Add all 23 GoF design pattern implementations

03-behavioral/interpreter/NumberExpression.java

@@ -0,0 +1,18 @@
+package interpreter;
+
+/**
+ * TerminalExpression — a number literal.
+ * Leaf node in the AST; has no child expressions.
+ */
+public class NumberExpression implements Expression {
+    private final int number;
+
+    public NumberExpression(int number) {
+        this.number = number;
+    }
+
+    @Override
+    public int interpret() {
+        return number;
+    }
+}