JAVA QUESTIONS

What will happen if we put a key object in a HashMap which is already there?

If you put the same key again than it will replace the old mapping because HashMap doesn't allow duplicate keys. Same key will result in same hashcode and will end up at same position in bucket. Each bucket contains a linked list of Map.Entry object, which contains both Key and Value. Now Java will take Key object form each entry and compare with this new key using equals() method, if that return true then value object in that entry will be replaced by new value.

What is difference between StringBuffer and StringBuilder in Java ?

StringBuffer:

StringBuffer is mutable means one can change the value of the object . The object created through StringBuffer is stored in the heap .  StringBuffer  has the same methods as the StringBuilder , but each method in StringBuffer is synchronized that is StringBuffer is thread safe . Due to this it does not allow  two threads to simultaneously access the same method . Each method can be accessed by one thread at a time . But being thread safe has disadvantages too as the performance of the StringBuffer hits due to thread safe property . Thus  StringBuilder is faster than the StringBuffer when calling the same methods of each class.

String Buffer can be converted to the string by using  toString() method.

StringBuffer demo1 = new StringBuffer("Hello") ;

demo1=new StringBuffer("Bye");

StringBuilder:

StringBuilder  is same as the StringBuffer , that is it stores the object in heap and it can also be modified . The main difference between the StringBuffer and StringBuilder is that StringBuilder is also not thread safe.

StringBuilder is fast as it is not thread safe . 

StringBuilder demo2= new StringBuilder("Hello");

demo2=new StringBuilder("Bye"); 

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