| A: | A final class can't be extended ie., final class may not be subclassed. A final method can't be overridden when its class is inherited. You can't change value of a final variable (is a constant).
| Q: | What if the main method is declared as private? | | A: | The program compiles properly but at runtime it will give "Main method not public." message. | | | [ Received from Sandesh Sadhale] | |
| Q: | What if the static modifier is removed from the signature of the main method? | | A: | Program compiles. But at runtime throws an error "NoSuchMethodError". | | | [ Received from Sandesh Sadhale] | |
| Q: | What if I write static public void instead of public static void? | | A: | Program compiles and runs properly. | | | [ Received from Sandesh Sadhale] | |
| Q: | What if I do not provide the String array as the argument to the method? | | A: | Program compiles but throws a runtime error "NoSuchMethodError". | | | [ Received from Sandesh Sadhale] | |
| Q: | What is the first argument of the String array in main method? | | A: | The String array is empty. It does not have any element. This is unlike C/C++ where the first element by default is the program name. | | | [ Received from Sandesh Sadhale] | |
| Q: | If I do not provide any arguments on the command line, then the String array of Main method will be empty or null? | | A: | It is empty. But not null. | | | [ Received from Sandesh Sadhale] | |
| Q: | How can one prove that the array is not null but empty using one line of code? | | A: | Print args.length. It will print 0. That means it is empty. But if it would have been null then it would have thrown a NullPointerException on attempting to print args.length. | | | [ Received from Sandesh Sadhale] | |
| Q: | What environment variables do I need to set on my machine in order to be able to run Java programs? | | A: | CLASSPATH and PATH are the two variables. | | | [ Received from Sandesh Sadhale] | |
| Q: | Can an application have multiple classes having main method? | | A: | Yes it is possible. While starting the application we mention the class name to be run. The JVM will look for the Main method only in the class whose name you have mentioned. Hence there is not conflict amongst the multiple classes having main method. | | | [ Received from Sandesh Sadhale] | |
| Q: | Can I have multiple main methods in the same class? | | A: | No the program fails to compile. The compiler says that the main method is already defined in the class. | | | [ Received from Sandesh Sadhale] | |
| Q: | Do I need to import java.lang package any time? Why ? | | A: | No. It is by default loaded internally by the JVM. | | | [ Received from Sandesh Sadhale] | |
| Q: | Can I import same package/class twice? Will the JVM load the package twice at runtime? | | A: | One can import the same package or same class multiple times. Neither compiler nor JVM complains abt it. And the JVM will internally load the class only once no matter how many times you import the same class. | | | [ Received from Sandesh Sadhale] | |
| Q: | What are Checked and UnChecked Exception? | | A: | A checked exception is some subclass of Exception (or Exception itself), excluding class RuntimeException and its subclasses.
Making an exception checked forces client programmers to deal with the possibility that the exception will be thrown. eg, IOException thrown by java.io.FileInputStream's read() method·
Unchecked exceptions are RuntimeException and any of its subclasses. Class Error and its subclasses also are unchecked. With an unchecked exception, however, the compiler doesn't force client programmers either to catch the
exception or declare it in a throws clause. In fact, client programmers may not even know that the exception could be thrown. eg, StringIndexOutOfBoundsException thrown by String's charAt() method· Checked exceptions must be caught at compile time. Runtime exceptions do not need to be. Errors often cannot be. | | | |
| Q: | What is Overriding? | | A: | When a class defines a method using the same name, return type, and arguments as a method in its superclass, the method in the class overrides the method in the superclass.
When the method is invoked for an object of the class, it is the new definition of the method that is called, and not the method definition from superclass. Methods may be overridden to be more public, not more private. | | | |
| Q: | What are different types of inner classes? | | A: | Nested top-level classes, Member classes, Local classes, Anonymous classes
Nested top-level classes- If you declare a class within a class and specify the static modifier, the compiler treats the class just like any other top-level class.
Any class outside the declaring class accesses the nested class with the declaring class name acting similarly to a package. eg, outer.inner. Top-level inner classes implicitly have access only to static variables.There can also be inner interfaces. All of these are of the nested top-level variety.
Member classes - Member inner classes are just like other member methods and member variables and access to the member class is restricted, just like methods and variables. This means a public member class acts similarly to a nested top-level class. The primary difference between member classes and nested top-level classes is that member classes have access to the specific instance of the enclosing class.
Local classes - Local classes are like local variables, specific to a block of code. Their visibility is only within the block of their declaration. In order for the class to be useful beyond the declaration block, it would need to implement a
more publicly available interface.Because local classes are not members, the modifiers public, protected, private, and static are not usable.
