This question already has answers here:
What is a NullPointerException, and how do I fix it?
(12 answers)
Closed 4 years ago.
I'm trying to use object oriented design and it's a bit confusing. Let's say I have 3 separate objects
public class Tool{
String name;
Float weight;
}
public class Drawer{
String name;
Tool[] tool;
}
public class Toolbox{
String name;
Drawer[] drawer;
public void setDrawers(String[] drawers) {
for(int i = 0; i < drawers.length; i++) {
drawer[i].name = drawers[i];
}
}
}
So a Tool has a String and Float. Drawer has a String and an array of Tools. Toolbox has a String and array of Drawers.
So let's say in my main code I say...
Toolbox myBox = new Toolbox();
myBox.setDrawers(drawers);
I'm getting null pointer issues. What's the proper way to instantiate everything if I want to instantiate a toolbox? Each class is in a separate Java file is this ok, or should they be in one file? Since all classes are in the same app do I need to use the import statement? What would my Constructors look like? Is there anything else I'm missing? Thanks for the help.
Please do not mark this as a duplicate of the "null pointer" question. That does not address objects that contain arrays of other objects and how to instantiate everything.
Here's a more standard way to do it.
public class Tool {
final String name;
final double weight;
public Tool(String name, double weight) {
this.name = name;
this.weight = weight;
}
}
public class Drawer {
final String name;
final List<Tool> tools;
public Drawer(String name) {
this.name = name;
this.tools = new ArrayList<>();
}
}
public class Toolbox {
final String name;
final List<Drawer> drawers;
public Toolbox(String name) {
this.name = name;
this.drawers = new ArrayList<>();
}
public void addDrawers(String... names) {
for (String name : names) {
drawers.add(new Drawer(name));
}
}
}
Now you can do
String[] drawers = {"Screwdrivers", "Files", "Spanners"};
Toolbox toolbox = new Toolbox("My toolbox");
toolbox.addDrawers(drawers);
Or just
Toolbox toolbox = new Toolbox("My toolbox");
toolbox.addDrawers("Screwdrivers", "Files", "Spanners");
I'm getting null pointer issues.
In myObject.callAMethod(aParameter); check myObject is not null.
Also refer to What is a NullPointerException, and how do I fix it?
What's the proper way to instantiate everything if I want to
instantiate a toolbox?
Each class can have it's own file. If you are starting with java I would go this way. There are ways to put everything in the same file, but you'll get to that in time.
The propper way to instantiate as in which order, what makes sense is from the bottom up. First Tool, then Drawer, then ToolBox. In this way the encolosed element already exists when you create the enclosing object.
do I need to use the import statement
Depends in which package you create the different classes. The IDE will highlight if you need to import the class.
I've read an entire book on programming Java on Android and it doesn't
cover the questions I asked.
This cannot be true.
I'm trying to analyse some bits of Java-code, looking if the code is written too complexly. I start with a String containing the contents of a Java-class.
From there I want to retrieve, given a function-name, the "inner code" by that function. In this example:
public class testClass{
public int testFunction(char x) throws Exception{
if(x=='a'){
return 1;
}else if(x=='{'){
return 2;
}else{
return 3;
}
}
public int testFunctionTwo(int y){
return y;
}
}
I want to get, when I call String code = getcode("testFunction");, that code contains if(x=='a'){ ... return 3; }. I've made the input code extra ugly, to demonstrate some of the problems one might encounter when doing character-by-character-analysis (because of the else if, the curly brackets will no longer match, because of the Exception thrown, the function declaration is not of the form functionName{ //contents }, etc.)
Is there a solid way to get the contents of testFunction, or should I implement all problems described manually?
You need to a java parser. I worked too with QDox. it is easy to use. example here:
import com.thoughtworks.qdox.JavaProjectBuilder;
import com.thoughtworks.qdox.model.JavaClass;
import com.thoughtworks.qdox.model.JavaMethod;
import java.io.File;
import java.io.IOException;
public class Parser {
public void parseFile() throws IOException {
File file = new File("/path/to/testClass.java");
JavaProjectBuilder builder = new JavaProjectBuilder();
builder.addSource(file);
for (JavaClass javaClass : builder.getClasses()) {
if (javaClass.getName().equals("testClass")) {
for (JavaMethod javaMethod : javaClass.getMethods()) {
if (javaMethod.getName().equals("testMethod")) {
System.out.println(javaMethod.getSourceCode());
}
}
}
}
}
}
Have you considered using a parser to read your code? There are a lot of parsers out there, the last time I worked on a problem like this http://qdox.codehaus.org made short work of these kinds of problems.
