
localArray = initialArray.clone();
for (int i = 0; i < initialArray.length; i++) {
localArray[i] = initialArray[i].clone();
}
}
public String toString() {
// ...
}
}

Insert Object into Java Array

I got from my teacher the next assignment -
I need to build a Course class and Student class
and to insert every Student into the Course class
Each Student has an Id,name and grade.
I have tried the next code:
public class Course {
Student[] android = new Student[100];
private void addStudent(Student a) {
for (int i=0;i<android.length;i++) {
if (android[i] == null) {
android[i] = a;
break;
}
}
}
public static void main(String[] args) {
addStudent(Joe);
}
}
I need to insert a Student that i have created in the Students class to the first null position in the array.
When i try addStudent(Joe); it gives me an error : "Joe cannot be resolved to a variable"
The Student class code:
public class Student {
private float grade;
private String name;
private long id;
public Student(long c,String b,float a) {
grade = a;
name = b;
id = c;
}
public static void main(String[] args) {
Student Joe = new Student(1,"Joe",40);
}
**The array holds 100 students (null at start)
When adding a student - i need to check for the first null value in the array and put it
there
When printing the students: i need to print only the non-null Objects in the array**

This code
Array[] Android = new Array[100];
is creating an array of type Array, so you can only store Array objects in it.
If you want to store Students, you need to create an array that, instead:
Student[] android = new Student[100];
Also, you need to realize that arrays in Java are indexed from 0. That is, you cannot reference a position that is the same as the size of the array. In your case, you have made
an array with 100 elements, but your for-loop is trying to put 101 objects in it.
Furthermore, your question text implies you only want to insert the new Student object once, but your loop puts it into every empty location in your array.
Try this instead:
for (int i=0;i<android.length;i++) { // < instead of <=, don't hardcode the length
if (android[i] == null) {
android[i] = a;
break; // once we insert a, stop looping
}
}
Update
The reason that the compiler can't find Joe is an issue of scope. You have declared Joe as a local variable in the main() method in the Student class. If you want the compiler to be able to see it, you need to declare it in the same method you are using it:
public static void main(String[] args) {
Student Joe = new Student(1,"Joe",40);
addStudent(Joe);
}
A quick google search of "Java variable scope tutorial" should give you plenty of reading about how and when you can use local and member variables.

insert method of student S (1111,"ahmed",3.5)in the position "p" in array of students(id,name,GBG)array size 100 and curent size =3.
-remove method that remove the student having GBG =2.5from the array of student .array size 100 and currentsize=3

First, change the
Array[] Android = new Array[100];
to
Student[] listOfStudent = new Student[100];
What this does is creates a array of size 100, and the type of the array is Students, which is what you will be inserting into the array. You are also going to have to change
for (int i=0;i<=100;i++) {
if (Android[i] == null) {
Android[i] = a;
}
}
to
for(int i=0; i < listOfStudents.length; i++){
if(listOfStudents[i] == null){
listOfStudent[i] = a;
}
}
otherwise you will get a "List Out of Bounds" error. It is better to not hard code values into your code - good practice for the future!
Good Luck!

Array in Method Header: error ']' expected (Java)

I'm quite new to arrays and methods, and I've been seeing this error recurring through several programs: error '[' expected.
In each occasion, it seems to correct itself as I adjust something else, but in this particular case, I am completely stumped.
By the way, I am using several methods and arrays to create a quiz (before you ask, yes, this is an assignment and I agree, a list is a better way to handle this data - but that is not an option).
It is possible that I am not passing the arrays correctly between methods, as I'm a little muddy on that process. From my understanding, in order to send/receive (i.e. import/export) an array or other variable between methods, I must declare that variable/array in the method header parameters.
import java.util.Scanner;
public class H7pseudo
{
public static void main(String[] args)
{
//call getAnswerkey method
getAnswerkey(answerkey[i]);
//call getAnswers method
getAnswers(answers[i]);
//call passed method? necessary or no?
boolean passed = passed(answerkey[i], answers[i], qMissed[i], points);
//Print results of grading
if (passed)
{
System.out.println("Congratulations! You passed.");
}
else
{
System.out.println("Try again, sucka. You FAILED.");
}
//call totalPoints
totalIncorrect(points);
//call questionsMissed
questionsMissed(qMissed[i]);
}
//get answer key (create answerkey array & export)
public static void getAnswerkey(answerkey[i])
{
//create answerkey array here
char[] answerkey;
//determine number of questions (indices)
answerkey = new char[20];
//input values (correct answers) for each index
//for our purposes today, the answer is always 'c'.
for (int i = 0; i <=20; i++)
{
answerkey[i] = 'c';
}
}
//get student answers (create answers array & export)
public static void getAnswers(answers[i])
{
//initialize scanner for user input
Scanner scan = new Scanner(System.in);
//create answer array here
char[] answers;
//determine number of questions (indices)
answers = new char[20];
//prompt for user input as values of each index
for (int i = 0; i <= 20; i++) {
answers[i] = scan.nextChar();
}
}
//grade student answers (import & compare index values of arrays:answers&answerkey
//create & export qMissed array
public static boolean passed(answerkey[i], answers[i], qMissed[i], points)
{
int points = 0;
//create new array: qMissed
boolean[] qMissed;
//determine number of questions to be graded
qMissed = new boolean[20];
//initialize values for array
for (int i = 0; i <= 20; i++) {
qMissed[i] = false;
}
//cycle through indices of answerkey[i] & answers[i];
for (int i = 0; i =< 20; i++)
{
if (answers[i] == answerkey[i])
{
correct = true;
points = points+1;
qMissed[i] = true;
}
else {
qMissed[i] = false;
}
}
//evaluate whether or not the student passed (15+ correct answers)
if (points >= 15)
{
passed = true;
}
else
{
passed = false;
}
return passed;
}
public static void totalIncorrect(points)
{
int missed = 20 - points;
System.out.println("You missed " + missed + " questions.");
}
public static void questionsMissed(qMissed[i])
{
// for each index of the array qMissed...
for (int i = 0; i < qMissed.length; i++)
{
//...print correct and false answers.
system.out.println(i + ": " + qMissed[i] + "\n");
}
}
}

