I think I spotted a mistake here.

The kata involves the operator () too, which is not included in the code.

The description says "Create a simple calculator that given a string of operators (), +, -, *, / and numbers separated by spaces returns the value of that expression".

So if I try
expression = "2 / ( 3 + 1 )"
It shows error since () isn't included.

Exception in thread "main" java.lang.NumberFormatException: For input string: "("
at java.base/jdk.internal.math.FloatingDecimal.readJavaFormatString(FloatingDecimal.java:2054)
at java.base/jdk.internal.math.FloatingDecimal.parseDouble(FloatingDecimal.java:110)
at java.base/java.lang.Double.parseDouble(Double.java:549)
at java.base/java.lang.Double.valueOf(Double.java:512)
at Test.main(Test.java:19)

Please check that up in the test cases.

Otherwise this is really a short and simple answer I would say.