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How do I loop through an array list and display the elements stored without a system.out.println statement? [duplicate]

This question already has answers here:
Best way to convert an ArrayList to a string
(27 answers)
Closed 3 years ago.
So I am having trouble trying to loop through an array list but i dont want to use a println statement to print the elements from the array list. Is it possible if i could store all the elements into a local variable through each loop and then return the local variable when i call the method later on?e
Here is my code:
public String displayProperties() {
String property = null;
for (int i = 0; i < properties.size(); i++) {
property = properties.get(i);
}
return property;
}

You would have to introduce an instance variable in order to cache the result of your method (your local variable doesn't live past the current execution of the method).
And you'd also need to change the logic of that method, to append all the elements into a single String.
I also suggest adding some separator between the elements.
private String cachedProperties = null;
public String displayProperties() {
if (cachedProperties == null) {
StringBuilder sb = new StringBuilder();
for (int i = 0; i < properties.size(); i++) {
if (i > 0) {
sb.append(',');
}
sb.append(properties.get(i));
}
cachedProperties = sb.toString();
}
return cachedProperties;
}
Note that if the properties List may change, you have to reset your cache instance variable each time that happens.

May be modify the code to get the final string as below
property += properties.get(i);
property += " "

understand Array Object, and what I want to do is call my array in every class I have

I'm trying to understand Array Object, and what I want to do is call my array in every class I have.
this is my code:
projectProva.java
public class ProjecteProva {
Scanner sc = new Scanner(System.in);
private final int maxContador = 4;
private final DadeArr LlistaUsuari[] = new DadeArr[maxContador];
int ContadorActual;
}
DadeArr.java
public class DadeArr {
private String nomUsuari;
private String cognomUsuari;
public DadeArr(String nU, String nC){
nomUsuari = nU;
cognomUsuari = nC;
}
Right now I'm working in projectProva.java , I have some method that saves into array a data input with scanner.
Here is an example of one of my method:
public int inserir(int aContadorActual){
ContadorActual = 1;
for (int i=1;i<=ContadorActual;++i){
System.out.println("Introdueix el nom del usuari: ");
String nU = sc.nextLine();
//sd.setNomUsuari(Name);
System.out.println("Introdueix el teu cognom : ");
String nI = sc.nextLine();
LlistaUsuari[ContadorActual] = new DadeArr(nU,nI);
System.out.println("El teu usuari s'ha creat satisfactoriament");
}
ContadorActual++;
return ContadorActual;
}
This method asks user his name and surname and saves it in array LlistaUsuari.
Then, I want to use this array(with the data in) in another .java file from the same package, but i don't know how to properly call the array.
I just started to learn this type of array, and i want to understand it.
After solving this , im looking forward to take all array info and send it to a data base or text file.
If I can't proceed with it i will switch to Array 2D.
Plus, I'm wondering if this type of Array ( array object ) is very usefull or not.
Thanks.
I also made this question at https://www.reddit.com/r/javahelp/comments/dsyu4b/array_object/?

You have multiple gotchas in your code.
1. you should always iterate an array from index 0 (unless you have special needs or you are programming in a language like Python where arrays start from 0)
2. You should set the condition in the loop to be less than the exact length of the array. So in your case it will be i < LlistaUsuari.length. You are getting a null because you are only filling up index #1 of the array.
All you need is a public getter method for the array that you want to access.
Something like:
public DadeArr[] getLlistaUsuari() {
return this.LlistaUsuari;
}
And that should do it.
In other classes, create an instance of ProjecteProva (lets's call it pPI) and to get the array there, simply do pPI.getLlistaUsuari() and you'll have it,

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!

Specific NullPointerException Java

i have short question, tell me just why first example don't work and second works.
Code before examples:
Tiles[] myTiles = new Tile[23];
number = 1;
First Example:
for(Tile tile : this.myTiles) {
if (number != this.myTiles.length) {
tile = new Tile(number, getResources().getColor(R.color.puzzle_default));
number++;
}
}
Second Example:
for(Tile tile : this.myTiles) {
if (number != this.myTiles.length){
this.myTiles[number-1] = new Tile(number, getResources().getColor(R.color.puzzle_default));
number++;
}
}
If i use code below in other method in class
this.myTiles[0].getNumber();
It's NullPointerException.
But with Second Example it nicely works.
I really don't know why. Thanks for any response

