I am new to Java and I am trying to print the student numbers and numbers (cijfer in this case) on 1 line. But for some reason I get weird signs etc. Also when I'm trying something else I get a non-static context error. What does this mean and how does this exactly work?
Down here is my code:
import java.text.DecimalFormat;
import java.util.Arrays;
public class Student {
public static final int AANTAL_STUDENTEN = 50;
public int[] studentNummer = new int[AANTAL_STUDENTEN];
public String[] cijfer;
public int[] StudentNummers() {
for (int i = 0; i < AANTAL_STUDENTEN; i++) {
studentNummer[i] = (50060001 + i);
}
return studentNummer;
}
public String[] cijfers(){
for (int i = 0; i < AANTAL_STUDENTEN; i++) {
DecimalFormat df = new DecimalFormat("#.#");
String cijferformat = df.format(Math.random() * ( 10 - 1 ) + 1);
cijfer[i++] = cijferformat;
}
return cijfer;
}
public static void main(String[] Args) {
System.out.println("I cant call the cijfer and studentnummer.");
}
}
Also I'm aware my cijfer array is giving a nullpointer exception. I still have to fix this.
I am not java developer but try
System.out.print
You could loop around System.out.print. Otherwise make your functions static to access them from main. Also initialize your cijfer array.
Besides the things I noted in the comments, your design needs work. You have a class Student which contains 50 studentNummer and cijfer members. Presumably, a Student would only have one studentNummer and and one cijfer. You need 2 classes: 1 for a single Student and one to hold all the Student objects (StudentBody for example).
public class StudentBody {
// An inner class (doesn't have to be)
public class Student {
// Just one of these
public int studentNummer;
public String cijfer;
// A constructor. Pass the student #
public Student(int id) {
studentNummer = id;
DecimalFormat df = new DecimalFormat("#.#");
cijfer = df.format(Math.random() * ( 10 - 1 ) + 1);
}
// Override toString
#Override
public String toString() {
return studentNummer + " " + cijfer;
}
}
public static final int AANTAL_STUDENTEN = 50;
public Student students[] = new Student[AANTAL_STUDENTEN];
// StudentBody constructor
public StudentBody() {
// Create all Students
for (int i = 0; i < AANTAL_STUDENTEN; i++) {
students[i] = new Student(50060001 + i);
}
}
// Function to print all Students
public void printStudents(){
for (int i = 0; i < AANTAL_STUDENTEN; i++) {
System.out.println(students[i]);
}
}
public static void main(String[] Args) {
// Create a StudentBody object
StudentBody allStudents = new StudentBody();
// Print
allStudents.printStudents();
}
}
Just make all your methods and class variables as static. And then you have access to them from main method. Moreover you have got some errors in code:
public class Student {
public static final int AANTAL_STUDENTEN = 50;
// NOTE: static variables can be moved to local ones
// NOTE: only static method are available from static context
public static int[] StudentNummers() {
int[] studentNummer = new int[AANTAL_STUDENTEN];
for (int i = 0; i < AANTAL_STUDENTEN; i++)
studentNummer[i] = 50060001 + i;
return studentNummer;
}
// NOTE: only static method are available from static context
public static String[] cijfers() {
// NOTE: it is better to use same `df` instance
DecimalFormat df = new DecimalFormat("#.#");
String[] cijfer = new String[AANTAL_STUDENTEN];
for (int i = 0; i < AANTAL_STUDENTEN; i++)
// NOTE: remove `i++`, because we have it in the loop
cijfer[i] = df.format(Math.random() * (10 - 1) + 1);
return cijfer;
}
// NOTE: this is `static` method, therefore it has access only to static methods and variables
public static void main(String[] Args) {
String[] cijfer = cijfers();
int[] studentNummer = StudentNummers();
// TODO you can pring two arrays one element per line
for(int i = 0; i < AANTAL_STUDENTEN; i++)
Sytem.out.println(cijfer[i] + '-' + studentNummer[i]);
// TODO as alternative, you can print whole array
System.out.println(Arrays.toString(cijfer));
System.out.println(Arrays.toString(studentNummer));
}
}
Related
I've started learning java some time ago. I'm reading through the Java Foundations book and doing exercises from the book to practice.
