Develop Reference

Why does static hashmap created for every instances?

I have a course class having one Hashmap. I'm trying to add values to the map with different objects. Map is common for all the objects so I marked it as Static still it shows weird behavior.
I have the following code -
class Course
{
static HashMap<Integer,List<String>> map;
Course()
{
map = new HashMap<>();
}
public boolean add(int id,String Course)
{
if(!map.containsKey(id))
{
map.put(id,new ArrayList<>());
}
try
{
List<String> temp = map.get(id);
temp.add(Course);
return true;
} catch (Exception e)
{
return false;
}
}
public void get()
{
System.out.println(map);
}
public static void main(String[] args)
{
Course c = new Course();
c.add(1, "abs");
c.add(2,"xyx");
c.add(1,"new");
c.add(3,"tye");
c.get();
Course c2 = new Course();
c2.add(1,"GP");
c2.add(2, "PT");
c2.get();
}
}
I have defined Hashmap as static because it is common for all the objects. But still, the new Hashmap is created for every instance.
Output
{1=[abs, new], 2=[xyx], 3=[tye]}
{1=[GP], 2=[PT]}

Because you initialize it in the constructor.
Don't. Just initialize it on the field:
static HashMap<Integer,List<String>> map = new HashMap<>();
(And remove the constructor).
And consider making the field final if you never intend to reassign it. This ensures that 1) you don't actually reassign it; 2) you actually do assign it once.

Java Array of InnerClass throwing java.lang.NoSuchFieldError

I am trying to brushup java after a long time.
Any help is much appreciated.
For demonstration I have Animal Class that has an array of innerclass of Organs.
public class Animal
{
String nameOfAnimal;
Organs [] vitalOrgans = new Organs[3];
public Animal()
{
}
public String getNameOfAnimal() {
return nameOfAnimal;
}
public void setNameOfAnimal(String nameOfAnimal) {
this.nameOfAnimal = nameOfAnimal;
}
#Override
public String toString() {
return "Animal{" + "nameOfAnimal=" + nameOfAnimal + "}";
}
class Organs{
String nameOfOrgan;
public String getNameOfOrgan() {
return nameOfOrgan;
}
public void setNameOfOrgan(String nameOfOrgan) {
this.nameOfOrgan = nameOfOrgan;
}
#Override
public String toString() {
return "Organs{" + "nameOfOrgan=" + nameOfOrgan + '}';
}
}
}
Now in driver file when I make call there is no syntactical error but I get "Exception in thread "main" java.lang.NoSuchFieldError: vitalOrgans"
Animal mamal = new Animal();
mamal.setNameOfAnimal("Chimp");
mamal.vitalOrgans[0].setNameOfOrgan("Heart");
System.out.println(mamal.vitalOrgans[0].getNameOfOrgan());
What would be the way to make this (or similar idea) to work.
Thanks.

You would need to initialize the vitalOrgrans with new Organs(). Like:
public Animal() {
for (int i = 0; i < vitalOrgans.length; i++) {
vitalOrgans[i] = new Organs();
}
}
Because when you say :
Organs[] vitalOrgans = new Organs[3];
You are creating an array of 3 null Organs. Hence the null pointer exception, when accessing "vitalOrgans[i].".

Taking the relevant bit of code:
public class Animal
{
//...
Organs [] vitalOrgans = new Organs[3];
//...
}
Since your declaration of vitalOrgans was never given an access modifier (i.e. one of private, public, protected) it took on default access, which means only other classes in the same package can see it. Since your other block of code is not in the same package, it cannot see the field.
A minimally viable modification to just make it work would be to set the access to public:
public class Animal
{
//...
public Organs [] vitalOrgans = new Organs[3];
//...
}
While this works, it's not necessarily the best solution, as if you ever change how vitalOrgans is represented, or need to perform any validation, those edits would have to be done throughout the application. Thus, a better solution (and also, a major stylistic convention in Java for those exact reasons) is to make it (and all your fields, in fact) private and access via methods:
public class Animal {
private String nameOfAnimal;
private Organs[] vitalOrgans = new Organs[3];
//...
public Organs[] getVitalOrgans() {
return vitalOrgans;
}
//Alternative accessor that fetches only one organ.
public Organs getVitalOrgan(int index) {
if(index >= 0 && index < vitalOrgans.length)
return vitalOrgans[index];
else
return null;
}
public void setVitalOrgans(Organs[] vitalOrgans) {
this.vitalOrgans = vitalOrgans
}
//...
}
Your caller could then access Organs via either form of the get method (note, you probably want Organs to be public):
Animal.Organs futureMammalHeart = mamal.getVitalOrgan(0); //Animal.Organs due to Organs being an inner class.
if(futureMammalHeart != null) //Demonstration of null check. Safety first!
futureMammalHeart.setNameOfOrgan("Heart");
Animal.Organs[] mammalianVitalOrgans = mamal.getVitalOrgans();
if(mammalianVitalOrgans != null) //Just in case...
System.out.println(mamal.mammalianVitalOrgans[0].getNameOfOrgan());
Also, as Ari mentioned in his answer, don't forget to initialize the organs in your array, otherwise you will get a NullPointerException!

