Different Index for getStackTrace()[2] vs[1], same results. One is accepted, one is not. Why !?

Question about the task

Under discussion

This one is accepted:

public class Solution {
    public static void main(String[] args) {
        log("In main method");
    }

    public static void log(String s) {
        //write your code here
        String fullClassName = Thread.currentThread().getStackTrace()[2].getClassName();
        String methodName = Thread.currentThread().getStackTrace()[2].getMethodName();
        System.out.println(fullClassName + ": " + methodName + ": " + s);
    }
}

This one is not accepted:

public class Solution {
    public static void main(String[] args) {
        log("In main method");
    }

    public static void log(String s) {
        //write your code here
        String fullClassName = Thread.currentThread().getStackTrace()[1].getClassName();
        String methodName = Thread.currentThread().getStackTrace()[2].getMethodName();
        System.out.println(fullClassName + ": " + methodName + ": " + s);
    }
}

Same requested output: "com.codegym.task.task09.task0906.Solution: main: In main method" Why is the only accepted answer the first version?

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Guadalupe Gagnon

Level 37 , Tampa, United States

10 November 2020, 16:47

useful

Its because the stack trace element at index 1 is going to be the stack trace for the current method (the log method), while the element at index 2 would be for the method that directly calls the current method. Consider this bit of code:

public class Solution {
    public static void main(String[] args) {
        log("In main method");
        Test.logTest();
    }

    public static void log(String s) {
        String fullClassName = Thread.currentThread().getStackTrace()[1].getClassName();
        String methodName = Thread.currentThread().getStackTrace()[2].getMethodName();
        System.out.println(fullClassName + ": " + methodName + ": " + s);
    }
}

class Test{
    static void logTest(){
        Solution.log("In the test class");
    }
}

Here i have created a second class that calls the log method from the solution class. If you run it then the output would show that both methods were called from the solution class, however since the Test class does call the log method it should output that this one is called from the Test class. If you change the element index to 2 instead of 1 then you will see the class changes because it gets the correct method from the Test class.

Guadalupe Gagnon

Level 37 , Tampa, United States

10 November 2020, 16:58

useful

The getStackTrace() element returns an array which you can print out with:

System.out.println(Arrays.toString(Thread.currentThread().getStackTrace()));

You will see that it always starts with the getStackTrace() method, element 1 would be the method that is called in, then the following elements would be the chain of methods in order going back to where you get the stackTrace, with the last element being a main method where the code starts. For example, take this bit of code:

public class Solution {
    public static void main(String[] args) {
        method1();
    }

    static void method1() {
        method2();
    }

    static void method2() {
        method3();
    }

    static void method3() {
        method4();
    }

    static void method4() {
        System.out.println(Arrays.toString(Thread.currentThread().getStackTrace()));
    }
}

If you ran it you would see the output starts with the Thread.getStackTrace() method, then method4 because that called the getStackTrace(), then method3 because that called method4, then method2, then method1, then finally the main method where the program started. You can add/remove methods to see the results change.

Andrei

Level 41

11 November 2020, 09:04

Yes, that makes more sense now! I also have a rested brain, haha! :) Thank you!