Develop Reference

Start coding from the debugging point Eclipse

I have a code like this
String s="Test-Code-Data";
String[] splitedData = s.split("-");
I have a break point at second line. When my code reaches that point, can i start coding below that and immediately see the output. For example. I want to see the output from below code, and i type it when the execution reaches the second line only.
System.out.println(splitedData[1])
Is this possible in eclipse?

To answer your question, NO you cannot effectively alter code while the code is in execution without making Eclipse warn you that the code is 'out of sync'.
However, you can look at the actual values of your various variables in the various stages of execution of your code. And, once the execution is complete, then you can edit the code and Run the program in Debug again and keep going that way.
You can use the Inspect Variable feature to look at the value of a variable while in execution and paused at a breakpoint by selecting the variable you'd like to inspect and using the keyboard shortcut Ctrl+Shift+I to get the value of the variable.

How can I access the Eclipse Java debugger from a plug-in?

I'm developing a plug-in for the Eclipse platform. This plug-in will be used to give information about the line of Java source code currently being debugged.
When debugging a Java program, as you hit a breakpoint, Eclipse switches to the standard Debug perspective. Inside this perspective, apart from the standard Console output, the stack trace and various other views, you can see source code of the Java program currently being debugged. Inside this 'source code view', you can see a highlighted line, which is the line of code currently being debugged/evaluated. This highlighted line of code is what I want to access.
Assuming I know when the debugger is running (I assess that through a DebugBreakpointListener class that implements IJavaBreakpointListener), I need to 'ask questions' to the debugger. What, I imagine, I will need, is to somehow ask the debugger directly either for the line of code it is currently highlighting/debugging/evaluating or for the line number of the said line of code.
I'm making a static access to the JDIDebugModel to add the Java Breakpoint Listener:
JDIDebugModel.addJavaBreakpointListener(new DebugBreakpointListener);
I thought I could access the debugger with static references to JDIDebugPlugin but I've yet to find what I'm looking for.
At Part 3 of this research paper, the authors suggested that:
The Eclipse Java debugger is built upon the API of Java Debug Interface (JDI), which is part of the Java Development Toolkit. This API enables adding requests to monitor JVM events such as BreakpointEvent. When an event occurs, the debugger gets a notification and the thread in which this event took place can be obtained. For each frame in the stack trace of this thread the following information can be obtained:
• The source Java file in which the execution at this frame has taken place (or null if the source is not available).
• The method and line number (if available).
• The this object or null if the method is static.
The Eclipse debugger uses this information when a breakpoint is hit. It shows the stack trace for the suspended thread in the ”Debug” view. For the selected frame in this trace, Eclipse highlights the corresponding line number in its source file, and displays the this variable in the ”Variables” view.
This bulletpoint-listed things are exactly what I'm looking for.
Unfortunately, I can't find detailed documentation on how to 'plug in' to the debugger.
If someone can give me information, point me to information or a sample code, or maybe provide me with contact information of someone from the Eclipse JDI project, it would be immensely appreciated.
Thanks in advance.
------Update & Answer:------
With the help of greg-449's answer, I did exactly what I wanted to do. Here's what I did:
The aformentioned breakpoint listener I wrote implements the interface method breakpointHit, which is as follows:
#Override
public int breakpointHit(IJavaThread thread, IJavaBreakpoint breakpoint) {
System.out.println("Just hit a breakpoint!");
// Save pointers to the thread & breakpoint for future use.
return 0;
}
With the pointers to the thread and breakpoint objects saved in one of my objects, I could query them to get up-to-date information on the state of the frame stack, the thread and about the particular breakpoint that I've hit. I can get the namea dn path of the class the debugger is currently debugging by calling:
IStackFrame topStackFrame = thread.getTopStackFrame();
int debuggedLineNumber = topStackFrame.getLineNumber();
String debuggedClassPath = topStackFrame.getLaunch().getSourceLocator().getSourceElement(thread.getTopStackFrame()).toString();
This was exactly what I was looking for. I imagine I will need to read the source code files manually, run them through a tokenizer by having the 'newline' character as a delimiter and get the corresponding token to read that specific line.

There is a huge amount of information available in the IJavaThread and IJavaBreakpoint arguments passed to the breakpointHit method of the IJavaBreakpointListener which should contain this information.
I think for breakpoints which have a line number (not all do) the IJavaBreakpoint argument also implements ILineBreakpoint containing the line information.

How can I set the default Scope in IntelliJ 12 for Find Usages?

I've returned to IntelliJ after a long hiatus for Android development so I'm getting used to it again. The problem I have is that for example when you want to see where is a class being used, you'd position the caret in the class declaration and issue cmdaltF7 (on Mac OS X) to Find Usages, which is returning stuff from mapping.txt and seeds.txt as well as the .java results, and even tho I can set up the defaults by doing shiftcmdaltF7 and un-tick the: search for text occurrences and even change the scope from Project Files to a custom scope (for example), these options are not saved when I invoke Find Usages again.
Does anybody know of a way to personalize the Find Usages so it's more close to what Eclipse would do? (I.e., find the real usages instead of a text search for occurrences).