Just come across this one "Modify the java program so that it works for the numbers in the range between -25 and 25." and I wonder if you have any different solutions to it or is it really that simple? :)
Here's the original code:
public class BasicArray
{
public static void main(String[] args)
{
final int LIMIT = 15;
final int MULTIPLE = 10;
int[] list = new int[LIMIT];
// Initialize the array values
for(int index = 0; index < LIMIT; index++)
list[index] = index * MULTIPLE;
list[5] = 999; // change one array value
// Print the array values
for(int value : list)
System.out.println(value + "");
}
}
And here's my solution to it:
public class BasicArray
{
public static void main(String[] args)
{
final int LIMIT = 51;
final int MULTIPLE = 1;
int[] list = new int[LIMIT];
// Initialize the array values
for(int index = 0; index < LIMIT; index++)
list[index] = (index - 25) * MULTIPLE;
list[5] = 999; // change one array value
// Print the array values
for(int value : list)
System.out.println(value + "");
}
}
Yes, basically it's really simple exercise.
Regarding to your solution we actually don't need MULTIPLE in code.
public class BasicArray {
public static void main(String[] args) {
final int LIMIT = 51;
int[] list = new int[LIMIT];
// Initialize the array values
for(int index = 0; index < LIMIT; index++) {
list[index] = (index - 25);
}
list[5] = 999; // change one array value
// Print the array values
for(int value : list) {
System.out.println(value + "");
}
}
}
If you are ready for a bit of advanced java, you can try following:
public class BasicArray {
public static void main(String[] args) {
IntStream.rangeClosed(-25, 25)
.forEach(System.out::println);
}
}
Or this if you need to replace one value:
public class BasicArray {
public static void main(String[] args) {
IntStream.rangeClosed(-25, 25)
.forEach(i -> {
if (i == -20) { // change one array value
System.out.println(999);
} else {
System.out.println(i);
}
});
}
}
I have been given an assignment to do the following
Generate id numbers for employees - Each id number must meet the following requirements:
a. It must be a prime number
b. It must not match a previously generated ID number
c. It must be exactly 5 digits in length
Store Employee data – for this feature the application must allow you to enter a new employee’s full name and their assigned ID number. These must be stored in the application.
So far am unsure of if am doing the correct thing but I have some code I started up below...
import java.util.Scanner;
public static void main(String[] args) {
// TODO Auto-generated method stub
primenum();
}
public static void primenum() {
int max = 20000;
System.out.println("Generate Prime num" + max);
for (int i = 10000; i < max; i++) {
boolean isPrimeNumber = true;
for (int j = 2; j < i; j++) {
if (i % j == 0) {
isPrimeNumber = false;
}
}
// print the number if prime
if (isPrimeNumber) {
System.out.println(i + " ");
}
}
}
public static void ID() {
String[] emp = new String[10];
emp[0] = "John";
emp[1] = "Mary";
emp[2] = "James";
emp[3] = "chris";
emp[4] = "charles";
}
I have just created an array that will hold some names.. but my main objective I want to get is for the next prime number generated to be stored in each of the emp[] associated with a name .. so for eg. emp[0] which is john I want him to be able to receive the next prime number for the primenum() method.. I am unsure of how to do this and will love all help apprecited.
