Does inbuilt String methods usage create new strings

String's are immutable - This is a fact in java (may be with exception from Reflection)

1. A String object once created will not be modified
2. Any operation performed using the reference variable pointing to the object either creates a new string object or points to an already existing different immutable string object

Do not do this

    public static void main(String[] args) throws Exception {
        String immutable = "immutable";
        String mutable = immutable;
        String anotherObject = ("immutable" + " ").trim();
        String internedCopy = anotherObject.intern();
        System.out.println(immutable + " - Initial object reference");
        System.out.println(mutable + " - Another reference to same object");
        System.out.println(anotherObject + " - Different object with same value");
        System.out.println(internedCopy + " - Reference to differently created object's internalized object");

        System.out.println("Now lets try something");
        Field field = String.class.getDeclaredField("value");
        field.setAccessible(true);
        char[] state = (char[]) field.get(mutable);
        state[0] = 'M';
        System.out.println(immutable + " - Initial object reference");
        System.out.println(mutable + " - Another reference to same object"); // this is the only acceptable change
        System.out.println(anotherObject + " - Different object with same value");
        System.out.println(internedCopy + " - Reference to differently created object's internalized object");
    }

As already stated, Strings are immutable and can and will never be changed. You can only change the reference stored in your variable.

You can already tell that the string itself does not get modified as when you call those functions, they return a new string and do not change the one you invoked the function on, as you need to do newString = oldString.trim();

When looking at the source code of the two functions you mentioned, there is a creation of Strings in both of those. In the first one (trim()), it just creates a new String from a byte[]-Copy of the original string, in the bounds of non-whitespace characters. The second function (replaceFirst()) calls a few helper functions, but the returning String instance comes from a StringBuilder instance.