Q1. How could Java classes direct program messages to the system console, but error messages, say to a file?
A. The class System has a variable out that represents the standard output, and the variable err that represents the standard error device. By default, they both point at the system console. This how the standard output and error could be re-directed:
PrintStream st = new PrintStream(new FileOutputStream("output.txt")); System.setErr(st); System.setOut(st);
Q2. What's the difference between an interface and an abstract class?
A. An abstract class may contain code in method bodies, which is not allowed in an interface. Java does not allow inheritance from multiple classes. On the other hand, you can implement multiple interfaces in your class. Also, an abstract class can contain private and protected members, and non-static member variables.
Q3. Why would you use a synchronized block vs. synchronized method?
A. Synchronized blocks place locks for shorter periods than synchronized methods, increasing the possibility of concurrent access to the protected code.
Q4. Explain the usage of the keyword transient?
A. This keyword indicates that the value of this member variable is not serialized with the object. When the class will be de-serialized, this variable will be initialized with a default value of its data type (e.g. zero for integers).
Q5. How can you force garbage collection?
A. You can't force GC, but could request it by calling System.gc(). JVM does not guarantee that GC will be started immediately, if at all.
Q6. How do you know if an explicit object casting is needed?
A. You can use an object anywhere it's declared class or a super-class is expected. You have to cast if a sub-class is expected, e.g. if you want to assign to a sub-class object or to use a sub-class's method.
Q7. What's the difference between the methods sleep() and wait()
A. The code sleep(1000) suspends the thread for at least one second. The code wait(1000), causes a wait of at most one second. A thread could stop waiting earlier if it receives the notify() or notifyAll() call. The method wait() is defined in the class Object and the method sleep() is defined in the class Thread.
Q8. Can you write a Java class that could be used both as an applet as well as an application?
A. Yes. Add a main() method to the applet's class.
Q9. What's the difference between constructors and other methods?
A. Constructors must have the same name as the class and can not return a value. They are only called once for a given object while regular methods could be called many times.
Q10. Can you call one constructor from another if a class has multiple constructors
A. Yes. Use this() syntax, passing any required parameters.
Q11. Explain the usage of Java packages.
A. This is a way to organize files when a project consists of multiple modules. It also helps resolve naming conflicts when different packages have classes with the same names. Package-private access level also allows you to protect data from being used by the non-authorized classes.
Q12. If a class is located in a package, what do you need to change in the OS environment to be able to use it?
A. You need to add a directory or a jar file that contains the package directories to the CLASSPATH environment variable. Let's say a class Employee belongs to a package com.xyz.hr; and is located in the file c:\dev\com\xyz\hr\Employee.java. In this case, you'd need to add c:\dev to the variable CLASSPATH. If this class contains the method main(), you could test it from a command prompt window as follows:
c:\>java com.xyz.hr.Employee
Alternatively you can pass the classpath as a parameter to the JRE, e.g. java -cp "the-classpath"
Q13. What's the difference between J2SDK 1.5 and J2SDK 5.0?
A.There's no difference, Sun Microsystems just re-branded this version.
Q14. What would you use to compare two String variables - the operator == or the method equals()?
A. I'd use the method equals() to compare the values of the Strings and the == to check if two variables point at the same instance of a String object.
Q15. Does it matter in what order catch statements for FileNotFoundException and IOExceptipon are written?
A. Yes, it does. The FileNoFoundException is inherited from the IOException. Exception's subclasses should be caught first, otherwise the super-class catch will hide the sub-class.
Q16. Can an inner class declared inside of a method access local variables of this method?
A. It's possible if these variables are final.
Q17. What can go wrong if you replace && with & in the following code:
String a=null; if (a!=null && a.length()>10) {...}
A. A single ampersand here would lead to a NullPointerException.
Q18. What's the main difference between a Vector and an ArrayList
A. Java Vector class is internally synchronized and ArrayList is not.
Q19. When should the method invokeLater()be used?
A. This method is used to ensure that the Runnable passed to invokeLater() is executed on the event-dispatching thread.
Q20. How can a subclass call a method or a constructor defined in a superclass?
A. Use the following syntax: super.myMethod(); To call a constructor of the superclass, just call super() in the first line of the subclass's constructor, passing any required parameters.
For senior-level developers:
Q21. What's the difference between a queue and a stack?
A. Stacks works by last-in-first-out rule (LIFO), while queues use the FIFO (first-in, first-out) rule.
Q22. You can create an abstract class that contains only abstract methods. On the other hand, you can create an interface that declares the same methods. So can you use abstract classes instead of interfaces?
A. Yes, but abstract classes are more limited because a class can only extend one class but can implement multiple interfaces.
Q23. What comes to mind when you hear about a young generation in Java?
A. Garbage collection; a young generation is a recent collection of newly created objects, which is reclaimed more aggressively than older ones.
Q24. What comes to mind when someone mentions a shallow copy in Java?
A. Object cloning; a shallow copy reuses references to the original's member variables, but a (complete) deep copy clones the entire object tree.
Q25. If you're overriding the method equals() of an object, which other method you might also consider?
A. hashCode(), because the hash algorithm should use the same member variables as equals().
Q26. You are planning to do an indexed search in a list of objects. Which of the two Java collections should you use:
ArrayList or LinkedList?
A. ArrayList
Q27. How would you make a copy of an entire Java object with its state?
A. Have this class implement Cloneable interface and call its method clone().
Q28. How can you minimize the need of garbage collection and make the memory use more effective?
A. Use object pooling to avoid creating new objects and weak object references to avoid keeping objects in memory that you can cheaply reconstruct.
Q29. There are two classes: A and B. The class B needs to inform a class A when some important event has happened. What Java technique would you use to implement it?
A. If these classes are threads I'd consider notify() or notifyAll(). For regular classes you can use the Observer interface. or some other implementation of the Listener pattern.
Q30. What access level do you need to specify in the class declaration to ensure that only classes from the same directory can access it?
A. You do not need to specify any access level, and Java will use a default package-private access level.
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