Java: message system needs to be able to pass various objects

I'm writing a messaging system to queue actions for my program to execute. I need to be able to pass various objects by the messages. I currently have a Msg object that accepts (Action enum, Data<?>...object). The Data object is intended to be a wrapper for any object I might pass.
Currently the Data object uses this code, with generics:
public class Data<T> {
private T data;
public Data(T data){
this.data = data;
}
public T getData(){
return data;
}
}
The Msg object takes Data<?>... type, so Msg has a Data<?>[] field.
If getData() is called on a Data<?> object, it returns the Object type. Obviously not ideal.
I need to be able to pass, say, Image objects as well as String objects. I'm certain there's a better way of passing arbitrary data.

The reason you're having trouble is that you're trying to get the static typing system of Java to do something that it can't. Once you convert from a Data<T> to a Data<?>, whatever T was is effectively lost. There's no clean way to get it back.
The quickest way to get it to work (from what you have right now) is to start throwing casts everywhere, like this:
Data<?> d = new Data("Hello");
String contents = (String)d.getData();
This is kind of a terrible idea, so let's go back to the drawing board.
If (ideally), you have all of the types you could ever need ahead of time (i.e. every Data is either a String or an Image or an Integer), then you can pretty easily (though it's a bit tedious) define a Sum type (aka a union if you're coming from C) of the different types of data you'll have to handle. As a class invariant, we assume that exactly one of the fields is non-null, and the rest are null. For this example I'll assume it can be either a String, an Image, or an Integer, but it's fairly simple to add or remove types from Data as necessary.
public class Data {
private Image imgData;
private String stringData;
private Integer intData;
public Data(Image img) {
this.imgData = img;
}
public Data(String stringData) {
this.stringData = stringData;
}
public Data(Integer intData) {
this.intData = intData;
}
public boolean isImage() {
return imageData != null;
}
public boolean isInteger() {
return intData != null;
}
public boolean isString() {
return stringData != null;
}
public Image asImage() {
if(! isImage()) throw new RuntimeException();
return imgData;
}
public Image asString() {
if(! isString()) throw new RuntimeException();
return stringData;
}
public Image asInt() {
if(! isInt()) throw new RuntimeException();
return intData;
}
}
One necessary side effect is that we cannot wrap null without causing exceptional behavior. Is this is desired, it isn't too difficult to modify the class to allow for it.
With this Data class, it's pretty easy to do if-else logic to parse it.
Data d = ....... //Get a data from somewhere
if(d.isImage()) {
Image img = d.asImage();
//...
} else if (d.isString()) {
String string = d.asString();
//...
} else if (d.isInteger()) {
Integer i = d.asInt();
//...
} else {
throw new RuntimeException("Illegal data " + d + " received");
}

If you call getData().getClass() you will get the class or type that was passed, which doesn't seem to me to be the same as an Object. You might not know what you are getting, but you can either find out or define a common interface for everything you might pass. You could for example, call toString() or getClass() on anything passed. Your question is that you are passing any conceivable object, so my question is what are you going to do with it? If you are going to serialize it into a database you don't need know anything about what type it is, otherwise you can test it or call a common interface.
public class PlayData {
class Msg {
private List<Data<?>> message = new ArrayList<Data<?>>();
public void addData(Data<?> datum) { message.add(datum); }
public void printTypes() { for ( Data<?> datum: message ) { System.out.println(datum.getData().getClass()); } }
}
class Data<T> {
private T value;
public Data(T value) { this.value = value; }
public T getData() { return value; }
}
class Listener {
public void receive(Msg msg) { msg.printTypes(); }
}
class Sender {
private Listener listener;
public Sender(Listener listener) { this.listener = listener; }
public void send(Msg msg) { listener.receive(msg); }
}
class MyPacket {
int i;
public MyPacket(int i) { this.i = i; }
}
public static void main(String[] args) throws Exception { new PlayData().run(); }
public void run() throws Exception {
Sender sender = new Sender(new Listener());
Msg msg = new Msg();
msg.addData(new Data<String>("testing") );
msg.addData(new Data<MyPacket>(new MyPacket(42)) );
sender.send(msg);
}
}