Reporting back from the future: the behaviour described in the question has now been implemented (Intellij issue mentioned in the comments).
To configure cmdaltF7 to run in a default scope, start by running it against some Symbol
Clicking on the wrench icon, one can select one of the pre-defined scopes, or create a new one (using the ... button).
The + creates a new scope. Find the folder in which to look, and click Include recursively. And voila!
Any consequent searches will use that scope until it is changed.

Instead of cmdaltF7, use the shortcut altF7. This will open a pop-up for you to make a selection about Scope, Test occurrences, and types of usage. You will have to make this selection one time. The next time you press altF7 then your choices are remembered.
The result is that altF7 followed by enter gives you what you need.

Inspect function of eclipse tricks me like a dummy, why does it seem to be that tricky while debugging?

Eclipse is awesome for writing java programs but today I find that it's awesome to trick new coders like me. #_#
I write a snippet as following,
public static void main(String[] args) {
for(int i=0;i<3;i++){
System.out.println("i = " + i);
}
}
then I add a breakpoint at the line of "System.out.print....", click "Debug" button, eclipse goes to the "Debug" perspective and the line of breakpoint is highlighted, then I move the cursor over variable "i", its value is "0" as expected.
And then, I select the "i++" and click "Inspect"(or press "Ctrl+Shift+I") once, I move the cursor over variable "i", its value changed to "1". I repeat "Inspect" again, the value of i changed to "2"......(its value will add by 1 every time I clicked the "Inspect")!!
Why does this happend!? I ONLY want to watch the value of "i" for debug propuse, DO NOT want to really change its value until I step into next statement. I think that "Inspect", as well as "Display" are only for viewing the variable/expression, they should not impact the value, but in this case, it doesn't work as I expect to.
Could anyone tell me what went wrong?
My eclipse version info:
Version: Indigo Service Release 2
Build id: 20120216-1857

If you inspect an expression, eclipse has to execute that expression so you can get the value. Therefore, if you inspect i++, eclipse adds one to i.
Think about it this way: If instead of i++, you inspected myFunction(i), would you expect eclipse to execute the function "myFunction" to get the value? It's the same with i++.

If you are concerned about displaying/showing values while debugging and do you want to be sure not affecting the value, you should select the variable or expression and use the "Watch" option.
This will track the variable/expression value without executing, just updating the new value each time this is run. I think is the most secure way.

As Pablo mentioned, it has to evaluate the code in order to tell you what value it returns. You could instead put a watch on "i+1" and that would give you the value you want without the side effect you don't want.
Basically, you need to be aware of any side effects of anything you launch, whether from the "main" code or from the debugging tools. As Erhannis mentioned, this is very useful at times for modifying values while debugging your code. (For example, you can verify that a tweak/fix is indeed helpful before actually tweaking your code.)
You were expecting "an isolate area" but this would be extremely hard to do, especially in an object-oriented context where many objects are linked to many other objects. Running the whole thing in parallel might sometimes work, but you'd lose the tweaking ability above. And in any context, you'd run into tons of problems with fighting over resources; e.g. both copies trying to read/write a particular file such as a log file. Also, the two execution paths could diverge, leading to incorrect/misleading watch values.
So, preventing such side effects is not really a feasible option here and would rarely be useful anyway. Just expect that the watches can both reflect and affect the code execution.

How to print Java object's current value in debug console in Eclipse?

I'm newbie on Eclipse.
In Objective-C, I could print value of an object in console window with this command.
po nameOfValue
Maybe gdb command. I'm using Eclipse now, what's the equivalent of this in Eclipse?
P.S. I'm debugging a Java app.

Eclipse has very robust debugging capabilities - much more so than Objective C.
First off, while debugging you can view the values of all variables in the Variables window. Additionally, in the lower part of the Variables window you can type arbitrary Java, select it, right click, then choose to Inspect or Execute. You can actually change the value of variables in your program this way, while its running.
You can do pretty much the same thing in your source pane. Highlight a variable, right click and choose to Inspect it. You can also type in a random expression and execute it. You can also places watches on variables (which I believe you can do in Objective-C), or on expressions.
There is an Expression view which is not displayed by default (on your menu select Window->Views->Expressions, while in Debug perspective). It allows you to add arbitrary (valid) Java expressions and the values of those expressions will then be watched over the lifetime of your debug session, very nifty. Thanks to #Baldrick for the reminder of this great tool.

System.out.println(nameOfValue);

I'm not aware of any printing option of the whole object state in console while debugging.
But you can override the toString() method of your object and there concatenate the string with the values of each field or whatever you want to print for that object. Then when calling somewhere in the code System.out.print(myObject); it will print the result of the toString() method which you've overridden.

Click Window menu select show view type display and show that view. In this view you can type java such as System.out.println(objectName); when the application has paused during debugging.

Override toString for that object/class. Then System.out.println(objectVariable).