Without layout managers, Java programmers are faced with determining how their GUI will be displayed across multiple windowing systems and finding a common sizing and positioning that will work within the constraints imposed by each windowing system.
StringBuffer is Synchronized where as StringBuilder is not
so StringBuffer is Secure and slow where as StringBuilder is viceversa
mutable object is an object whose state unaltered after it is created i.e an object doesn't change by the operations performed on it, but by the assigning of it.
mutable is the contrary of immutable