问不能从静态上下文错误引用非静态变量

EN

public static void main(String[] args) {

Scanner input = new Scanner(System.in);
System.out.println("Enter expression: ");
String exp = input.nextLine().trim();

ExpressionTree exptree = new ExpressionTree1(); //new instance of ExpressionTree
TreeNode root = new TreeNode();
root = exptree.parseOutsideExpression(exp);
int evaluate = exptree.Compute(root);
String answer = Integer.toString(evaluate);
System.out.println("The evaluated expression is:" + answer);

}

class ExpressionTree{