How can I remove a substring from a given String?

replace('regex', 'replacement');
replaceAll('regex', 'replacement');

In your example,

String hi = "Hello World!"
String no_o = hi.replaceAll("o", "");

You could easily use String.replace():

String helloWorld = "Hello World!";
String hellWrld = helloWorld.replace("o","");

You can use StringBuffer

StringBuffer text = new StringBuffer("Hello World");
text.replace( StartIndex ,EndIndex ,String);

Check out Apache StringUtils:

• static String replace(String text, String searchString, String replacement) Replaces all occurrences of a String within another String.
• static String replace(String text, String searchString, String replacement, int max) Replaces a String with another String inside a larger String, for the first max values of the search String.
• static String replaceChars(String str, char searchChar, char replaceChar) Replaces all occurrences of a character in a String with another.
• static String replaceChars(String str, String searchChars, String replaceChars) Replaces multiple characters in a String in one go.
• static String replaceEach(String text, String[] searchList, String[] replacementList) Replaces all occurrences of Strings within another String.
• static String replaceEachRepeatedly(String text, String[] searchList, String[] replacementList) Replaces all occurrences of Strings within another String.
• static String replaceOnce(String text, String searchString, String replacement) Replaces a String with another String inside a larger String, once.
• static String replacePattern(String source, String regex, String replacement) Replaces each substring of the source String that matches the given regular expression with the given replacement using the Pattern.DOTALL option.