src/main/java/Job/Job.java

@@ -20,6 +20,7 @@ public abstract class Job implements HasExperience {
 
     public void levelUp() {
         level++;
+        experience = 0;
     }
 
     @Override

src/main/java/Job/Miner.java

@@ -23,5 +23,8 @@ public class Miner extends Job {
     @Override
     public void gainExperience(int exp) {
         experience += exp;
+        if (experience >= 100) {
+            levelUp();
+        }
     }
 }

src/test/java/MinerTest.java

@@ -20,6 +20,14 @@ public class MinerTest {
         assertEquals(25, job.getExperience());
     }
 
+    @Test
+    void miner_level_up_when_experience_is_100() {
+        var job = new Miner();
+        job.gainExperience(100);
+        assertEquals(0, job.getExperience());
+        assertEquals(2, job.getLevel());
+    }
+
 }