I create a form with Play framework. but I get a error: cannot find symbol
I reviewed the example codes in play directory, still can not figure it out. by the way, can I use Play to access PostgresSQL in heroku?
This is following code:
This is piece of code in /controllers/Application.java
final static Form<Geo> geoForm = form(Geo.class);
public static Result showDBpage(){
//get problem here :-<
Form<Geo> filledForm = geoForm.bindFormRequest();
Geo loc = filledForm.get();
return ok(database.render(loc));
}
This is conf/routes:
POST /database controllers.Application.showDBpage()
views/database.scala.html
#(loc: Geo)
#main("") {
<p>This is Database pages</p>
<p>#loc.longitute and #loc.latitute</p>
<a href=#routes.Application.index>Back to form</a>
}
models/Geo.java:
package models;
import java.util.*;
import javax.validation.*;
import play.data.validation.Constraints.*;
public class Geo
{
#Required
public String longitute;
#Required
public String latitute;
public Geo()
{
}
public Geo(String longitude,String latitute)
{
this.longitute = longitute;
this.latitute = latitute;
//this.length = length;
}
}
There is NO method bindFormRequest() but bindFromRequest() - you have a typo in your code.
Check the API http://www.playframework.org/documentation/api/2.0.2/java/play/data/Form.html
I have set of classes and (struts)jsp files which uses Message bundle. I want to find properties which are not used in project.One simple way would to search for each property (in given project) and if 0 result, delete it.
In eclipse there is a function like
Source > Find Broken Externalized Strings
I don't think it works. Not sure what its for. I get message
"no nls property files with corresponding accessor of class found in selection"
It seems like Eclipse expects to find the class that it generates in the "Externalize Strings..." command:
package com.foo.bar;
import java.util.MissingResourceException;
import java.util.ResourceBundle;
public class Messages {
private static final String BUNDLE_NAME = "com.foo.bar.messages"; //$NON-NLS-1$
private static final ResourceBundle RESOURCE_BUNDLE = ResourceBundle.getBundle(BUNDLE_NAME);
private Messages() {
}
public static String getString(String key) {
try {
return RESOURCE_BUNDLE.getString(key);
} catch (MissingResourceException e) {
return '!' + key + '!';
}
}
}
Actually, after digging a little deeper, I found that Eclipse really wants the following:
private static final String BUNDLE_NAME = "com.foo.bar.messages"; //$NON-NLS-1$
public static String getString(String key) {
...
}
The package "com.foo.bar" must contain the file "messages.properties". You can run "Find Broken Externalized Strings" on any source file (or on the package) that you add these lines to.
I've run through the Google Web Toolkit StockWatcher Tutorial using Eclipse and the Google Plugin, and I'm attempting to make some basic changes to it so I can better understand the RPC framework.
I've modified the "getStocks" method on the StockServiceImpl server-side class so that it returns an array of Stock objects instead of String objects. The application compiles perfectly, but the Google Web Toolkit is returning the following error:
"No source code is available for type com.google.gwt.sample.stockwatcher.server.Stock; did you forget to inherit a required module?"
It seems that the client-side classes can't find an implementation of the Stock object, even though the class has been imported. For reference, here is a screenshot of my package hierarchy:
I suspect that I'm missing something in web.xml, but I have no idea what it is. Can anyone point me in the right direction?
EDIT: Forgot to mention that the Stock class is persistable, so it needs to stay on the server-side.
After much trial and error, I managed to find a way to do this. It might not be the best way, but it works. Hopefully this post can save someone else a lot of time and effort.
These instructions assume that you have completed both the basic StockWatcher tutorial and the Google App Engine StockWatcher modifications.
Create a Client-Side Implementation of the Stock Class
There are a couple of things to keep in mind about GWT:
Server-side classes can import client-side classes, but not vice-versa (usually).
