title: Lambda expressions "x cannot be resolved to variable" error
body: <p>So I'm currently learning java and the problem lies within this example class: 
  <a href="https://docs.oracle.com/javase/tutorial/java/javaOO/examples/Calculator.java" rel="nofollow noreferrer">https://docs.oracle.com/javase/tutorial/java/javaOO/examples/Calculator.java</a></p>
  
  <p>When I try to copy the code into eclipse(newest version, jdk 1.8, java compliance 1.7) it says that a can't be resolved to variable.
  <a href="https://i.stack.imgur.com/3OnER.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/3OnER.png" alt="enter image description here"></a></p>
  
  <p>Can someone please tell me what I'm doing wrong?</p>