ADD file via upload

src/main/java/com/example/myapp/model/Expression.java

@@ -0,0 +1,14 @@
+// Expression.java
+package com.example.myapp.model;
+
+public class Expression {
+  public String expression;
+  public double value;  // 改为 double 类型支持小数答案
+  public String mainOperator;
+
+  public Expression(String expression, double value, String mainOperator) {
+    this.expression = expression;
+    this.value = value;
+    this.mainOperator = mainOperator;
+  }
+}