Develop Reference

Struggling with exceptions, try/catch block and singleton in Java

I am trying to learn to work with error handling/throwing exceptions in Java.
I have a class, UserDatabase, holding a collection of students, and it should also save the collection to a file.
What I'm fairly sure that I haven't done correctly, is the methods for file handling. Method public boolean saveDatabase should not throw exceptions and be handled in a try-catch, and use the encode method from the student class on Student objects, to write every object as a line in the file. This one is compiling, but it looks off to me. In the book it says write the method public boolean saveDatabase(); and as you can see I changed the header for it to make sense to me. This, mainly because I don't know how to write a method with the header ending with ();
Method public boolean loadDatabase should also handle possible IO errors and return false if one occurs. For every line in the field it should create a student object by the constructor public Student(String encodedStudent) from the sudent class. If a line in the file cannot be decoded as a student it should throw a new exception, DatabaseFormatException (this is a seperate class). This one is also listed as public boolean loadDatabase(); in the book. Let's face it, this one is completely off. I have no idea what to do and I have been working with it for hours and hours, reading books, reading online, I am lost.
Here's my code:
/**
* This method should not throw exceptions.
* By using a try/catch block, we anticipate possible failures.
* We recognize that these actions might fail and throw errors.
*/
public boolean saveDatabase() throws IOException {
//This method is using the encode method on student objects and should
//write each object as a line in the file.
String encode = null;
boolean saved;
try {
encode = null;
userdatabase.saveDatabase();
saved = false;
}
catch (IOException e) {
System.out.println("Error");
saved = false;
}
finally {
if(encode.equals(students)) {
//System.out.println("Students" + students)?;
saved = true;
}
}
return saved;
}
/**
* Method loadDatabase should handle possible IO errors, and return false
* if one occurs. Otherwise, it should return true and create a new
Student object
* by using the constructor public Student(String encodedStudent).
* If a line cannot be decoded as a student, the method should throw a
new
* exception "DatabaseFormatException".
*
*/
public boolean loadDatabase() throws IOException,DatabaseFormatException {
//Attempting to use the String encodedStudent constructor from Student class
String encodedStudent = null;
boolean loaded;
try {
//Attempting to create possible IO errors returning false if they occur
enco dedStudent = null;
//UserDatabase.loadDatabase(encodedStudent);
loaded = false;
}
catch(IOException e) {
if (encodedStudent == null) {
System.out.println("Error");
loaded = false;
}
}
//Trying a for each loop to throw a DatabaseFormatException
for(Student student : students) {
//This is supposed to throw a DatabaseFormatException if a
//line in the file cannot be decoded as a student!
if(student.getName.isEmpty() && this.course.isEmpty()) {
throw new DatabaseFormatException(
"No student found");
}
}

Your code should be
public boolean saveDatabase() {
try {
// maybe do some more work...
userdatabase.saveDatabase();
return true;
} catch (IOException e) {
return false;
}
}
Simply return true/false depending on wether an exception occurred or not. Drop the saved since you no longer need it. And drop the encode since you did not need it in the first place and never assigned a value to it.

alter dictionary object in local variable

I'm trying to both remove the method from the method ArrayList and check to see if the ArrayList is empty in one lookup.
This uses two look ups.
private Map<String, List<Method>> events;
public void removeEvent(String eventName, Method method){
try{
events.get(eventName).remove(method);
if(events.get(eventName).size() == 0){
events.remove(eventName);
}
}
catch (Exception e){
}
}
As you can see it looks up the ArrayList of methods to remove a method then looks it up again to see if its length is zero then looks it up again to remove the HashMap entry. Is their a way to combine at least the first two look ups?

You may change it to this way:
List<Method> methods = events.get(eventName);
if (methods == null) {
return;
}
methods.remove(method);
if (methods.isEmpty()) {
events.remove(eventName);
}

Below code might be helpful in your case.
It does not require to lookup second times to check the size of list.
public void removeEvent(String eventName, Method method){
try{
List<Method> methods = events.get(eventName);
methods.remove(method);
if(methods.size() == 0){
events.remove(eventName);
}
}
catch (Exception e){
}
}

