String . - String is immutable. - It seems mutable in certain cases, but it is a misconception. String hi = "Hi" hi += "Hello" //it seems that we just changed the object itself from "Hi" to "HiHello" //but what really happened is that we created a new String value "HiHello" and //changed the reference of the object hi from "Hi" to "HiHello" StringBuffer - StringBuffer is a data type that is used..