The first loop makes a copy of each object and is equivalent to
for (int i=0; i < myTiles.length; i++) {
Tile tile;
...
tile = new Tile(...); // set local reference only
}
As elements in an Object array are null by default these would remain unassigned outside the scope of the loop. The original elements of the myTiles remain at their default null values

The for each loop uses an Iterator internally to fetch items from the collection and return you a new reference to a local variable containing each element - overwriting this reference is completely useless, as it is only valid for one for-loop and will be replaced on the next.
"Internally", your first loop would translate to
for (Iterator<Tile> iterator = myTiles.iterator(); iterator.hasNext;){
Tile tile = iterator.next();
tile = new Tile(number, getResources().getColor(R.color.puzzle_default));
number++;
}

In Java, there is no such thing as manipulating a pointer directly. Any time you get a reference to an object, you are getting a copy to a reference, like a pointer to a pointer. For this reason if you do something like:
String s = "hello";
modify(s);
System.out.println(s); // still hello!
void modify(String s){
s = s + " world";
}
You can't actually change the original reference s because the function is manipulating a copy to that reference. In the example above you would need something like this:
String s = "hello";
s = modify(s);
System.out.println(s); // prints 'hello world'
String modify(String s){
return s + " world";
}
The same happens in your for comprehension. The variable tile is bound to the loop and is a copy of a reference in the array. The original reference (the array at the given position) can't be changed directly this way. That's why you need to call directly:
myTiles[i] = // something
Read this article for more information.
So the idiomatic way of modifying an array in java is something like:
for(int i = 0; i < myTiles.length; i++){
myTiles[i] = new Tile(...); // directly reassigning the reference in the array!
}

Get information outside a loop

I've a problem. I want to fill an array with objects containing different Informations.
here is my loop
public FileRecord [] calcPos() throws IOException{
for (int i = 0; i < getEFSFATmaxRecords(); i++){
int blockNumber = i/5;
int recordOffset = i%5;
pos = (recordOffset*100+(getFsatPos() + 512 + 512*blockNumber));
FileRecord rec = new FileRecord(pos,getHeader());
array = new FileRecord[header.getMaxFileRecords()];
array[i] = rec;
System.out.println("FileName: " + array[i].getFileName());
}
return array;
}
It should make different objects of FileRecord. The position depends on the running variable i. t
Then the loop stores everything in the array and returns the array. Ive declared array as a global variable in this calss so I thought the changes inside the loop would directly affect the global array. But it doesnt work. what I'm doing wrong?

Within the array you are doing:
array = new FileRecord[header.getMaxFileRecords()];
This will re-create the array every interation and you'll lose the records stored in it.
You'll need to do this before the loop

You are re initializing your array in every iteration. Below is a correct version of the code you want:
public FileRecord [] calcPos() throws IOException{
FileRecord[] array = new FileRecord[header.getMaxFileRecords()];
for (int i = 0; i < getEFSFATmaxRecords(); i++){
int blockNumber = i/5;
int recordOffset = i%5;
pos = (recordOffset*100+(getFsatPos() + 512 + 512*blockNumber));
FileRecord rec = new FileRecord(pos,getHeader());
array[i] = rec;
System.out.println("FileName: " + array[i].getFileName());
}
return array;
}
As vogel says if the header.getMaxFileRecords() changes within the loop then your array may run out of bound.
Solution: An ArrayList should work.

The problem is that you do:
array = new FileRecord[header.getMaxFileRecords()];
INSIDE the method every time it is invoked (in fact, inside the loop!).
This way, you are "setting" a new FileRecord[] object to the variable (and even worse, this happens many times in your method as the initialization is done in the loop).
Each time this initialization happens, the variable "points to the new FileRecord[] object allocated in memory. The Object that was "pointed to" by array before is not used anymore, and will be destroyed, the when is responsibility of the garbage collector.
(http://javabook.compuware.com/content/memory/how-garbage-collection-works.aspx).
In simple words, you are "recreating" the array again and again inside your loop.
Initialize the object only ONCE before using it in your method (for example in class constructor or in main, before using it in a sense).
Generally, I suggest that you don't use global variables. Search more on class encapsulation, a very important Object-Oriented Programming principle:
(http://www.tutorialspoint.com/java/java_encapsulation.htm).