The client-side can't import any Google App Engine libraries (i.e. com.google.appengine.api.users.User)
Due to both items above, the client can never implement the Stock class that we created in com.google.gwt.sample.stockwatcher.server. Instead, we'll create a new client-side Stock class called StockClient.
StockClient.java:
package com.google.gwt.sample.stockwatcher.client;
import java.io.Serializable;
import java.util.Date;
public class StockClient implements Serializable {
private Long id;
private String symbol;
private Date createDate;
public StockClient() {
this.createDate = new Date();
}
public StockClient(String symbol) {
this.symbol = symbol;
this.createDate = new Date();
}
public StockClient(Long id, String symbol, Date createDate) {
this();
this.id = id;
this.symbol = symbol;
this.createDate = createDate;
}
public Long getId() {
return this.id;
}
public String getSymbol() {
return this.symbol;
}
public Date getCreateDate() {
return this.createDate;
}
public void setId(Long id) {
this.id = id;
}
public void setSymbol(String symbol) {
this.symbol = symbol;
}
}
Modify Client Classes to Use StockClient[] instead of String[]
Now we make some simple modifications to the client classes so that they know that the RPC call returns StockClient[] instead of String[].
StockService.java:
package com.google.gwt.sample.stockwatcher.client;
import com.google.gwt.sample.stockwatcher.client.NotLoggedInException;
import com.google.gwt.user.client.rpc.RemoteService;
import com.google.gwt.user.client.rpc.RemoteServiceRelativePath;
#RemoteServiceRelativePath("stock")
public interface StockService extends RemoteService {
public Long addStock(String symbol) throws NotLoggedInException;
public void removeStock(String symbol) throws NotLoggedInException;
public StockClient[] getStocks() throws NotLoggedInException;
}
StockServiceAsync.java:
package com.google.gwt.sample.stockwatcher.client;
import com.google.gwt.sample.stockwatcher.client.StockClient;
import com.google.gwt.user.client.rpc.AsyncCallback;
public interface StockServiceAsync {
public void addStock(String symbol, AsyncCallback<Long> async);
public void removeStock(String symbol, AsyncCallback<Void> async);
public void getStocks(AsyncCallback<StockClient[]> async);
}
StockWatcher.java:
Add one import:
import com.google.gwt.sample.stockwatcher.client.StockClient;
All other code stays the same, except addStock, loadStocks, and displayStocks:
private void loadStocks() {
stockService = GWT.create(StockService.class);
stockService.getStocks(new AsyncCallback<String[]>() {
public void onFailure(Throwable error) {
handleError(error);
}
public void onSuccess(String[] symbols) {
displayStocks(symbols);
}
});
}
private void displayStocks(String[] symbols) {
for (String symbol : symbols) {
displayStock(symbol);
}
}
private void addStock() {
final String symbol = newSymbolTextBox.getText().toUpperCase().trim();
newSymbolTextBox.setFocus(true);
// Stock code must be between 1 and 10 chars that are numbers, letters,
// or dots.
if (!symbol.matches("^[0-9a-zA-Z\\.]{1,10}$")) {
Window.alert("'" + symbol + "' is not a valid symbol.");
newSymbolTextBox.selectAll();
return;
}
newSymbolTextBox.setText("");
// Don't add the stock if it's already in the table.
if (stocks.contains(symbol))
return;
addStock(new StockClient(symbol));
}
private void addStock(final StockClient stock) {
stockService.addStock(stock.getSymbol(), new AsyncCallback<Long>() {
public void onFailure(Throwable error) {
handleError(error);
}
public void onSuccess(Long id) {
stock.setId(id);
displayStock(stock.getSymbol());
}
});
}
Modify the StockServiceImpl Class to Return StockClient[]
Finally, we modify the getStocks method of the StockServiceImpl class so that it translates the server-side Stock classes into client-side StockClient classes before returning the array.
