Q: What is the difference between "==" and "equals()" in comparing Java String objects?

"==" is shallow comparison. It's actually comparing the two object references to see if they point to the same object.
"equals()" is deep comparison. It compares the actual string values



Q: What is pass-by-value and pass-by-reference in Java?
If the passed in argument was a primitive value like int, char, etc, the passed in primitive value is copied to the method parameter.
Modifying the copied parameter will not modify the original primitive value.
if the passed in argument was an object reference, the passed in reference is copied to the method parameter. The copied reference will still be pointing to the same object.
So, if you modify the object value through the copied reference, the original object will be modified.

Q: Why String class has been made immutable in Java?
For performance & thread-safety.

• If string is mutable, changing the string with one reference will lead to the wrong value for the other references.
• Synchronization and concurrency: making String immutable automatically makes them thread safe thereby solving the synchronization issues.

Q: What is the main difference between String, StringBuffer, and StringBuilder?

• String is immutable in Java, and this immutability gives the benefits like security and performance discussed above.
• StringBuffer is mutable, hence you can add strings to it, and when required, convert to an immutable String with the toString( ) method.
• StringBuilder is very similar to a StringBuffer, but StringBuffer has one disadvantage in terms of performance as all of its public methods are synchronized for thread-safety. StringBuilder in Java is a copy of StringBuffer but without synchronization to be used in local variables which are inherently thread-safe. So, if thread-safety is required, use StringBuffer, otherwise use StringBuilder.