Lclassname.classname; cannot be cast to classname.classname

i'm writing a program for a game called 'Trivia'. Below is the source code:
Trivia.java
public class Trivia implements Serializable {
private String question;
private String answer;
private int points;
public Trivia() {
question = " ";
answer = " ";
points = 0;
}
public String getQuestion() {
return question;
}
public String getAnswer() {
return answer;
}
public int getPoints() {
return points;
}
public void setQuestion(String q) {
question = q;
}
public void setAnswer(String a) {
answer = a;
}
public void setPoints(int p) {
points = p;
}
}
Driver.java
public class Driver {
public static void main(String[] args) {
Trivia[] t = new Trivia[5];
for (int i = 0; i < 5; i++) {
t[i] = new Trivia();
}
t[0].setQuestion("How many states are in the US?");
t[0].setAnswer("50");
t[0].setPoints(1);
t[1].setQuestion("Who is the richest person in the US");
t[1].setAnswer("You");
t[1].setPoints(1);
t[2].setQuestion("How many senators come from each state?");
t[2].setAnswer("2");
t[2].setPoints(2);
t[3].setQuestion("What is the largest state?");
t[3].setAnswer("Alaska");
t[3].setPoints(2);
t[4].setQuestion("Who was the thrid president?");
t[4].setAnswer("Thomas Jefferson");
t[4].setPoints(3);
ObjectOutputStream outputStream = null;
try {
outputStream = new ObjectOutputStream(new FileOutputStream("C:\\Work\\workspace\\aman\\src\\trivia\\trivia.dat"));
} catch (IOException e) {
System.out.println("Could not open file");
System.exit(0);
}
try {
outputStream.writeObject(t);
outputStream.close();
} catch (IOException e) {
System.out.println("Writing error");
System.exit(0);
}
ArrayList<Trivia> triviaQuestions = new ArrayList<Trivia>();
try {
ObjectInputStream inputStream = new ObjectInputStream(new FileInputStream("C:\\Work\\workspace\\aman\\src\\trivia\\trivia.dat"));
for(int i=0; i<5; i++){ // Repeats the content of the loop five times
triviaQuestions.add((Trivia) inputStream.readObject());
}
inputStream.close(); // Closes the input stream because it is not longer needed
} catch (IOException | ClassNotFoundException e) {
System.out.println("File not found.");
System.exit(0);
}
Trivia yourRandomTrivia = triviaQuestions.get((new Random()).nextInt(triviaQuestions.size())); // This will be your random question
}
// You did not get an auto complete suggestion because you typed outside of a method
}
noe when I try to run this program, I get an error saying "Ltrivia.Trivia; cannot be cast to trivia.Trivia". The error is thrown in class Driver on line " triviaQuestions.add((Trivia) inputStream.readObject());". I did some research on this and found that 'L' means array of a datatype. But, I have simple created an arrayList of type Trivia and trying to add each element I get from the inputStream by casting them to Trivia class.
Does anybody have any suggestions on this?

Your code is writing an Array of Trivia objects.
Then you try to read and add that to a list of Trivia objects.
You cant add arrays of Trivia to an List of Trivia!
And that is what the message is telling you: you cant cast the type Trivia[] to Trivia. Because an array of X is not the same as a single X.
One solution: instead of writing t as a whole, you can simply iterate t and write the members of the array. Of course that means that you have to somehow remember how many elements you wrote into that stream. You could get there by first writing an Integer object representing the number of Trivia objects that will follow.
The other solution: just read back that Trivia[]; and iterate it then; to add the various Trivia objects one by one.
Edit: on your comment: when you read from an ObjectInputStream you get back those things that you put into your file/stream earlier on. As said: your code puts a single object of type ARRAY of Trivia into bytes ... and then you want to read that thing back as a single Trivia object! That does not work!

"Cannot return a value with void result type" error

The following code comes from this answer
try {
// get all the interfaces
List<NetworkInterface> all = Collections.list(NetworkInterface.getNetworkInterfaces());
//find network interface wlan0
for (NetworkInterface networkInterface : all) {
if (!networkInterface.getName().equalsIgnoreCase("wlan0")) continue;
//get the hardware address (MAC) of the interface
byte[] macBytes = networkInterface.getHardwareAddress();
if (macBytes == null) {
return "";
}
StringBuilder res1 = new StringBuilder();
for (byte b : macBytes) {
//gets the last byte of b
res1.append(Integer.toHexString(b & 0xFF) + ":");
}
if (res1.length() > 0) {
res1.deleteCharAt(res1.length() - 1);
}
return res1.toString();
}
} catch (Exception ex) {
ex.printStackTrace();
}
I get error Cannot return a value with void result type on those 2 lines: return ""; and return res1.toString(); I put the code inside public void onStart() How do I fix this, and can you tell me the cause of this problem?

Instead of returning an empty String, just return;
Methods that are void do not return anything but you can use return statement to terminate an operation if some condition is not met!
I hope this helps!

You need to change the line
public void onStart()
To
public String onStart()
This is because you are returning a String, whereas a void function does not return any data.
If the method cannot be changed to a String return type then you could just put the string into a variable that you declare earlier in the program and then use
return;
To exit the method.

