Can someone explain what's happening?

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Thanks sir, lately i read and research about StackTrace i also watched videos of it. but somehow i got confused and pressured so i end up saying this cause i desperately want to learn :(

The first place to visit is the Oracle api documentation if you want to know something about a specific class of that api. The easiest way to do that is to setup IntelliJ to open the docs when pressing F1 on your keyboard while the curser is on the class (or a method of it) in question (Edit - Project Settings - SDKs - now select the SDK you're using - Documentation Paths - and click the plus with the little globe [specify URL is the mouse over help text] - and finally OK [IntelliJ should fill in an apropriate path automatically]). That code you already know from the previous lesson. Add it to a test document and run it

StackTraceElement[] stackTraceElements = Thread.currentThread().getStackTrace();
for (StackTraceElement element : stackTraceElements)
{
    System.out.println(element.getMethodName());
}

output is: getStackTrace main now click on StackTraceElement and press Shift-F1 and read the help that pops up An element in a stack trace, as returned by Throwable.getStackTrace(). Each element represents a single stack frame. All stack frames except for the one at the top of the stack represent a method invocation. The frame at the top of the stack represents the execution point at which the stack trace was generated. Typically, this is the point at which the throwable corresponding to the stack trace was created. So the stack trace is a representation of a mthod hierarchy. With the help of the stack the JVM has knowledge where in the method invocation history it resides and if it finds a return statement where to go back to. The entry point of your program was the main method as the bottom method and the last method invocation was getStackTrace. That's the output of the the stack trace.

Now change your program a little bit to

public static void main(String[] args) throws IOException {
    method1();
}

public static void method1() {
    StackTraceElement[] elements = Thread.currentThread().getStackTrace();
    for (int i = 0; i < elements.length; i++) {
        System.out.printf("Array index: %s - value: %s%n", i, elements[i].getMethodName());
    }
}

I changed it to a standard for loop to make it easier to understand. In the loop you access each index of the StackTraceElement array and invoke the getMethodName method on it. That'll give you the name saved in that element. The last element is, as before, the getStackTrace method. Index 1 is the method inside that you called getStackTrace and the following indexes called the caller method. That's the stack trace.

sad to say im just using an android phone since i cant afford to buy atleast a low end laptop, im just a student but thank you i somehow understand your explanation :)

Then just do a google search for the api or open it here: Oracle Api docs Docs for StackTraceElement

@john This course, while not impossible, will be VERY VERY difficult to complete on your phone. The problem with the online compiler and the mobile app, is that it hardly alerts you to errors. IntelliJ not only alerts you but tells you how to fix them. One app that I have used, which you can find on google play, is called "grasshopper": their website. This teaches JavaScript from your mobile with lessons and guided exercises. The have 4 levels with something like 25 tasks each. While not as complete as Codegym, it is quite a bit more forgiving with the tasks. You can really gain some experience. Once you complete that you will have less problems here, though without a full pc its just going to be hard in general.