Anonymous classes - Anonymous inner classes extend local inner classes one level further. As anonymous classes have no name, you cannot provide a constructor. |
| Q: | Are the imports checked for validity at compile time? e.g. will the code containing an import such as java.lang.ABCD compile? | | A: | Yes the imports are checked for the semantic validity at compile time. The code containing above line of import will not compile. It will throw an error saying,can not resolve symbol
symbol : class ABCD
location: package io
import java.io.ABCD; | | | [ Received from Sandesh Sadhale] | |
| Q: | Does importing a package imports the subpackages as well? e.g. Does importing com.MyTest.* also import com.MyTest.UnitTests.*? | | A: | No you will have to import the subpackages explicitly. Importing com.MyTest.* will import classes in the package MyTest only. It will not import any class in any of it's subpackage. | | | [ Received from Sandesh Sadhale] | |
| Q: | What is the difference between declaring a variable and defining a variable? | | A: | In declaration we just mention the type of the variable and it's name. We do not initialize it. But defining means declaration + initialization.
e.g String s; is just a declaration while String s = new String ("abcd"); Or String s = "abcd"; are both definitions. | | | [ Received from Sandesh Sadhale] | |
| Q: | What is the default value of an object reference declared as an instance variable? | | A: | null unless we define it explicitly. | | | [ Received from Sandesh Sadhale] | |
| Q: | Can a top level class be private or protected? | | A: | No. A top level class can not be private or protected. It can have either "public" or no modifier. If it does not have a modifier it is supposed to have a default access.If a top level class is declared as private the compiler will complain that the "modifier private is not allowed here". This means that a top level class can not be private. Same is the case with protected. | | | [ Received from Sandesh Sadhale] | |
| Q: | What type of parameter passing does Java support? | | A: | In Java the arguments are always passed by value . | | | [ Update from Eki and Jyothish Venu] | |
| Q: | Primitive data types are passed by reference or pass by value? | | A: | Primitive data types are passed by value. | | | [ Received from Sandesh Sadhale] | |
| Q: | Objects are passed by value or by reference? | | A: | Java only supports pass by value. With objects, the object reference itself is passed by value and so both the original reference and parameter copy both refer to the same object . | | | [ Update from Eki and Jyothish Venu] | |
| Q: | When a thread is created and started, what is its initial state? | | A: | A thread is in the ready state after it has been created and started. | | | | |
| Q: | What is the purpose of finalization? | | A: | The purpose of finalization is to give an unreachable object the opportunity to perform any cleanup processing before the object is garbage collected. | | | | |
| Q: | What is the Locale class? | | A: | The Locale class is used to tailor program output to the conventions of a particular geographic, political, or cultural region. | | | | |
| Q: | What is the difference between a while statement and a do statement? | | A: | A while statement checks at the beginning of a loop to see whether the next loop iteration should occur. A do statement checks at the end of a loop to see whether the next iteration of a loop should occur. The do statement will always execute the body of a loop at least once. | | | | |
| Q: | What is the difference between static and non-static variables? | | A: | A static variable is associated with the class as a whole rather than with specific instances of a class. Non-static variables take on unique values with each object instance. | | | | |
| Q: | How are this() and super() used with constructors? | | A: | This() is used to invoke a constructor of the same class. super() is used to invoke a superclass constructor. | | | | |
| Q: | What are synchronized methods and synchronized statements? | | A: | Synchronized methods are methods that are used to control access to an object. A thread only executes a synchronized method after it has acquired the lock for the method's object or class. Synchronized statements are similar to synchronized methods. A synchronized statement can only be executed after a thread has acquired the lock for the object or class referenced in the synchronized statement. | | | | |
| Q: | What is daemon thread and which method is used to create the daemon thread? | | A: | Daemon thread is a low priority thread which runs intermittently in the back ground doing the garbage collection operation for the java runtime system. setDaemon method is used to create a daemon thread. |
| Q: | How does a try statement determine which catch clause should be used to handle an exception? | | A: | When an exception is thrown within the body of a try statement, the catch clauses of the try statement are examined in the order in which they appear. The first catch clause that is capable of handling the exceptionis executed. The remaining catch clauses are ignored. |
| Q: | What method must be implemented by all threads? | | A: | All tasks must implement the run() method, whether they are a subclass of Thread or implement the Runnable interface.
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| Q: | What are synchronized methods and synchronized statements? | | A: | Synchronized methods are methods that are used to control access to an object. A thread only executes a synchronized method after it has acquired the lock for the method's object or class. Synchronized statements are similar to synchronized methods. A synchronized statement can only be executed after a thread has acquired the lock for the object or class referenced in the synchronized statement.
