coreJava3 Flashcards
What is immutable object in Java? Can you change values of a immutable object?
A Java object is considered immutable when its state cannot change after it is created.
Use of immutable objects is widely accepted as a sound strategy for creating simple, reliable code. Immutable objects are particularly useful in concurrent applications. Since they cannot change state, they cannot be corrupted by thread interference or observed in an inconsistent state.
java.lang.String and java.lang.Integer classes are the Examples of immutable objects from the Java Development Kit.
Immutable objects simplify your program due to following characteristics :
- are simple to use test and construct.
- are automatically thread-safe.
- do not require a copy constructor.
- do not require an implementation of clone.
- allow hashCode to use lazy initialization, and to cache its return value.
- do not need to be copied defensively when used as a field.
- are good Map keys and Set elements (Since state of these objects must not change while stored in a collection).
- have their class invariant established once upon construction, and it never needs to be checked again.
- always have “failure atomicity” (a term used by Joshua Bloch) : if an immutable object throws an exception, it’s never left in an undesirable or indeterminate state.
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 created 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?
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.
- You can create substrings without copying.
- You just create a pointer into an existing base String guaranteed never to change.
- Immutability is the secret that makes Java substring implementation very fast.
- Immutable objects are good fit for becoming Hashtable keys.
- If you change the value of any object that is used as a hash table key without removing it and re-adding it you will lose the object mapping.
- Since String is immutable, inside each String is a char[] exactly the correct length.
- Unlike a StringBuilder there is no need for padding to allow for growth.
- If String were not final, you could create a subclass and have two strings that look alike when “seen as Strings”, but that are actually different.
What is OutOfMemoryError in java? How to deal with java.lang.OutOfMemoryError error?
This Error is thrown when the Java Virtual Machine cannot allocate an object because it is out of memory, and no more memory could be made available by the garbage collector.
Note: Its an Error (extends java.lang.Error) not Exception.
Two important types of OutOfMemoryError are often encountered
- java.lang.OutOfMemoryError: Java heap space
The quick solution is to add these flags to JVM command line when Java runtime is started: -Xms1024m -Xmx1024m
- java.lang.OutOfMemoryError: PermGen space
The solution is to add these flags to JVM command line when Java runtime is started: -XX:+CMSClassUnloadingEnabled-XX:+CMSPermGenSweepingEnabled
Long Term Solution: Increasing the Start/Max Heap size or changing Garbage Collection options may not always be a long term solution for your Out Of Memory Error problem. Best approach is to understand the memory needs of your program and ensure it uses memory wisely and does not have leaks.
You can use a Java memory profiler to determine what methods in your program are allocating large number of objects and then determine if there is a way to make sure they are no longer referenced, or to not allocate them in the first place.
Why there are two Date classes; one in java.util package and another in java.sql?
From the JavaDoc of java.sql.Date: A thin wrapper around a millisecond value that allows JDBC to identify this as an SQL DATE value. A milliseconds value represents the number of milliseconds that have passed since January 1, 1970 00:00:00.000 GMT.
To conform with the definition of SQL DATE, the millisecond values wrapped inside a java.sql.Date instance must be ‘normalized’ by setting the hours, minutes, seconds, and milliseconds to zero. Explanation:
A java.util.Date represents date and time of day, a java.sql.Date only represents a date (the complement of java.sql.Date is java.sql.Time, which only represents a time of day, but also extends java.util.Date).
What is Marker interface? How is it used in Java?
The marker interface is a design pattern, used with languages that provide run-time type information about objects.
It provides a way to associate metadata with a class where the language does not have explicit support for such metadata. To use this pattern, a class implements a marker interface, and code that interact with instances of that class test for the existence of the interface. Whereas a typical interface specifies methods that an implementing class must support, a marker interface does not do so. The mere presence of such an interface indicates specific behavior on the part of the implementing class. There can be some hybrid interfaces, which both act as markers and specify required methods, are possible but may prove confusing if improperly used. Java utilizes this pattern very well and the example interfaces are
java.io.Serializable - Serializability of a class is enabled by the class implementing the java.io.Serializable interface. The Java Classes that do not implement Serializable interface will not be able to serialize or deserializ their state. All subtypes of a serializable class are themselves serializable. The serialization interface has no methods or fields and serves only to identify the semantics of being serializable.
java.rmi.Remote - The Remote interface serves to identify interfaces whose methods may be invoked from a non-local virtual machine. Any object that is a remote object must directly or indirectly implement this interface. Only those methods specified in a “remote interface”, an interface that extends java.rmi.Remote are available remotely.
java.lang.Cloneable - A class implements the Cloneable interface to indicate to the Object.clone() method that it is legal for that method to make a field-for-field copy of instances of that class. Invoking Object’s clone method on an instance that does not implement the Cloneable interface results in the exception CloneNotSupportedException being thrown.
javax.servlet.SingleThreadModel - Ensures that servlets handle only one request at a time. This interface has no methods. java.util.EvenListener - A tagging interface that all event listener interfaces must extend.
The “instanceof” keyword in java can be used to test if an object is of a specified type. So this keyword in combination with Marker interface can be used to take different actions based on type of interface an object implements.
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.
What is the difference between an Interface and an Abstract class?
An abstract class can have instance methods that implement a default behavior.
An Interface can only declare constants and instance methods, but cannot implement default behavior and all methods are implicitly abstract.
An interface has all public members and no implementation.
An abstract class is a class which may have the usual flavors of class members (private, protected, etc.), but has some abstract methods.
What is the purpose of garbage collection in Java, and when is it used?
The purpose of garbage collection is to identify and discard objects that are no longer needed by a program so that their resources can be reclaimed and reused.
A Java object is subject to garbage collection when it becomes unreachable to the program in which it is used.
Describe synchronization in respect to multithreading.
With respect to multithreading, synchronization is the capability to control the access of multiple threads to shared resources.
Without synchonization, it is possible for one thread to modify a shared variable while another thread is in the process of using or updating same shared variable.
This usually leads to significant errors.
Explain different way of using thread?
The thread could be implemented by using runnable interface or by inheriting from the Thread class.
The former is more advantageous, ‘cause when you are going for multiple inheritance..the only interface can help.
What is HashMap and Map?
Map is Interface and Hashmap is class that implements that.
Difference between Swing and Awt?
AWT are heavy-weight componenets. Swings are light-weight components. Hence swing works faster than AWT.
What is the difference between a constructor and a method?
A constructor is a member function of a class that is used to create objects of that class. It has the same name as the class itself, has no return type, and is invoked using the new operator.
A method is an ordinary member function of a class. It has its own name, a return type (which may be void), and is invoked using the dot operator.
