diff --git a/src/main/java/com/example/Expression.java b/src/main/java/com/example/Expression.java
new file mode 100644
index 0000000..7868dd1
--- /dev/null
+++ b/src/main/java/com/example/Expression.java
@@ -0,0 +1,120 @@
+package de.tudortmund.cs.mnp.project2;
+
+import java.util.Random;
+
+/**
+ * Interface for mathematical Expressions
+ */
+public sealed interface Expression permits Expression.Add, Expression.Mul, Expression.Sub, Expression.Val {
+    /**
+     * Addition term: left + right
+     * @param left
+     * @param right
+     */
+    record Add(Expression left, Expression right) implements Expression{
+        public String toString(){
+            return "(" + left.toString() + "+" + right.toString() + ")";
+        }
+        public int eval() {
+            return left.eval() + right.eval();
+        }
+        public int runtime(){
+            return 1 + Math.max(left.runtime(), right.runtime());
+        }
+    }
+
+    /**
+     * Multiplication term: left * right
+     * @param left
+     * @param right
+     */
+    record Mul(Expression left, Expression right) implements Expression{
+        public String toString(){
+            return "(" + left.toString() + "*" + right.toString() + ")";
+        }
+        public int eval() {
+            return left.eval() * right.eval();
+        }
+
+        public int runtime(){
+            return 1 + Math.max(left.runtime(), right.runtime());
+        }
+    }
+
+    /**
+     * Subtraction term: left - right
+     * @param left
+     * @param right
+     */
+    record Sub(Expression left, Expression right) implements Expression{
+        public int runtime(){
+            return 1 + Math.max(left.runtime(), right.runtime());
+        }
+        public String toString(){
+            return "(" + left.toString() + "-" + right.toString() + ")";
+        }
+        public int eval() {
+            return left.eval() - right.eval();
+        }
+    }
+
+    /**
+     * Encapsulates a single integer
+     * @param inner
+     */
+    record Val(int inner) implements Expression{
+        public int runtime(){
+            return 0;
+        }
+        public String toString(){
+            return String.valueOf(inner);
+        }public int eval() {
+            return inner;
+        }
+    }
+
+    /**
+     * @return A string representation of the expression
+     */
+    String toString();
+
+    /**
+     * @return The computed result of the expression
+     */
+    int eval();
+
+    /**
+     * @return The estimated maximal runtime
+     */
+    int runtime();
+
+    /**
+     * Build a random expression of max depth `max_depth` and has integer values up to `max_val`.
+     * @param max_depth
+     * @param max_val
+     * @return A random expression
+     */
+    static Expression generateExpression(int max_depth, int max_val){
+        var x = new Random();
+        int choice = x.nextInt(4);
+        if (max_depth <= 1) {
+            choice = 4;
+        }
+
+        return switch (choice) {
+            case 0 -> new Add(
+                    generateExpression(max_depth-1, max_val),
+                    generateExpression(max_depth-1, max_val)
+            );
+            case 1 -> new Mul(
+                    generateExpression(max_depth-1, max_val),
+                    generateExpression(max_depth-1, max_val)
+            );
+            case 2 -> new Sub(
+                    generateExpression(max_depth-1, max_val),
+                    generateExpression(max_depth-1, max_val)
+            );
+            default -> new Val(x.nextInt(max_val+1)+1);
+        };
+    }
+}