Java – why link string Will trim () throw an exception?

Sorry, if this is a stupid question, but I can't find an explanation

I have a string like this:

String str ="This is 50 test. Try it !!";

I want to get this number before the test

If I do:

str = str.substring(0,str.indexOf("test")).trim();
str = str.substring(str.lastIndexOf(" ")).trim();
System.out.println(str);

I see: "50"

But if I do this:

str = str.substring(0,str.indexOf("test")).trim().substring(str.lastIndexOf(" ")).trim();

I get: exception Java in thread "main" Lang. stringindexoutofboundsexception: string index out of range: - 13

Why is this exception thrown because trim() returns a copy of the string and omits leading and trailing spaces? Why can't I link my method calls on STR? I can't understand

Solution

str = str.substring(0,str.indexOf("test")).trim().substring(str.lastIndexOf(" ")).trim();

str = str.substring(0,str.indexOf("test")).trim().substring(str.lastIndexOf(" ")).trim();
                                                             ^

This STR refers to the original string

str = str.substring(0,str.indexOf("test")).trim();  // first operation
str = str.substring(str.lastIndexOf(" ")).trim();
                    ^

This STR refers to the string after the first operation is applied Therefore, the two methods are not the same