use this solution:
import java.util.HashMap;
import java.util.Map;
import java.util.Map.Entry;
public class PrimeNumber {
public static void main(String[] args) {
Map<String,Integer> map=new HashMap<String,Integer>();
map.put("John", nextPrime(1));
map.put("Mary", nextPrime(map.get("John")));
map.put("James", nextPrime(map.get("Mary")));
map.put("chris", nextPrime(map.get("James")));
map.put("charles", nextPrime(map.get("chris")));
for(Entry<String, Integer> e:map.entrySet()) {
System.out.println(e.getKey()+": "+e.getValue());
}
}
public static int nextPrime(int input){
int counter;
input++;
while(true){
int l = (int) Math.sqrt(input);
counter = 0;
for(int i = 2; i <= l; i ++){
if(input % i == 0) counter++;
}
if(counter == 0)
return input;
else{
input++;
continue;
}
}
}
}
To associate a prime number with your employee name:
Create a custom object which is responsible for relation between employee object and prime number(ID) like below :
public class EmployeeNameAndId {
private Employee employee;
private Integer id;
// constructors, getters, setters here
}
Now you need to use this object in your main class like below :
public class MainClass {
public static void main(String[] args) {
List<EmployeeNameAndId> l = new ArrayList<>();
l.add(new EmployeeNameAndId("John", generatePrimeId());
l.add(new EmployeeNameAndId("Mary", generatePrimeId());
l.add(new EmployeeNameAndId("James", generatePrimeId());
l.add(new EmployeeNameAndId("Chris", generatePrimeId());
}
}
public static int arraysize=1;
public String namabuku;
public String penulis;
public String Kategori;
public String buku[][]=new String[arraysize][3];
public static int a=0;
public void isiData(String kategori, String buku, String penulis){
this.buku[a][0]=kategori;
this.buku[a][1]=buku;
this.buku[a][2]=penulis;
arraysize++;
a++;
}
Hi guys I tried to increase my array length every time I call a method named "isiData", but it didn't work. I already checked the increment, but nothing wrong with it. Is there any way to increase its length every time I use the method? I want to make a simple way to input book, category, and its author using array.
You cannot increase the size of array.
There are 3 approaches to solve this problem:
Use ArrayList as suggested by others.
You can create another temp array of size one greater than the previous array and then copy the temp array to already created array.
You can use the copyOf(array, size) function of Arrays in Java
For example:
previousArray = Arrays.copyOf(previousArray , arraysize + 1);
arraysize += 1
Just try this Approach:
import java.util.ArrayList;
import java.util.List;
import java.util.Arrays;
/**
*
* #author Maverick
*/
public class Buku {
public static int arraysize;
public String namabuku;
public String penulis;
public String Kategori;
public List<List<String>> bukuList = new ArrayList<List<String>>();
public static void main(String[] args) {
Buku n = new Buku();
for (int i = 0; i < 5; i++) {
n.isiData("ab" + i, "cd" + i, "ef" + i);
}
n.arraysize = n.bukuList.size();
for (int i = 0; i < n.bukuList.size(); i++) {
System.out.println(n.bukuList.get(i).toString());
}
}
public void isiData(String kategori, String buku, String penulis) {
bukuList.add(Arrays.asList(kategori, buku, penulis));
}
}
Output:
[ab0, cd0, ef0]
[ab1, cd1, ef1]
[ab2, cd2, ef2]
[ab3, cd3, ef3]
[ab4, cd4, ef4]
You have to call new array to change the size of an array. I assume this is an exercise to practice using an array, so I'm going to avoid the classes like Arrays and System in the isiData method. You should look at those classes though.
So something like this:
public class BukuTest
{
public String namabuku;
public String penulis;
public String Kategori;
public String buku[][] = new String[ 0 ][ 3 ];
public void isiData( String kategori, String buku, String penulis )
{
String[][] temp = this.buku;
final int len = temp.length;
this.buku = new String[ len + 1 ][];
for( int i = 0; i < len; i++ )
this.buku[i] = temp[i];
this.buku[len] = new String[ 3 ];
this.buku[len][0] = kategori;
this.buku[len][1] = buku;
this.buku[len][2] = penulis;
// not needed
// arraysize++;
// a++;
}
public static void main(String[] args) {
BukuTest b = new BukuTest();
b.isiData( "test1", "test2", "test3" );
b.isiData( "test4", "test5", "test6" );
b.isiData( "test7", "test8", "test9" );
System.out.println(b);
}
#Override
public String toString()
{
return "BukuTest{" + "namabuku=" + namabuku + ", penulis=" + penulis +
", Kategori=" + Kategori + ", buku=" +
Arrays.deepToString(buku) + '}';
}
}
Using an ArrayList is definitely the way to go here as others have commented and displayed but, if you are absolutely bent on using a Two Dimensional String Array then this can be done with a custom method conveniently named redimPreserve() as I have shown below.
