I was writing some basic test cases in Java using IntelliJ and noticed something strange:

• The tests don’t run in the order I wrote them
• The order is NOT alphabetical
• But it’s also NOT random — it stays the same every time

Example: I defined tests in this order:

simpleTest
validMobileNo
missingPlus
tooShort
tooLong
nullNumber
emptyNumber

But execution order looks like:

tooShort
nullNumber
emptyNumber
tooLong
simpleTest
validMobileNo
missingPlus

So I went down a rabbit hole to understand why 😅

What I found:

1. JUnit does NOT reorder tests by default (unless you use u/TestMethodOrder)
2. It relies on reflection (getDeclaredMethods())
3. Reflection returns methods in the order they exist in the compiled .class file
4. The .class file order is determined by the Java compiler — not necessarily your source code order

So effectively:

Execution order = JVM reflection order = class file method order

That’s why:

• It looks random ❌
• But is consistent every run ✅

Key takeaway:

If you care about order, don’t rely on this behavior. Use:

[u/TestMethodOrder](/u/TestMethodOrder)(MethodOrderer.OrderAnnotation.class)

https://preview.redd.it/5x8a3ulihrsg1.png?width=575&format=png&auto=webp&s=64a815b6495995cb5f524617fe75c8d21cc3747d

class MainTest {


    [u/Test](/u/Test)
    void simpleTest() {
        int result = 2 + 3;
        
assertEquals
(5, result);
    }

    [u/Test](/u/Test)
    void validMobileNo_ShouldReturnTrue(){
        boolean result = Main.
isValidMobileNo
("+919313625910");
        
assertTrue
(result);
    }

    [u/Test](/u/Test)
    void missingPlus_ShouldReturnFalse(){
        boolean result = Main.
isValidMobileNo
("919313625910");
        
assertFalse
(result);
    }

    [u/Test](/u/Test)
    void tooShort_ShouldReturnFalse(){
        boolean result = Main.
isValidMobileNo
("+7408512");
        
assertFalse
(result);
    }

    [u/Test](/u/Test)
    void tooLong_ShouldReturnFalse(){
        boolean result = Main.
isValidMobileNo
("+96388574788968570");
        
assertFalse
(result);
    }

    [u/Test](/u/Test)
    void nullNumber_ShouldReturnFalse(){
        boolean result = Main.
isValidMobileNo
(null);
        
assertFalse
(result);
    }

    


this is my java class 
    void emptyNumber_ShouldReturnFalse(){
        boolean result = Main.
isValidMobileNo
("");
        
assertFalse
(result);
    }


}

package org.example;

import org.junit.jupiter.api.Test;

import org.junit.jupiter.engine.*;

import static org.junit.jupiter.api.Assertions.*;

//import org.junit.jupiter.api.TestMethodOrder;
//import org.junit.jupiter.api.MethodOrderer;


//@TestMethodOrder(MethodOrderer.MethodName.class)
class MainTest {


    [u/Test](/u/Test)
    void simpleTest() {
        int result = 2 + 3;
        assertEquals(5, result);
    }

    [u/Test](/u/Test)
    void validMobileNo_ShouldReturnTrue(){
        boolean result = Main.isValidMobileNo("+911234567890");
        assertTrue(result);
    }

    [u/Test](/u/Test)
    void missingPlus_ShouldReturnFalse(){
        boolean result = Main.isValidMobileNo("911234567890");
        assertFalse(result);
    }

    [u/Test](/u/Test)
    void tooShort_ShouldReturnFalse(){
        boolean result = Main.isValidMobileNo("+7408512");
        assertFalse(result);
    }

    [u/Test](/u/Test)
    void tooLong_ShouldReturnFalse(){
        boolean result = Main.isValidMobileNo("+96388574788968570");
        assertFalse(result);
    }

    [u/Test](/u/Test)
    void nullNumber_ShouldReturnFalse(){
        boolean result = Main.isValidMobileNo(null);
        assertFalse(result);
    }

    [u/Test](/u/Test)
    void emptyNumber_ShouldReturnFalse(){
        boolean result = Main.isValidMobileNo("");
        assertFalse(result);
    }


}

this is java class 
can anyone explain why at execution i found this order and every time it follow same order so how it happens