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| Q: | What modifiers are allowed for methods in an Interface? | | A: | Only public and abstract modifiers are allowed for methods in interfaces.
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| Q: | What are some alternatives to inheritance? | | A: | Delegation is an alternative to inheritance. Delegation means that you include an instance of another class as an instance variable, and forward messages to the instance. It is often safer than inheritance because it forces you to think about each message you forward, because the instance is of a known class, rather than a new class, and because it doesn't force you to accept all the methods of the super class: you can provide only the methods that really make sense. On the other hand, it makes you write more code, and it is harder to re-use (because it is not a subclass).
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| Q: | What does it mean that a method or field is "static"? | | A: | Static variables and methods are instantiated only once per class. In other words they are class variables, not instance variables. If you change the value of a static variable in a particular object, the value of that variable changes for all instances of that class. Static methods can be referenced with the name of the class rather than the name of a particular object of the class (though that works too). That's how library methods like System.out.println() work out is a static field in the java.lang.System class.
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| Q: | Is Empty .java file a valid source file? | | A: | Yes, an empty .java file is a perfectly valid source file. | | | [Received from Sandesh Sadhale] | |
| Q: | Can a .java file contain more than one java classes? | | A: | Yes, a .java file contain more than one java classes, provided at the most one of them is a public class. | | | [ Received from Sandesh Sadhale] | |
| Q: | Is String a primitive data type in Java? | | A: | No String is not a primitive data type in Java, even though it is one of the most extensively used object. Strings in Java are instances of String class defined in java.lang package. | | | [ Received from Sandesh Sadhale] | |
| Q: | Is main a keyword in Java? | | A: | No, main is not a keyword in Java. | | | | |
| Q: | Is next a keyword in Java? | | A: | No, next is not a keyword. | | | [ Received from Sandesh Sadhale] | |
| Q: | Is delete a keyword in Java? | | A: | No, delete is not a keyword in Java. Java does not make use of explicit destructors the way C++ does. | | | [ Received from Sandesh Sadhale] | |
| Q: | Is exit a keyword in Java? | | A: | No. To exit a program explicitly you use exit method in System object. | | | | |
| Q: | What happens if you dont initialize an instance variable of any of the primitive types in Java? | | A: | Java by default initializes it to the default value for that primitive type. Thus an int will be initialized to 0, a boolean will be initialized to false. | | | | |
| Q: | What will be the initial value of an object reference which is defined as an instance variable? | | A: | The object references are all initialized to null in Java. However in order to do anything useful with these references, you must set them to a valid object, else you will get NullPointerExceptions everywhere you try to use such default initialized references. | | | [ Received from Sandesh Sadhale] | |
| Q: | What are the different scopes for Java variables? | | A: | The scope of a Java variable is determined by the context in which the variable is declared. Thus a java variable can have one of the three scopes at any given point in time.
1. Instance : - These are typical object level variables, they are initialized to default values at the time of creation of object, and remain accessible as long as the object accessible.
2. Local : - These are the variables that are defined within a method. They remain accessbile only during the course of method excecution. When the method finishes execution, these variables fall out of scope.
3. Static: - These are the class level variables. They are initialized when the class is loaded in JVM for the first time and remain there as long as the class remains loaded. They are not tied to any particular object instance. | | | [ Received from Sandesh Sadhale] | |
| Q: | What is the default value of the local variables? | | A: | The local variables are not initialized to any default value, neither primitives nor object references. If you try to use these variables without initializing them explicitly, the java compiler will not compile the code. It will complain abt the local varaible not being initilized.. | | | [ Received from Sandesh Sadhale] | |
| Q: | How many objects are created in the following piece of code?
MyClass c1, c2, c3;
c1 = new MyClass ();
c3 = new MyClass ();
| | A: | Only 2 objects are created, c1 and c3. The reference c2 is only declared and not initialized. | | | | |
| Q: | Can a public class MyClass be defined in a source file named YourClass.java? | | A: | No the source file name, if it contains a public class, must be the same as the public class name itself with a .java extension. | | | | |
| Q: | Can main method be declared final? | | A: | Yes, the main method can be declared final, in addition to being public static. | | | | |
| Q: | What will be the output of the following statement?
System.out.println ("1" + 3); | | A: | It will print 13. | | | | |
| Q: | What will be the default values of all the elements of an array defined as an instance variable? | | A: | If the array is an array of primitive types, then all the elements of the array will be initialized to the default value corresponding to that primitive type. e.g. All the elements of an array of int will be initialized to 0, while that of boolean type will be initialized to false. Whereas if the array is an array of references (of any type), all the elements will be initialized to null. | | | |
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