Add all 23 GoF design pattern implementations (2026-06-13)

03-behavioral/interpreter/MultiplyExpression.java

@@ -0,0 +1,19 @@
+package interpreter;
+
+/**
+ * NonTerminalExpression — multiplication: left * right.
+ */
+public class MultiplyExpression implements Expression {
+    private final Expression left;
+    private final Expression right;
+
+    public MultiplyExpression(Expression left, Expression right) {
+        this.left = left;
+        this.right = right;
+    }
+
+    @Override
+    public int interpret() {
+        return left.interpret() * right.interpret();
+    }
+}