Add all 23 GoF design pattern implementations

03-behavioral/interpreter/AddExpression.java

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