TestListPop.java

import java.util.ArrayList;
import java.util.List;

public class TestListPop {

    private final List<String> states = new ArrayList<>();

    public void push(String state) {
        states.add(state);
    }

    public String pop() {
        int lastIndex = states.size() - 1;
        String lastState = states.get(lastIndex);
        states.remove(lastState);

        return lastState;
    }

    public String pop_nobug() {
        return states.remove(states.size() - 1);
    }

    @Override
    public String toString() {
        return "TestList{" + "states=" + String.join("-", states) + '}';
    }

    public static void main(String[] args) {
        test1();
        test2();
    }
    
    public static void test1() {
        System.out.println("--== Test1 ==--");
        
        TestListPop testList = new TestListPop();

        testList.push("a");
        testList.push("a");
        testList.push("b");
        testList.push("a");

        System.out.println("before pop: testList = " + testList);

        testList.pop();
        System.out.println("after pop:  testList = " + testList);
    }

    public static void test2() {
        System.out.println("--== Test2 ==--");
        
        TestListPop testList = new TestListPop();

        testList.push("a");
        testList.push("a");
        testList.push("b");
        testList.push("a");

        System.out.println("before pop: testList = " + testList);

        testList.pop_nobug();
        System.out.println("after pop:  testList = " + testList);
    }
    
}