Stack Trace. what am I missing?

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what am i missing here. array length -1 is the top element in the stack. the method that called the top stack element should be arraylength -2 , so why is this not validating ? in the code gym solution they used [2] for everything. That doesnt make much sense to me. if there are like 5 method that calls each others, how does the [2] used for all the methods gets the method's caller Name??? public class Solution { public static void main(String[] args) { method1(); } public static String method1() { method2(); return Thread.currentThread().getStackTrace()[2].getMethodName(); } public static String method2() { method3(); return Thread.currentThread().getStackTrace()[2].getMethodName(); }

package en.codegym.task.jdk13.task09.task0902; /* Stack trace revisited */ public class Solution { public static void main(String[] args) { method1(); } public static String method1() { method2(); //write your code here StackTraceElement[] elements = Thread.currentThread().getStackTrace(); return elements[elements.length -2].getMethodName() ; } public static String method2() { method3(); //write your code here StackTraceElement[] elements = Thread.currentThread().getStackTrace(); return elements[elements.length -2].getMethodName(); } public static String method3() { method4(); //write your code here StackTraceElement[] elements= Thread.currentThread().getStackTrace(); return elements[elements.length-2].getMethodName(); } public static String method4() { method5(); //write your code here StackTraceElement[] elements = Thread.currentThread().getStackTrace() ; return elements[elements.length -2].getMethodName(); } public static String method5() { //write your code here StackTraceElement[] elements = Thread.currentThread().getStackTrace(); return elements[elements.length-2].getMethodName(); } }

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Jesufemi Adejare

Level 20 , Nigeria

5 June, 15:01

After spending time with the concept and testing the result by printing on console , i finally understood. The mistake i made initially was not outputting the result cause i was trying to solve the task on phone. Thank you both ❤

Jesufemi Adejare

Level 20 , Nigeria

4 June, 13:02

So youre saying that the top element on the stack actually starts from index 0 and not the array.length - 1. i thought it would be the last index(array.length-1), as in last in, first out i guess i should have outputted the result like you mentioned.

Guadalupe Gagnon

Level 37 , Tampa, United States

4 June, 13:35

Thomas' explanation is really good. I like to include an example to go along with it:

package en.codegym.task.jdk13.task09.task0902;
import java.util.Arrays;
public class Solution {
    public static void main(String[] args) {
        m1();
    }

    public static String m1() {
        StackTraceElement[] e = Thread.currentThread().getStackTrace();

        System.out.println("\tIn m1, stacktrace: " + Arrays.toString(e));
        m2();

        return e[2].getMethodName();
    }

    public static String m2() {
        StackTraceElement[] e = Thread.currentThread().getStackTrace();

        System.out.println("\tIn m2, stacktrace: " + Arrays.toString(e));
        m3();

        return e[2].getMethodName();
    }

    public static String m3() {
        StackTraceElement[] e = Thread.currentThread().getStackTrace();

        System.out.println("\tIn m3, stacktrace: " + Arrays.toString(e));
        m4();

        return e[2].getMethodName();
    }

    public static String m4() {
        StackTraceElement[] e = Thread.currentThread().getStackTrace();

        System.out.println("\tIn m4, stacktrace: " + Arrays.toString(e));
        m5();

        return e[2].getMethodName();
    }

    public static String m5() {
        StackTraceElement[] e = Thread.currentThread().getStackTrace();

        System.out.println("\tIn m5, stacktrace: " + Arrays.toString(e));

        return e[2].getMethodName();
    }
}

If you look at the output at each method the stack trace is exactly how Thomas describes it, with the main method being the very last and each method "stacking" from it in the order they are called.

Guadalupe Gagnon

Level 37 , Tampa, United States

4 June, 13:52

Another neat thing the stack trace gives you is the line numbers of the individual lines of code responsible for the elements in the stack trace. For example, here is one line of output from the above code I shared: In m3, stacktrace: [java.base/java.lang.Thread.getStackTrace(Thread.java:1610), Test.Solution.m3(Solution.java:27), Test.Solution.m2(Solution.java:21), Test.Solution.m1(Solution.java:12), Test.Solution.main(Solution.java:5)] So the getStackTrace() method starts in the Thread class at line 1610. That was called from line 27 of the m3 class, which was called from line 21 of the m2 class, which was called from line 12 of the m1 class which was called from line 5 of the main class. When you have an error/exception that occurs you can print the stack trace and figure out where it is, that is if you had caught it. If you didn't catch it then the program outputs the stack trace in red and the program crashes. You can use that stack trace information to find where the problem occurred using the line numbers. I created one more example and took a screen shot. If I added an error in at line 37 and ran the program it would crash but I could use the stack trace that is output to find the problem:

Thomas

Level 41 , Bayreuth, Germany

4 June, 11:46

You are no friend of testing :) ? Or did you try to output your return values? Or maybe even better, the stacktrace incl. index? Then it is self explanatory why it has to be 2. A stack works like a, hmm, stack. At the bottom is the first entry, the main method. If you call other methods, you stack them on top of main. So above main comes method1, above method1 comes method2 and so on. The last method on the stack is the getStackTrace method itself. If now you convert the stack to an array you end up with smth like that

0 getStackTrace
1 method5       the method invoking getStackTrace()
2 method4       the method invoking the last called method
3 method3
4 method2
5 method1       elements.length -2
6 main          elements.length -1