You can't define array size in the method signature, in Java.

public static void getAnswerkey(answerkey[i])
You can't put anything inside the [] in a method declaration. Also, you have to mention the type:
public static void getAnswerKey(char[] answerkey)
This is not the only reason your code won't work as intended, but I'll leave the rest as part of the exercise.

Look at your method definitions:
public static void questionsMissed(qMissed[i])
This is wrong. You should define the type of the variable and it should not contain [i] like an element of an array. It should be something like this:
public static void questionsMissed(int qMissed)
Or if you want to pass the array, write it like this:
public static void questionsMissed(int[] qMissed)
Apart of this, there are other several errors in your code:
getAnswerkey(answerkey[i]); //answerkey is not defined
getAnswers(answers[i]); //answers is not defined
It would be better if you start reading a Java tutorial first.

I want to vote up Luiggi's answer, but I don't have enough reputation to do that :)
Congrats, cordivia, on getting started with Java!
Here is how an array is declared:
type[] arrayName = new type[numberOfElements]
For example, you did this right in your method definition for getAnswerkey():
char[] answerkey;
answerkey = new char[20];
The part in the method definition inside the parentheses defines the kind of data the method is willing to accept from the outside. So if you don't need to put something into the method to get something out of it, you don't need to put anything in the parentheses. Define the method like this:
getAnswerkey() {
...But that's not the whole story. If you want to get something out of the method, it needs to have a return type as well. A return type is what you're gonna get out of the method when the method's done doing it's magic. For example, if you want to get an int array out of a method you would do something like this:
public static int getTheInteger() {
Since you want an array of chars from the method, you'll want to do something like this:
public static char[] getAnswerkey() {
So that's how you get a method to give you something back. If don't want anything back, you put void:
public static void noMarshmallows() {
Now, when you use the method, you're gonna need to do something with what it gives you, or it did all that work for nothing. So you need to store the return value in a variable when you call the array (calling methods is what you've been doing in main). You know how to store something in a variable. You use the '=' operator:
int myVeryFavoriteNumber;
myVeryFavoriteNumber = 5;
So, you do the same thing when you're getting something out of an array. You assign the return value to a variable. If you want to do this with an array, do this:
int[] myFavs;
myFavs = getMyFavoriteNumbers();
Same with chars:
char[] answerKey;
answerKey = getAnswerKey();
Voila! Your answer key is now right out in the open for the rest of main to see :)
Now, if you want to put something into a method and have it do something with what you put in, you define a parameter. You know how this works. It's just like declaring a variable, and that's exactly what it is. Parameters go in the parentheses and only the method using the parameter sees that variable name (it's local). Something like this:
public static void plusOneToAll (int[] numbers) {
for (int i = 0; i < numbers.length; i++) {
numbers[i] = numbers[i] + 1;
}
}
Notice int[] numbers in the parentheses. The type being passed in is int[] or integer array. numbers is just the parameter name. It functions just like a variable, but it is declared locally (inside the parentheses) and use locally (inside the method). So, if you wanted to compare the answers from two arrays and return the number of matches (like a total score for instance), you would do something like this:
public static int getTotalScore (char[] correctAnswers, char[] userAnswers) {
int totalScore = 0;
for (int i = 0; i < correctAnswers.length; i++) {
if (userAnswers[i] == correctAnswers[i]) {
totalScore = totalScore + 1;
}
}
return totalScore;
}
Notice the return type: int (written before the method name). Inside the array I'm using the variable totalScore to keep track of the number of times the answers match. The method takes two char arrays from the outside and uses them on the inside. Finally, I return an int: totalScore. That kicks the value of totalScore back out to whatever called it (main).
If I might add one last thing: Do yourself a favor and go pick up a copy of Head First Java. It's hard to find a good tutorial online and Java textbooks are just plain boring. The Head First series are kind of like coloring books for adults.
Best of luck!

