test: adding subject and main

subjects/java/checkpoints/first-unique/ExerciseRunner.java

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+public class ExerciseRunner {
+    public static void main(String[] args) {
+        FirstUnique firstUnique = new FirstUnique();
+
+        // Test case 1
+        String s1 = "leetcode";
+        System.out.println("First unique character: " + firstUnique.findFirstUnique(s1)); // Expected output: 'l'
+
+        // Test case 2
+        String s2 = "loveleetcode";
+        System.out.println("First unique character: " + firstUnique.findFirstUnique(s2)); // Expected output: 'v'
+
+        // Test case 3
+        String s3 = "aabbcc";
+        System.out.println("First unique character: " + firstUnique.findFirstUnique(s3)); // Expected output: '_'
+    }
+}

subjects/java/checkpoints/first-unique/README.md

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+## First Unique
+
+### Instructions
+
+Write a function that finds the first non-repeating character in a string. If there is no such character, return an underscore (\_).
+
+### Expected Class
+
+```java
+public class FirstUnique {
+    public char findFirstUnique(String s) {
+        // Implementation to find the first non-repeating character
+    }
+}
+```
+
+### Usage
+
+Here is a possible `ExerciseRunner.java` to test your class:
+
+```java
+public class ExerciseRunner {
+    public static void main(String[] args) {
+        FirstUnique firstUnique = new FirstUnique();
+
+        // Test case 1
+        String s1 = "leetcode";
+        System.out.println("First unique character: " + firstUnique.findFirstUnique(s1)); // Expected output: 'l'
+
+        // Test case 2
+        String s2 = "loveleetcode";
+        System.out.println("First unique character: " + firstUnique.findFirstUnique(s2)); // Expected output: 'v'
+
+        // Test case 3
+        String s3 = "aabbcc";
+        System.out.println("First unique character: " + firstUnique.findFirstUnique(s3)); // Expected output: '_'
+    }
+}
+```
+
+### Expected Output
+
+```shell
+$ javac *.java -d build
+$ java -cp build ExerciseRunner
+First unique character: l
+First unique character: v
+First unique character: _
+$
+```