As #Jdman1699 had mentioned in his comment under your post, this is a very inefficient way of doing this sort of thing especially for larger arrays but since you asked, here is how it can be done:
// YOUR METHOD:
public int arraysize = 1;
public String[][] buku = new String[arraysize][3];
public void isiData(String kategori, String buka, String penulis){
// I have renamed the buku argument for this method to buka
// since you can not have a parameter variable named the
// same as a Class Global variable.
buku = redimPreserve(buku, arraysize, 3);
buku[arraysize-1][0] = kategori;
buku[arraysize-1][1] = buka;
buku[arraysize-1][2] = penulis;
arraysize++;
}
// THE redimPreserve() METHOD:
public static String[][] redimPreserve(String[][] yourArray, int newRowSize, int... newColSize) {
int newCol = 0;
if (newColSize.length != 0) { newCol = newColSize[0]; }
// The first row of your supplied 2D array will always establish
// the number of columns that will be contained within the entire
// scope of the array. Any column value passed to this method
// after the first row has been established is simply ignored.
if (newRowSize > 1 && yourArray.length != 0) { newCol = yourArray[0].length; }
if (newCol == 0 && newRowSize <= 1) {
throw new IllegalArgumentException("\nredimPreserve() Method Error!\n"
+ "No Column dimension provided for 2D Array!\n");
}
if (newCol > 0 && newRowSize < 1 && yourArray.length != 0) {
throw new IllegalArgumentException("\nredimPreserve() Method Error!\n"
+ "No Row dimension provided for 2D Array!\n");
}
String[][] tmp = new String[newRowSize][newCol];
if (yourArray.length != 0) {
for(int i = 0; i < yourArray.length; i++) {
System.arraycopy(yourArray[i], 0, tmp[i], 0, yourArray[i].length);
}
}
return tmp;
}
So I'm writing a basic MasterMind game that is... mostly functional. However, its exhibiting odd behavior and I'm unsure why.
The idea is that what defines a Code and its behavior is one file, the gameplay is another, and the Main just creates a new game and starts playing. When I initialize the game, the computer creates a new random string of 4 (the "secret code"), as expected; but then once I get input for the User guess, it seems to rewrite the secret code into whatever I've input. Further, my methods for evaluating matches don't work at all, but considering that the secret code keeps changing means that it's not being set to begin with, and I'm unsure why.
All three classes below. Why is my class variable in Game not setting properly and accessible to the other methods?
Main.java
class Main {
public static void main(String[] args) {
Game newGame = new Game();
newGame.play();
}
}
Code.java
import java.util.Random;
import java.util.HashMap;
import java.util.Collection;
import java.util.ArrayList;
import java.util.Set;
import java.lang.Math;
import java.lang.StringBuilder;
class Code {
private static HashMap<String,String> PEGS;
private static ArrayList<String> pegStrings;
protected static String secretCodeString;
public static void main(String[] args) {
}
public Code(String input){
this.secretCodeString = input;
}
public Code(){
randomize();
}
//literally just creates the peghash
public static void setPegs(){
PEGS = new HashMap<String,String>();
PEGS.put("C","c");
PEGS.put("Y","y");
PEGS.put("R","r");
PEGS.put("P","p");
PEGS.put("O","o");
PEGS.put("G","g");
}
//turns the pegs ito something randomize can use
public static ArrayList<String> makePegArray(){
setPegs();
pegStrings = new ArrayList<String>();
Collection<String> pegValues = PEGS.values();
Object[] pegObjects = pegValues.toArray();
for (int i = 0; i < pegObjects.length; i++){
pegStrings.add(pegObjects[i].toString());
}
return pegStrings;
}
// sets Class Variable secretCode to a four letter combination
public static Code randomize(){
secretCodeString = new String();
Random rand = new Random();
int randIndex = rand.nextInt(makePegArray().size());
for (int i = 0; i < 4; i++){
randIndex = rand.nextInt(makePegArray().size());
secretCodeString = secretCodeString.concat(makePegArray().get(randIndex));
}
Code secretCode = parse(secretCodeString);
return secretCode;
}
public static Code parse(String input) {
setPegs();
makePegArray();
String[] letters = input.split("");
StringBuilder sb = new StringBuilder();
for (String letter : letters) {
if (pegStrings.contains(letter)) {
sb.append(letter);
} else {
System.out.println(letter);
throw new RuntimeException();
}
}
String pegListString = sb.toString();
Code parsedCode = new Code(pegListString);
//System.out.println(parsedCode);
