java – How can I remove a substring from a given String?

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);

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

In your example,

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