docs: adding subject and main

subjects/java/checkpoints/distinct-substring-lenght/ExerciseRunner.java

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+public class ExerciseRunner {
+    public static void main(String[] args) {
+        DistinctSubstringLength finder = new DistinctSubstringLength();
+
+        // Test cases
+        System.out.println(finder.maxLength("abcabcbb")); // Expected output: 3
+        System.out.println(finder.maxLength("bbbbb")); // Expected output: 1
+        System.out.println(finder.maxLength("pwwkew")); // Expected output: 3
+        System.out.println(finder.maxLength("")); // Expected output: 0
+    }
+}

subjects/java/checkpoints/distinct-substring-lenght/README.md

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+## Distinct Substring length
+
+### Instructions
+
+Create a class `DistinctSubstringLength` that provides a method to find the length of the longest substring without repeating characters in a given string.
+
+### Expected Class
+
+```java
+public class DistinctSubstringLength {
+    public int maxLength(String s) {
+        // Implementation to find the length of the longest substring without repeating characters
+    }
+}
+```
+
+### Usage
+
+Here is a possible `ExerciseRunner.java` to test your class:
+
+```java
+public class ExerciseRunner {
+    public static void main(String[] args) {
+        DistinctSubstringLength finder = new DistinctSubstringLength();
+
+        // Test cases
+        System.out.println(finder.maxLength("abcabcbb")); // Expected output: 3
+        System.out.println(finder.maxLength("bbbbb")); // Expected output: 1
+        System.out.println(finder.maxLength("pwwkew")); // Expected output: 3
+        System.out.println(finder.maxLength("")); // Expected output: 0
+    }
+}
+```
+
+### Expected Output
+
+```shell
+$ javac *.java -d build
+$ java -cp build ExerciseRunner
+3
+1
+3
+0
+$
+```