ifference between @RestController and @Controller

How to create a immutable object in Java? Does all property of immutable object needs to be final?

To create a object immutable You need to make the class final and all its member final so that once objects gets crated no one can modify its state. You can achieve same functionality by making member as non final but private and not modifying them except in constructor. Also its NOT necessary to have all the properties final since you can achieve same functionality by making member as non final but private and not modifying them except in constructor.

What is difference between String, StringBuffer and StringBuilder? When to use them?

The main difference between the three most commonly used String classes as follows.

• StringBuffer and StringBuilder objects are mutable whereas String class objects are immutable.
• StringBuffer class implementation is synchronized while StringBuilder class is not synchronized.
• Concatenation operator "+" is internally implemented by Java using either StringBuffer or StringBuilder.

Criteria to choose among String, StringBuffer and StringBuilder

• If the Object value will not change in a scenario use String Class because a String object is immutable.
• If the Object value can change and will only be modified from a single thread, use a StringBuilder because StringBuilder is unsynchronized(means faster).
• If the Object value may change, and can be modified by multiple threads, use a StringBuffer because StringBuffer is thread safe(synchronized).

Why String class is final or immutable?

It is very useful to have strings implemented as final or immutable objects. Below are some advantages of String Immutability in Java

• Immutable objects are thread-safe. Two threads can both work on an immutable object at the same time without any possibility of conflict.
• Security: the system can pass on sensitive bits of read-only information without worrying that it will be altered
• You can share duplicates by pointing them to a single instance.

Is Java Pass by Reference or Pass by Value?

The Java Spec says that everything in Java is pass-by-value. There is no such thing as "pass-by-reference" in Java. The difficult thing can be to understand that Java passes "objects as references" passed by value. This can certainly get confusing and I would recommend reading this article from an expert: http://javadude.com/articles/passbyvalue.htm Also read this interesting thread with example on StackOverflow : Java Pass By Ref or Value

The problem we're facing here is statements like

In Java, Objects are passed by reference, and primitives are passed by value.
This is half incorrect. Everyone can easily agree that primitives are passed by value; there's no such thing in Java as a pointer/reference to a primitive.
However, Objects are not passed by reference. A correct statement would be Object references are passed by value.
This may seem like splitting hairs, bit it is far from it. There is a world of difference in meaning. The following examples should help make the distinction.
In Java, take the case of

Figure 5: (Java) Pass-by-value example

public void foo(Dog d) {
    d = new Dog("Fifi"); // creating the "Fifi" dog
}



Dog aDog = new Dog("Max"); // creating the "Max" dog
// at this point, aDog points to the "Max" dog
foo(aDog);
// aDog still points to the "Max" dog

the variable passed in (aDog) is not modified! After calling foo, aDog still points to the "Max" Dog!
Many people mistakenly think/state that something like

Figure 6: (Java) Still pass-by-value...

public void foo(Dog d) { 
    d.setName("Fifi");
}

shows that Java does in fact pass objects by reference.

Why main() in java is declared as public static void main? What if the main method is declared as private?

Public - main method is called by JVM to run the method which is outside the scope of project therefore the access specifier has to be public to permit call from anywhere outside the application static - When the JVM makes are call to the main method there is not object existing for the class being called therefore it has to have static method to allow invocation from class. void - Java is platform independent language therefore if it will return some value then the value may mean different to different platforms so unlike C it can not assume a behavior of returning value to the operating system. If main method is declared as private then - Program will compile properly but at run-time it will give "Main method not public." error.