Add JavaDoc comments to ValueHolder public methods and constructors

Added JavaDoc with Example Usage sections to all 9 non-private
methods and constructors in ValueHolder.java:
- Default and parameterized constructors
- getValue(), setValue(), reset()
- equals(), hashCode(), toString()
- Static factory method of()

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: Copilot

microsphere-java-core/src/main/java/io/microsphere/util/ValueHolder.java

@@ -48,25 +48,100 @@ public class ValueHolder<V> {
 
     private V value;
 
+    /**
+     * Constructs an empty {@link ValueHolder} with a {@code null} initial value.
+     *
+     * <h3>Example Usage</h3>
+     * <pre>{@code
+     *   ValueHolder<String> holder = new ValueHolder<>();
+     *   System.out.println(holder.getValue()); // Output: null
+     * }</pre>
+     *
+     * @since 1.0.0
+     */
     public ValueHolder() {
     }
 
+    /**
+     * Constructs a {@link ValueHolder} with the specified initial value.
+     *
+     * <h3>Example Usage</h3>
+     * <pre>{@code
+     *   ValueHolder<Integer> holder = new ValueHolder<>(42);
+     *   System.out.println(holder.getValue()); // Output: 42
+     * }</pre>
+     *
+     * @param value the initial value to hold, may be {@code null}
+     * @since 1.0.0
+     */
     public ValueHolder(V value) {
         this.value = value;
     }
 
+    /**
+     * Returns the currently held value.
+     *
+     * <h3>Example Usage</h3>
+     * <pre>{@code
+     *   ValueHolder<String> holder = ValueHolder.of("Hello");
+     *   String value = holder.getValue(); // "Hello"
+     * }</pre>
+     *
+     * @return the held value, or {@code null} if no value has been set
+     * @since 1.0.0
+     */
     public V getValue() {
         return value;
     }
 
+    /**
+     * Sets the held value to the specified value.
+     *
+     * <h3>Example Usage</h3>
+     * <pre>{@code
+     *   ValueHolder<Double> holder = new ValueHolder<>();
+     *   holder.setValue(3.14);
+     *   System.out.println(holder.getValue()); // Output: 3.14
+     * }</pre>
+     *
+     * @param value the value to hold, may be {@code null}
+     * @since 1.0.0
+     */
     public void setValue(V value) {
         this.value = value;
     }
 
+    /**
+     * Resets the held value to {@code null}.
+     *
+     * <h3>Example Usage</h3>
+     * <pre>{@code
+     *   ValueHolder<String> holder = ValueHolder.of("data");
+     *   holder.reset();
+     *   System.out.println(holder.getValue()); // Output: null
+     * }</pre>
+     *
+     * @since 1.0.0
+     */
     public void reset() {
         setValue(null);
     }
 
+    /**
+     * Checks whether this {@link ValueHolder} is equal to another object. Two {@link ValueHolder}
+     * instances are considered equal if they hold equal values as determined by {@link Objects#equals(Object, Object)}.
+     *
+     * <h3>Example Usage</h3>
+     * <pre>{@code
+     *   ValueHolder<Integer> holder1 = ValueHolder.of(100);
+     *   ValueHolder<Integer> holder2 = ValueHolder.of(100);
+     *   System.out.println(holder1.equals(holder2)); // Output: true
+     * }</pre>
+     *
+     * @param o the object to compare with
+     * @return {@code true} if the other object is a {@link ValueHolder} holding an equal value, {@code false} otherwise
+     * @since 1.0.0
+     */
     @Override
     public boolean equals(Object o) {
         if (this == o) {
@@ -79,18 +154,57 @@ public boolean equals(Object o) {
         return Objects.equals(value, that.value);
     }
 
+    /**
+     * Returns the hash code of the held value using {@link Objects#hashCode(Object)}.
+     *
+     * <h3>Example Usage</h3>
+     * <pre>{@code
+     *   ValueHolder<String> holder = ValueHolder.of("key");
+     *   int hash = holder.hashCode(); // hash code of "key"
+     * }</pre>
+     *
+     * @return the hash code of the held value, or {@code 0} if the value is {@code null}
+     * @since 1.0.0
+     */
     @Override
     public int hashCode() {
         return Objects.hashCode(value);
     }
 
+    /**
+     * Returns a string representation of this {@link ValueHolder} in the format
+     * {@code ValueHolder{value=...}}.
+     *
+     * <h3>Example Usage</h3>
+     * <pre>{@code
+     *   ValueHolder<String> holder = ValueHolder.of("test");
+     *   System.out.println(holder.toString()); // Output: ValueHolder{value=test}
+     * }</pre>
+     *
+     * @return a string representation of this holder and its value
+     * @since 1.0.0
+     */
     @Override
     public String toString() {
         return "ValueHolder{" +
                 "value=" + value +
                 '}';
     }
 
+    /**
+     * Creates a new {@link ValueHolder} containing the specified value.
+     *
+     * <h3>Example Usage</h3>
+     * <pre>{@code
+     *   ValueHolder<String> holder = ValueHolder.of("Hello World");
+     *   System.out.println(holder.getValue()); // Output: Hello World
+     * }</pre>
+     *
+     * @param value the value to hold, may be {@code null}
+     * @param <V>   the type of the value
+     * @return a new {@link ValueHolder} containing the given value
+     * @since 1.0.0
+     */
     public static <V> ValueHolder<V> of(V value) {
         return new ValueHolder<>(value);
     }