return parsedCode;
}
public int countExactMatches(Code guess){
String guessString = guess.secretCodeString;
int exactMatches = 0;
String[] guessArray = guessString.split("");
String[] winningCodeArray = (this.secretCodeString).split("");
for(int i = 0; i < 4; i++){
if(guessArray[i] == winningCodeArray[i]){
exactMatches++;
}
}
return exactMatches;
}
public int countNearMatches(Code guess) {
String guessString= guess.secretCodeString;
HashMap<String,Integer> guessCount = new HashMap<String,Integer>();
HashMap<String,Integer> secretCodeCount = new HashMap<String,Integer>();
Set<String> codeKeys = guessCount.keySet();
int matches = 0;
int keys = guessCount.keySet().size();
String[] keyArray = new String[keys];
for(int i = 0; i < guessString.length(); i++) {
//removes character from string
String codeCharacter = String.valueOf(guessString.charAt(i));
String guessShort = guessString.replace(codeCharacter,"");
//counts instances of said character
int count = guessString.length() - guessShort.length();
guessCount.put(codeCharacter, count);
}
for(int i = 0; i < secretCodeString.length(); i++) {
//removes character from string
String winningString = this.secretCodeString;
String winningCodeCharacter = String.valueOf(winningString.charAt(i));
String winningCodeShort = guessString.replace(winningCodeCharacter,"");
//counts instances of said character
int count = winningString.length() - winningCodeShort.length();
secretCodeCount.put(winningCodeCharacter, count);
}
for (int i = 0; i < keys; i++) {
codeKeys.toArray(keyArray);
String keyString = keyArray[i];
if (secretCodeCount.containsKey(keyString)) {
matches += Math.min(secretCodeCount.get(keyString), guessCount.get(keyString));
}
}
int nearMatches = matches - countExactMatches(guess);
return nearMatches;
}
}
Game.java
import java.util.Scanner;
class Game {
protected static Code winningCode;
public static void main(String[] args){
}
public Game(){
winningCode = new Code();
}
protected static Code getGuess() {
Scanner userInput = new Scanner(System.in);
int count = 0;
int maxTries = 5;
while(true){
try {
String codeToParse = userInput.next();
Code guess = Code.parse(codeToParse);
return guess;
} catch(RuntimeException notACode) {
System.out.println("That's not a valid peg. You have " + (maxTries - count) + " tries left.");
if (++count == maxTries) throw notACode;
}
}
}
protected static void displayMatches(Code guess){
int nearMatches = winningCode.countNearMatches(guess);
int exactMatches = winningCode.countExactMatches(guess);
System.out.println("You have " + exactMatches + " exact matches and " + nearMatches + " near matches.");
}
protected static void play(){
int turnCount = 0;
int maxTurns = 10;
System.out.println("Greetings. Pick your code of four from Y,O,G,P,C,R.");
while(true){
Code guess = getGuess();
displayMatches(guess);
if (guess == winningCode) {
System.out.print("You win!!");
break;
} else if (++turnCount == maxTurns) {
System.out.print("You lose!!");
break;
}
}
}
}
On every guess, you call Code.parse, Code.parse creates a new Code (new Code(pegListString);) and that constructor sets the secretCodeString and because that's static, all instances of Code share the same variable. You need to avoid mutable static members.
Another tip is to either have a method return a value, or mutate state (of either its input, or its own instance, this), but avoid doing both.
"Why is my class variable rewriting itself after an unrelated method runs?"
Because, actually, it is not unrelated. The "mess" that you have created by declaring variables and methods as static has lead to unwanted coupling between different parts of your code.
It is difficult to say what the correct solution is here because your code has gotten so confused by the rewrites that it is hard to discern the original "design intent".
My advice would be to start again. You now should have a clearer idea of what functionality is required. What you need to do is to redo the object design so that each class has a clear purpose. (The Main and Game classes make sense, but Code seems to be a mashup of functionality and state that has no coherent purpose.)
I'm fairly new to Java and this may sound a bit strange.
Okay basically I'm taking in 9 values and storing them as Integers in an ArrayListScores.
I have verified that they are storing in there and all looks okay with that.
I'm developing a simple androids app.
So we take it as if I take in the 9 values in class 1 as such from text views parseInt them. This works fine that is not my issue.
