Fragile test

FooBarTest.java

import org.junit.Rule;
import org.junit.Test;
import org.junit.rules.ExpectedException;

import java.util.Arrays;
import java.util.List;

public class FooBarTest {

    @Rule
    public ExpectedException exceptionRule = ExpectedException.none();
    private FooBar toTest = new FooBar();

    @Test
    public void getMethodThrowsException_whenListHasTooManyElements() throws Exception {
        List<String> listWith101Elements = Arrays.asList(new String[101]);

        exceptionRule.expect(IllegalArgumentException.class);
        toTest.getMethod(listWith101Elements);
    }

    public class FooBar {

        public int getMethod(List<String> code) throws IllegalArgumentException {

            code.add("Foo");  // this change fail whole test

            if (code.size() > 100)
                throw new IllegalArgumentException();

            return 0;
        }
    }
}