Check duplicate variable in a method scope Java

I read java file using FileReader that contains some method. How i can read method scope (method area inside) to find duplicate variable?
For example, this is a java file that i read:
public double[] copyArray(double[] data) {
int _nn = data.length;
double[] _tmp = new double[_nn];
System.arraycopy(data, 0, _tmp, 0, _nn);
int _nn;
_nn = tmp;
return _tmp;
}
How to know method scope?, i mean between { and }, if scope has found, find duplicate variable such as example above (int _nn) duplicated.
[addition]
i have tried using java parser, then is success. Then i should send the results using list, but only last method in the list. What's wrong with my code?
This is a MethodVisitor:
private static class MethodVisitor extends VoidVisitorAdapter {
private List<String> list = new ArrayList<String>();
#Override
public void visit(MethodDeclaration n, Object file) {
list.add(n.getName());
}
public List<String> getList() {
return list;
}
}
Then, this is a method to call MethodVisitor:
private MethodVisitor mv;
public void doIt(File file) throws Exception {
CompilationUnit cu;
try {
// parse the file
cu = JavaParser.parse(file);
} finally {
//file.close();
}
// visit and print the methods names
mv = new MethodVisitor();
mv.visit(cu, file);
List<String> list = mv.getList();
for(String item:list){
System.out.println(item);
}
}

1 you need to parse java code:
http://code.google.com/p/javaparser/
or read this:
Java source code parsers/generators
2 after that, try something and show your code

How can I merge three similar methods with different argument types into one generic one?

I am relatively inexperienced with java & generics, so please excuse me if this is a stupid question.
I have 3 very similar helper methods called verifyTextualSort, verifyNumericSort and verifyDateSort.
The 3 methods follow the same pattern with only a slight difference in them:
private boolean verifyTextualSort(...) {
ArrayList<String> list = new ArrayList<String>();
// Do common stuff with the list
// Do textual-specific stuff
// Do common stuff with the list
}
private boolean verifyNumericSort(...) {
ArrayList<Integer> list = new ArrayList<Integer>();
// Do common stuff with the list
// Do Numeric-specific stuff
// Do common stuff with the list
}
Is there some way I can combine them into one method, passing somehow the type (Integer, String, Date) as a parameter? I have to be able to know which is the type from inside the method so that I can do the correct specific stuff.

You need three method for the specific stuff. However for the common stuff you can create a common method they both call.
private boolean verifyNumericSort(...) {
List<Integer> list = new ArrayList<Integer>();
commonStuff1(list);
// Do Numeric-specific stuff
commonStuff2(list);
}

You could pass a Class as a parameter, if that is what you want (as you said, passing the type as a parameter):
public <T> void test(List<T> l, T t, Class<T> c) {
System.out.println(c.getName());
System.out.println(l.get(0).getClass().getName());
System.out.println(t.getClass().getName());
}
All the sysouts above will print out the name of the class, so you'll be able to choose which one suits you the best.

You can't do that by introspection using the Generics because of type erasure. But if the list is not empty, you can check the type of the first element and then invoke appropriate method.

since you have 3 fields you can do this..
class A
{
private Date date = null;
private Integer int = null;
private String text = null;
//add getters and setters for these fields
}
and now pass this class Object as an arguement to that method
public boolean verify(A a){
a.getDate();
a.getInt()
//etc and do your stuff
}

You need generics and refactoring:
private boolean verifyTextualSort(List<String> strings) {
commonStuffA(strings);
// Do textual-specific stuff
commonStuffB(strings);
return true; // ?
}
private boolean verifyNumericSort(List<Integer> ints) {
commonStuffB(ints);
// Do Numeric-specific stuff
commonStuffB(ints);
return true; // ?
}
private void commonStuffA(List<?> things) { // This method accept a list of anything
// Do common stuff A with the list
}
private void commonStuffB(List<?> things) { // This method accept a list of anything
// Do common stuff B with the list
}
private void someCallingMethod() {
List<String> strings = new ArrayList<String>();
verifyTextualSort(strings);
List<Integer> ints = new ArrayList<Integer>();
verifyTextualSort(ints);
}

I think you could possibly do something similar to this:
public <T extends Object> boolean verify(T t)
{
if(!(t==null))
{
if(t instanceof Date)
{
//Do date verify routine
return true;
}
else if(t instanceof String)
{
//Do String verify routine
return true;
}
else
{
//Do default verify routine which could be Integer
return true;
}
}
return false;
}
NOTE:
This is not tested.

As others have mentioned, you can't do that with generics because of type erasure (see the other answers for a link to type erasure). I believe you can get a reasonable solution (without instanceof) with polymorphism. Here is an example:
public class VerifySort
{
public static void main(String[] args)
{
VerifySort verifySort = new VerifySort();
Date testDate = new Date();
Integer testInteger = 17;
String testString = "Blammy";
verifySort.verify(testString);
verifySort.verify(testInteger);
verifySort.verify(testDate);
}
private boolean verify(Date parameter)
{
SimpleDateFormat dateFormat = new SimpleDateFormat();
System.out.print("Date parameter: ");
System.out.println(dateFormat.format(parameter));
return true;
}
private boolean verify(Integer parameter)
{
System.out.print("Integer parameter: ");
System.out.println(parameter);
return true;
}
private boolean verify(String parameter)
{
System.out.print("String parameter: ");
System.out.println(parameter);
return true;
}