An excepció és un error no desitjat i inesperat llançat al programa. La majoria de vegades, es produeix una excepció quan hi ha un error al nostre codi però es pot gestionar. Pertorba el flux normal del codi.
Per exemple, el codi llança una excepció si l'usuari ha introduït informació no vàlida, si el codi no pot llegir el fitxer situat al lloc remot o si la connexió de xarxa es perd enmig de la comunicació.
IllegalStateException a Java
IllegalStateException és la subclasse de la classe RuntimeException i, per tant, és una excepció no marcada. El programador o el desenvolupador de l'API la planteja explícitament. Es llança quan una trucada de mètode és il·legal o quan es crida un mètode en un moment incorrecte.
Per exemple, un cop iniciem un fil, no podem reiniciar el mateix fil; si intentem fer-ho, llançarà una excepció d'execució, és a dir, IllegalStateException .
L'excepció maig sorgeixen al codi normalment quan estem treballant amb el marc de col·leccions. La llista, la cua, els mapes, l'arbre són algunes de les col·leccions. D'aquests, la llista i les cues tendeixen a llançar l'excepció d'estat il·legal a les condicions específiques.
Nota: l'excepció IllegalStateException no es limita només al marc de col·leccions.
Vegem alguns dels escenaris en què IllegalStateException serà llençat.
Exemple 1:
El següent programa Java mostra la situació en què intentem cridar el mètode start() quan el mètode run() ja s'està executant.
IllegalStateExceptionTest1.java
// importing necessary packages import java.io.*; import java.util.*; // creating a new thread in NewThread class by extending Thread class // below class is acts as a helper class class NewThread extends Thread { // declaring the run() method // executes 3 times using for loop public void run() { for (int i = 0; i <3; i++) { displaying the text system.out.println('this is example of illegalstateexception'); } creating main class public illegalstateexceptiontest1 method static void main(string[] args) object above helper newthread t="new" newthread(); starting created thread using start() t.start(); thread'); again when it already running this gives an exception < pre> <p> <strong>Output:</strong> </p> <img src="//techcodeview.com/img/java-tutorial/19/how-resolve-illegalstateexception-java.webp" alt="How to resolve IllegalStateException in Java"> <h3>Example 2:</h3> <p>The following code depicts the situation where we call the start() method on a thread when the execution of run() method is over.</p> <p> <strong>IllegalStateExceptionTest2.java</strong> </p> <pre> // importing necessary packages import java.io.*; import java.util.*; // creating a new thread in NewThread class by extending Thread class // below class is acts as a helper class class NewThread extends Thread { // declaring the run() method // executes 3 times using for loop public void run() { for (int i = 0; i <3; i++) { displaying the text system.out.println('this is example of illegalstateexception'); } creating main class public illegalstateexceptiontest2 method static void main(string[] args) object above helper newthread t="new" newthread(); starting created thread using start() t.start(); try system.out.println('main going to sleep'); putting on sleep for 4000ms t.sleep(4000); awaken'); catch (exception e) system.out.println(e); message thread'); calling over a dead this also gives an exception < pre> <p> <strong>Output:</strong> </p> <img src="//techcodeview.com/img/java-tutorial/19/how-resolve-illegalstateexception-java-2.webp" alt="How to resolve IllegalStateException in Java"> <h3>Example 3:</h3> <p>The following code explains the situation where we are using the remove() method to remove the element from the ArrayList, before moving to the first element.</p> <p> <strong>IllegalStateExceptionTest3.java</strong> </p> <pre> // importing necessary packages import java.util.ArrayList; import java.util.ListIterator; // creating the main class public class IllegalStateExceptionTest3 { // main method public static void main(String args[]) { // instantiating the object of ArrayList ArrayList list = new ArrayList(); // adding elements to the ArrayList list.add('Nirnay'); list.add('Anu'); list.add('Swara'); list.add('Pavan'); // creating the iterator object to iterate the list ListIterator it = list.listIterator(); // removing the element without moving to first position // gives an exception it.remove(); } } </pre> <p> <strong>Output:</strong> </p> <img src="//techcodeview.com/img/java-tutorial/19/how-resolve-illegalstateexception-java-3.webp" alt="How to resolve IllegalStateException in Java"> <h2>Solution for the IllegalStateException</h2> <p>To avoid the <strong>java.lang.IllegalStateException</strong> in Java we should take care that any method in our code cannot be called at inappropriate or illegal time.</p> <p> <strong>Solution for example 1 and 2:</strong> </p> <p>Consider the above example 1 and 2 where we have called the start() method more than once. If we call it only once, we will not get this exception. Because start() method is not called after starting the thread.</p> <p> <strong>IllegalStateExceptionSolution.java</strong> </p> <pre> // importing necessary packages import java.io.*; import java.util.