EDIT:
This was my original response, but in hindsight @iccthedral's answer is probably the best.
One approach would be to split the text into words, then just concatenate the first word to the end of everything that came after it.
String input = "I love cats and hate dogs";
String[] words = input.split("\\s+");
String firstWord = words[0];
StringBuilder everythingAfterFirstWord = new StringBuilder();
for(int i = 1 ; i < words.length ; i++){
String word = words[i];
everythingAfterFirstWord.append(word);
everythingAfterFirstWord.append(" ");
}
String switched = everythingAfterFirstWord + firstWord;
Another approach would be to use regular expressions. Match the first word, and everything else then use String.replaceAll to switch the two groups.
String switched = input.replaceAll("^(\\w+)\\s(.*)$", "$2 $1")