Core Java Interview Questions
16 Q what is a the difference between System.err and System.out
A

We can redirect System.out to another file but we cannot redirect System.err stream

17 Q What are  the differences between an abstract class and an interface?
A

An abstract class can have concrete method, which is not allowed in an interface. Abstract class can have private or protected methods and variables and only public methods and variables are allowed in interface. We can implement more than one interface , but we can extend only one abstract class. Interfaces provides loose coupling where as abstract class provides tight coupling.

18 Q What is the difference between synchronized block and synchronized method ?
  A Synchronized blocks place locks for the specified block where as synchronized methods place locks for the entire method.
19 Q How can you force garbage collection in java?
  A

You cannot force Garbage Collection, but you can request for it by calling the method System.gc().  But it doesn't mean that Garbage Collection will start immediately. The garbage collection is a low priority thread of JVM.

20 Q How can you call a constructor from another constructor ?
A By using this() reference.
21 Q How can you call the constructor of super class ?
A By using super() syntax.
22 Q What's the difference between normal methods and constructors?
  A

Constructors must have the same name of the class and can not have a return type. They are called only once,  while regular methods can be called whenever required. We cannot explicitly call a constructor.

23 Q What is the use of packages in java ?
A

Packages are a way to organize files in java when a project consists of more than one module. It helps in resolving name conflicts when different modules have classes with the same names.

24 Q What must be the order of catch blocks when catching more than one exception?
A

The sub classes must come first. Otherwise it will give a compile time error.

25 Q

How can we call a method or variable of the super class from child class ?

A

We can use super.method() or super.variable syntax for this purpose.

26 Q If you are overriding equals() method of a class, what other methods you might need to override ?
A hashCode
27 Q How can you create your own exception ?
A

Our class must extend either Exception or its sub class

28 Q What is serialization ?
A

Serialization is the process of saving the state of an object.

29 Q What is de-serialization?
A De-serialization is the process of restoring the state of an object.
30 Q What is externalizable ?
A

It is an interface that extends Serializable. It is having two different methods writeExternal() and readExternal. This interface allows us to customize the output.

   
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