Class 1
ArrayList<Integer> Scores = new ArrayList<Integer>();
Scores.add(Integer.parseInt(et1.getText().toString()));
Scores.add(Integer.parseInt(et2.getText().toString()));
Scores.add(Integer.parseInt(et3.getText().toString()));
Scores.add(Integer.parseInt(et4.getText().toString()));
Scores.add(Integer.parseInt(et5.getText().toString()));
Scores.add(Integer.parseInt(et6.getText().toString()));
Scores.add(Integer.parseInt(et7.getText().toString()));
Scores.add(Integer.parseInt(et8.getText().toString()));
Scores.add(Integer.parseInt(et9.getText().toString()));
My Problem is that I have another class which I want to do some basic calculations, just add up all the scores as such.
Class 2
public class AddNumbers {
private static AddNumbers instance;
private AddNumbers(){
}
public static AddNumbers getInstance() {
if(instance == null) {
instance = new AddNumbers();
}
return instance;
}
public int getFinalScore() {
ArrayList<Integer> Scores = new ArrayList<Integer>();
int final_score = 0;
for(int s: Scores){
final_score += s;
}
return final_score;
}
}
I was to do the basic adding up of all the scores in class 2 and send the result back to class 1 but I don't know how.
Do you really need another class for this? Why not just put this in a method in Class 1?
It would look like:
public int getFinalScore(){
You want to put in your ArrayList here. This should look like:
public int getFinalScore(ArrayList<Integer> Scores) {
Then put your for loop, and return final_score:
int final_score = 0;
for (int s: Scores) {
final_score += s;
}
return final_score;
So your final method would look like this:
public int getFinalScore(ArrayList<Integer> Scores) {
int final_score = 0;
for (int s: Scores) {
final_score += s;
}
return final_score;
}
You would call it just via getFinalScore(Scores).
Pass Scores from Class 1 as a parameter into the getFinalScore method in Class 2
public int getFinalScore(List<Score> scores) {
int final_score = 0;
for(int s: scores){
final_score += s;
}
return final_score;
}
Then use the return value as your sum in Class 1.
What I would do is make a variable/instance of the ArrayList from class 1. so first you need to make Scores public so your other class can access it:
public static ArrayList<Integer> Scores = new ArrayList<Integer>(); //add public and static
Scores.add(Integer.parseInt(et1.getText().toString()));
Scores.add(Integer.parseInt(et2.getText().toString()));
Scores.add(Integer.parseInt(et3.getText().toString()));
Scores.add(Integer.parseInt(et4.getText().toString()));
Scores.add(Integer.parseInt(et5.getText().toString()));
Scores.add(Integer.parseInt(et6.getText().toString()));
Scores.add(Integer.parseInt(et7.getText().toString()));
Scores.add(Integer.parseInt(et8.getText().toString()));
Scores.add(Integer.parseInt(et9.getText().toString()));
Then, you need to refer back to it like so:
public int getFinalScore() {
ArrayList<Integer> scores = Class1.Scores; //make a new variable referring to the Scores (Case Matters!)
int final_score = 0;
for(int s: scores){ //use the new variable
final_score += s;
}
return final_score;
}
Alternatively, you can make the new variable scores (CASE matters) public and static if you want to use it in yet another class again (if you want to, this isn't necessary). However, you still need to make the ArrayList public! The public scores would look like this:
public class AddNumbers {
private static AddNumbers instance;
public static ArrayList<Integer> scores = Class1.Scores //made the variable public and static (optional)
private AddNumbers(){
}
public static AddNumbers getInstance() {
if(instance == null) {
instance = new AddNumbers();
}
return instance;
}
//same as before
public int getFinalScore() {
ArrayList<Integer> scores = Class1.Scores; //make a new variable referring to the Scores (Case Matters!)
int final_score = 0;
for(int s: scores){ //use the new variable
final_score += s;
}
return final_score;
}
}
Alternatively again, you can make a new parameter and set it to Scores (Again, you still need to make Scores public):
public int getFinalScore(ArrayList<Integer> scores) {
scores = Class1.Scores //set local variable to public variable
int final_score = 0;
for(int s: scores){
final_score += s;
}
return final_score;
}