Issue is clear, you are returning a value for a function public void onStart(). You are declaring the return type as void and yet you have return statements.
Try different ways to return the value, like put it in request/session or static variable(not recommanded) etc

Can we get the exact location where the condition fails, in an If case having multiple conditions?

I am new to Java,
Here is my code,
if( a.name == b.name
&& a.displayname == b.displayname
&& a.linkname == b.linkname
......... )
return true;
else
return false;
I will call this method and have to check that all properties of objects 'a' and 'b'.
Each object will have more than 20 properties. So, it is will be tidy if i use if case for each property.
An exception is throwed if the return is false and I have to report which property fails.
Is there any easy method to find where the condition fails within the if case.
Pls help. Ask if you are not clear about the question.

The question is, would you like to continue checking if one of the conditions fails?
You could do something like comparator where you have interface:
public interface IComparator {
boolean compare(YourObject o1, YourObject o2);
String getComparatorName();
}
Next you create set of implementations of that interface:
NameComparator implements IComparator {
private name="Name Comparator";
#Override
public boolean compare(YourObject o1, YourObjecto2) {
return o1.getName().equals(o2.getName());
}
#Override
public String getComparatorName() {
return name;
}
}
Next you store set of these comparators in arrayList and you iterate through them and record which one fails by adding them to some other collection.. Hope that helps!
For instance you create array:
IComparator[] comparators = new IComparator[]{ new NameComparator, new DisplayNameComparator};
List<IComparator> failedComparationOperations = new ArrayList<IComparator>();
for(IComparator currentComparator : comparators) {
if(!currentComparator.compare(o1, o2)) {
failedComparationOperations.add(currentComparator);
}
}
for(IComparator currentComparator: failedComparationOperations)
{
System.out.println("Failed Comparation at: "+currentComparator.getComparatorName());
}

You may use reflection: browse what fields are defined, and check each of them using method equals. Print error message if they're not equal, give summary at the end.
boolean equals = true;
Field[] fields = a.getClass().getDeclaredFields();
for (Field f: fields){
f.setAccessible(true);
try {
if (!f.get(a).equals(f.get(b))){
System.out.println(f.getName() + ": " + f.get(a) + "!="+ f.get(b));
equals = false;
};
} catch (Exception e) {
e.printStackTrace();
}
}
System.out.println("equals?: " + equals);

If you need to know which of the conditions has failed you should check each of the conditions independently.

It might be a little overkill if you are dealing with this single requirement, but what about the Strategy Design Pattern?
http://sourcemaking.com/refactoring/replace-conditional-with-polymorphism
It should be an interesting option if you have other business rules that you can combine with this check.

If a and b are instances of the same class, let's assume A, and the fields are visible, then you can use reflections:
for (Field f : A.class.getFields()) {
try {
if (!f.get(a).equals(f.get(b))) {
throw new RuntimeException("Field " + f.getName() + " is different.");
}
} catch (Exception e) {
e.printStackTrace();
}
}

Without reflection you can't get maximum conciseness, but the followincg can help you to some extent. Make this kind of class:
class NamedEquals {
final String name;
final Object left, right;
NamedCondition(String name, Object left, Object right) { ...assign them... }
boolean areEqual() { return left.equals(right); }
}
Then make a List<NamedEquals>:
List<NamedEquals> conds = Arrays.asList(
new NamedEquals("name", left.name, right.name),
new NamedEquals("displayname", left. displayname, right.displayname),
...
);
And you can find if some of them fail:
for (NamedEquals eq : conds)
if (!eq.areEqual()) throw new ValidationException(eq.name);
Using a factory method can shorten the construction code:
static NamedEquals eq(String name, Object left, Object right) {
return new NamedEquals(name, left, right);
}
With that you can have
List<NamedEquals> conds = Arrays.asList(
eq("name", left.name, right.name),
eq("displayname", left. displayname, right.displayname),
...
);

How about?
// Adapted from your example:
if(!equalTo(a.name, b.name))
fail("name");
if(!equalTo(a.displayname, b.displayname))
fail("displayname");
... etc ...
...
// Allow for null values.
public boolean equalTo(Object a, Object b) {
return a != null ? a.equals(b) : b == null;
}
public void fail(String which) throws SomeException {
throw new SomeException("Failed on '"+which+"'!");
}
Another possible might be to turn each object into a Map<String,?>, perhaps by adding a Map<String,?> toMap() method to the value object, and implementing this by constructing a new map and dumping the value's fields into it. Then you can get the maps and do equals() on them.