Find and print the product of all the entries in array

Okay I have tried to write a simple Java code in BlueJ, that finds and prints the product of all the entries in data such as if data is {1,2,3,4} then the result will be 24.
And my code is below:
public class Product {
public static int[] product(int[] a) {
int [] s = new int[a.length];
for (int i =0; i< a.length; i++)
s[i] = a[i]*a[i];
return s; //the definition of your method...
}
public static void main(String[] args) {
//calling the method to seek if compiles
int[] results = Product.product(new int[] { 1,2,3,4 });
//printing the results
System.out.println(java.util.Arrays.toString(results));
}
}
The above code is giving me the square of each number, which is not what I want to have, somehow I have modify the code that the result will be 24 but I couldn't figure it out, anyone knows how to do it?

First of all, if you are first writing Java it is important to know that variable, function and class names are quite important. Please note that having Product.product() is not a good idea, since the function name is almost the same as the class name. Anyway, regarding your code. Your code is indeed returning the square of your input, what you would want is the following:
public class Product {
public static int getProduct(int[] input) {
int total = 1;
for (int v : input) {
total *= v;
}
return total;
}
}
This will return an integer value with the product of your input array. This also uses a for-each loop instead of a regular for-loop for readability. Also you don't need the index in this case. Good luck with it!

First, your product method needs to return an int rather than an int [].
You need to maintain the product as a variable. You can set it to 1 initially, and then multiply it by each element of the a array in turn; then you just return this value.

Return multiple arrays to the main method

My program is suppose to take a text file, read the first four names, create a random number between 1-4, and then assign the names to 4 different teams based on what the random number was. For instance, if the number was 3, then the first name would go to team 3, second name to team 4, etc. etc.(repeat process until there are no more names) I believe I have all of the code for that correct, the problem is I can't figure out how to return all the names I have put into the arrays that were brought into the method. Here is my code:
public static void main(String args[]) throws IOException
{
BufferedReader girlFile = new BufferedReader(new FileReader("girls40.txt"));
PrintWriter teamFile = new PrintWriter(new FileWriter("xxxxxxx-teamlist.txt"));
String team1[] = new String[20];
String team2[] = new String[20];
String team3[] = new String[20];
String team4[] = new String[20];
int n;
n = loadTeams(team1,team2,team3,team4,girlFile);
girlFile.close();
teamFile.close();
}
public static String[] loadTeams(String team1[],String team2[],String team3[],String team[],BufferedReader girlFile)
{
int n;
int random;
String name1;
String name2;
String name3;
String name4;
while((name1=girlFile.readLine())!=null)
{
name2=girlFile.readLine();
name3=girlFile.readLine();
name4=girlFile.readLine();
random = 1 + (int)(Math.random() * 4);
if(random==1)
{
team1[n]=name1;
team2[n]=name2;
team3[n]=name3;
team4[n]=name4;
}
if(random==2)
{
team1[n]=name4;
team2[n]=name1;
team3[n]=name2;
team4[n]=name3;
}
if(random==3)
{
team1[n]=name3;
team2[n]=name4;
team3[n]=name1;
team4[n]=name2;
}
if(random==4)
{
team1[n]=name2;
team2[n]=name3;
team3[n]=name4;
team4[n]=name1;
}
n++;
}
return team1[],team2[],team3[],team4[];
}`
The main method was given to me, so it cannot be changed.

If there is more code in main method than you've posted here. You'll have to mention what is the variable n and how is it being used else follow the answer.
main Method can't be changed
In your main method,
int n;
n = loadTeams(team1,team2,team3,team4,girlFile);
girlFile.close();
teamFile.close();
} // End of Main Method
You have not used returned value n for nothing. So it really doesn't matter what you return from method loadTeams() as long as it is an int.
Also, here loadTeams() returns an String[] which can't be assigned be int n, you'll have to change return type of loadTeams() to int as
public static int loadTeams(String team1[],String team2[],String team3[],String team[],BufferedReader girlFile) {
/*
...
*/
return 0; // whatever, it isn't being used
}
This the solution if you can't change the main method.

The call to loadTeams() expects a return value of type int. Not an array or multiple arrays. If you can't change the main method then loadTeams should return an integer.
// ...
int n;
n = loadTeams(team1,team2,team3,team4,girlFile);
// ...

you don't have to return anything, arrays created in main() will be passed to your method by reference, you can fill them there, and after execution of your method, values will be kept in these arrays

The loadTeams should return an int and not String[].
There is no need to return arrays. Changes made in the arrays in the loadTeams methods will be reflected back to the array in main method.