StockServiceImpl.java
import com.google.gwt.sample.stockwatcher.client.StockClient;
We need to change the addStock method slightly so that the generated ID is returned:
public Long addStock(String symbol) throws NotLoggedInException {
Stock stock = new Stock(getUser(), symbol);
checkLoggedIn();
PersistenceManager pm = getPersistenceManager();
try {
pm.makePersistent(stock);
} finally {
pm.close();
}
return stock.getId();
}
All other methods stay the same, except getStocks:
public StockClient[] getStocks() throws NotLoggedInException {
checkLoggedIn();
PersistenceManager pm = getPersistenceManager();
List<StockClient> stockclients = new ArrayList<StockClient>();
try {
Query q = pm.newQuery(Stock.class, "user == u");
q.declareParameters("com.google.appengine.api.users.User u");
q.setOrdering("createDate");
List<Stock> stocks = (List<Stock>) q.execute(getUser());
for (Stock stock : stocks)
{
stockclients.add(new StockClient(stock.getId(), stock.getSymbol(), stock.getCreateDate()));
}
} finally {
pm.close();
}
return (StockClient[]) stockclients.toArray(new StockClient[0]);
}
Summary
The code above works perfectly for me when deployed to Google App Engine, but triggers an error in Google Web Toolkit Hosted Mode:
SEVERE: [1244408678890000] javax.servlet.ServletContext log: Exception while dispatching incoming RPC call
com.google.gwt.user.server.rpc.UnexpectedException: Service method 'public abstract com.google.gwt.sample.stockwatcher.client.StockClient[] com.google.gwt.sample.stockwatcher.client.StockService.getStocks() throws com.google.gwt.sample.stockwatcher.client.NotLoggedInException' threw an unexpected exception: java.lang.NullPointerException: Name is null
Let me know if you encounter the same problem or not. The fact that it works in Google App Engine seems to indicate a bug in Hosted Mode.
GWT needs the .java file in addition to the .class file. Additionally, Stock needs to be in the "client" location of a GWT module.
The GWT compiler doesn't know about Stock, because it's not in a location it looks in. You can either move it to the client folder, or if it makes more sense leave it where it is and create a ModuleName.gwt.xml that references any other classes you want, and get your Main.gwt.xml file to inherit from that.
eg: DomainGwt.gwt.xml
<module>
<inherits name='com.google.gwt.user.User'/>
<source path="javapackagesabovethispackagegohere"/>
</module>
and:
<module rename-to="gwt_ui">
<inherits name="com.google.gwt.user.User"/>
<inherits name="au.com.groundhog.groundpics.DomainGwt"/>
<entry-point class="au.com.groundhog.groundpics.gwt.client.GPicsUIEntryPoint"/>
</module>
There's a better answer here: GWT Simple RPC use case problem : Code included
Basically, you can add parameters to your APPNAME.gwt.xml file so the compiler to give the compiler a path to the server-side class.
I was getting the same issue and the "mvn gwt:compile" output was not very helpful.
Instead, when I tried deploying to tomcat (via the maven tomcat plugin: mvn tomcat:deploy) I got helpful error messages.
A few things I had to fix up:
Make the object that is sent from the client to the server implement Serializable
Add an empty-arg constructor to that same object
Yes, it is sure that we need to use the Serialization for getting the server objects to the client. These modile?? file settings won't work to use the Stock class in the client side.
In your case you have only one class Stock and you can create a StockClient in client side. It is easy. But what will be the solution if anyone having more classes. Something like the properties of this class are also some other type of classes.
Example: stock.getEOD(date).getHigh();
getEOD will return another class with the given date and that class has the getHigh method.
What to do in such big cases? I don't think creating all classes implementing serialization in client side is good for that. Then we have to write code in both server and client. all classes two times.
Keying off of rustyshelf's answer above ...
In my case I needed to edit the ModuleName.gwt.xml file and add the following:
<source path='client'/>
<source path='shared'/>
I created my project with the New->Web Application Project wizard but unchecked the Generate project sample code option. I then created the shared package. Had I not unchecked that, the package would have been created for me and the xml file modified per the above.
There is a far more simple and easy solution for that. If you want to send an object of your custom designed class from server side to client side you should define this custom class in shared package.
For example for your case the you just have to carry the Stock.java class (by drag and drop) into
com.google.gwt.sample.stockwatcher.shared
package. However from your package hierarchy screenshot it seems that you had deleted this shared package. Just re-create this package and drop the Stock.java inside it and let the game begin.