*; // creating a new thread in NewThread class by extending Thread class // below class is acts as a helper class class NewThread extends Thread { // declaring the run() method // executes 3 times using for loop public void run() { for (int i = 0; i <3; i++) { displaying the text system.out.println('this is example of illegalstateexception'); } creating main class public illegalstateexceptionsolution method static void main(string[] args) object above helper newthread t="new" newthread(); starting created thread using start() t.start(); try system.out.println('main going to sleep'); putting on sleep for 4000ms t.sleep(4000); awaken'); catch (exception e) system.out.println(e); message thread'); we do not call again < pre> <p> <strong>Output:</strong> </p> <img src="//techcodeview.com/img/java-tutorial/19/how-resolve-illegalstateexception-java-4.webp" alt="How to resolve IllegalStateException in Java"> <p> <strong>Solution for Example 3:</strong> </p> <p>The remove() method of the ArrayList class is used to remove the last element after calling the next() method.</p> <ul> <li>After removing element at current index we have to move next element to remove it. (for every call of the next() method, we have to invoke the remove() method only once).</li> <li>As the initial position of list will be before the first element, we cannot call the remove() method without calling the next() method.</li> </ul> <p>In order to prevent the exception we need to follow the above steps in our Java code.</p> <p> <strong>IllegalStateExceptionSolution2.java</strong> </p> <pre> // importing necessary packages import java.util.ArrayList; import java.util.ListIterator; // creating the main class public class IllegalStateExceptionSolution2 { // main method public static void main(String args[]) { // instantiating the object of ArrayList ArrayList list = new ArrayList(); // adding elements to the ArrayList list.add('Nirnay'); list.add('Anu'); list.add('Swara'); list.add('Pavan'); // creating the iterator object to iterate the list ListIterator it = list.listIterator(); // using the next() method before remove() it.next(); // removing the element without moving to first position it.remove(); // displaying new ArrayList System.out.println('List after removing the first element: ' + list); } } </pre> <p> <strong>Output:</strong> </p> <img src="//techcodeview.com/img/java-tutorial/19/how-resolve-illegalstateexception-java-5.webp" alt="How to resolve IllegalStateException in Java"> <hr></3;></pre></3;></pre></3;> Sortida:
Solució per a IllegalStateException
Per evitar el java.lang.IllegalStateException a Java hem de tenir cura que cap mètode del nostre codi no es pugui cridar en un moment inadequat o il·legal.
Solució per exemple 1 i 2:
Considereu l'exemple anterior 1 i 2 on hem cridat el mètode start() més d'una vegada. Si l'anomenem només una vegada, no obtindrem aquesta excepció. Com que el mètode start() no es crida després d'iniciar el fil.
IllegalStateExceptionSolution.java
// importing necessary packages import java.io.*; import java.util.*; // creating a new thread in NewThread class by extending Thread class // below class is acts as a helper class class NewThread extends Thread { // declaring the run() method // executes 3 times using for loop public void run() { for (int i = 0; i <3; i++) { displaying the text system.out.println(\'this is example of illegalstateexception\'); } creating main class public illegalstateexceptionsolution method static void main(string[] args) object above helper newthread t="new" newthread(); starting created thread using start() t.start(); try system.out.println(\'main going to sleep\'); putting on sleep for 4000ms t.sleep(4000); awaken\'); catch (exception e) system.out.println(e); message thread\'); we do not call again < pre> <p> <strong>Output:</strong> </p> <img src="//techcodeview.com/img/java-tutorial/19/how-resolve-illegalstateexception-java-4.webp" alt="How to resolve IllegalStateException in Java"> <p> <strong>Solution for Example 3:</strong> </p> <p>The remove() method of the ArrayList class is used to remove the last element after calling the next() method.</p> <ul> <li>After removing element at current index we have to move next element to remove it. (for every call of the next() method, we have to invoke the remove() method only once).</li> <li>As the initial position of list will be before the first element, we cannot call the remove() method without calling the next() method.</li> </ul> <p>In order to prevent the exception we need to follow the above steps in our Java code.</p> <p> <strong>IllegalStateExceptionSolution2.java</strong> </p> <pre> // importing necessary packages import java.util.ArrayList; import java.util.ListIterator; // creating the main class public class IllegalStateExceptionSolution2 { // main method public static void main(String args[]) { // instantiating the object of ArrayList ArrayList list = new ArrayList(); // adding elements to the ArrayList list.add('Nirnay'); list.add('Anu'); list.add('Swara'); list.add('Pavan'); // creating the iterator object to iterate the list ListIterator it = list.listIterator(); // using the next() method before remove() it.next(); // removing the element without moving to first position it.remove(); // displaying new ArrayList System.out.println('List after removing the first element: ' + list); } } </pre> <p> <strong>Output:</strong> </p> <img src="//techcodeview.com/img/java-tutorial/19/how-resolve-illegalstateexception-java-5.webp" alt="How to resolve IllegalStateException in Java"> <hr></3;> Sortida:
