Primitive Data Types

The Java programming language is statically-typed, which means that all variables must first be declared before they can be used. 


A primitive type is predefined by the language and is named by a reserved keyword. Primitive values do not share state with other primitive values. The eight primitive data types supported by the Java programming language are:

• byte: The byte data type is an 8-bit signed two's complement integer. It has a minimum value of -128 and a maximum value of 127 (inclusive). The byte data type can be useful for saving memory in large arrays, where the memory savings actually matters. They can also be used in place of int where their limits help to clarify your code; the fact that a variable's range is limited can serve as a form of documentation.
• short: The short data type is a 16-bit signed two's complement integer. It has a minimum value of -32,768 and a maximum value of 32,767 (inclusive). As with byte, the same guidelines apply: you can use a short to save memory in large arrays, in situations where the memory savings actually matters.
• int: By default, the int data type is a 32-bit signed two's complement integer, which has a minimum value of -2³¹ and a maximum value of 2³¹-1. In Java SE 8 and later, you can use theint data type to represent an unsigned 32-bit integer, which has a minimum value of 0 and a maximum value of 2³²-1. Use the Integer class to use int data type as an unsigned integer. See the section The Number Classes for more information. Static methods like compareUnsigned, divideUnsigned etc have been added to the Integer class to support the arithmetic operations for unsigned integers.
• long: The long data type is a 64-bit two's complement integer. The signed long has a minimum value of -2⁶³ and a maximum value of 2⁶³-1. In Java SE 8 and later, you can use the longdata type to represent an unsigned 64-bit long, which has a minimum value of 0 and a maximum value of 2⁶⁴-1. Use this data type when you need a range of values wider than those provided by int. The Long class also contains methods like compareUnsigned, divideUnsigned etc to support arithmetic operations for unsigned long.
• float: The float data type is a single-precision 32-bit IEEE 754 floating point. Its range of values is beyond the scope of this discussion, but is specified in the Floating-Point Types, Formats, and Values section of the Java Language Specification. As with the recommendations for byte and short, use a float (instead of double) if you need to save memory in large arrays of floating point numbers. This data type should never be used for precise values, such as currency. For that, you will need to use the java.math.BigDecimal class instead.Numbers and Strings covers BigDecimal and other useful classes provided by the Java platform.
• double: The double data type is a double-precision 64-bit IEEE 754 floating point. Its range of values is beyond the scope of this discussion, but is specified in the Floating-Point Types, Formats, and Values section of the Java Language Specification. For decimal values, this data type is generally the default choice. As mentioned above, this data type should never be used for precise values, such as currency.
• boolean: The boolean data type has only two possible values: true and false. Use this data type for simple flags that track true/false conditions. This data type represents one bit of information, but its "size" isn't something that's precisely defined.
• char: The char data type is a single 16-bit Unicode character. It has a minimum value of '\u0000' (or 0) and a maximum value of '\uffff' (or 65,535 inclusive).
  
  
  The following chart summarizes the default values for the above data types.

  Data Type	Default Value (for fields)
  byte	0
  short	0
  int	0
  long	0L
  float	0.0f
  double	0.0d
  char	'\u0000'
  String (or any object)	null
  boolean	false

• 
  

Common Observations:

       final int i =100;
        byte b = i;
        System.out.println(b);  // valid prints 100


        /*
        final int x =1000;
        byte y = x;   //compile error
        System.out.println(y);  */


        /*
         int x =100;
        byte y = (byte) x;   //valid
        System.out.println(y); */


        /*int x =100;
        byte y =  x;   //compile error. Nee to cast x --  byte y =  (byte) x
        System.out.println(y); */

        char c = 9;   //valid
        char d = 'A';  //valid
        char e = 0x12; //valid

        int j = 0345;  //valid
   
        System.out.println(1+2f+3d); // print 6.0

        /*final long test = 1;
        byte b1 = test; //compile error
        System.out.println(b1);*/
 

Interview Question on Singleton Design Pattern

What is Singleton class? Have you used Singleton before?
Singleton is a class which has only one instance in whole application and provides a getInstance() method to access the singleton instance. There are many classes in JDK which is implemented using Singleton pattern like java.lang.Runtime which provides getRuntime() method to get access of it and used to get free memory and total memory in Java

What is double checked locking in Singleton?One of the most hyped question on Singleton pattern and really demands complete understanding to get it right because of Java Memory model caveat prior to Java 5. If a guy comes up with a solution of using volatile keyword with Singleton instance and explains it then it really shows it has in depth knowledge of Java memory model and he is constantly updating his Java knowledge.

Answer: Double checked locking is a technique to prevent creating another instance of Singleton when call to getInstance() method is made in multi-threading environment. In Double checked locking pattern as shown in below example, singleton instance is checked two times before initialization.

public static Singleton getInstance(){
      if(_INSTANCE == null){
          synchronized(Singleton.class){
          //double checked locking - because second check of Singleton instance with lock
                if(_INSTANCE == null){
                    _INSTANCE = new Singleton();
                }
            }
          }
      return _INSTANCE;
}

Double checked locking should only be used when you have requirement for lazy initialization otherwise use Enum to implement singleton or simple static final variable.

How do you prevent for creating another instance of Singleton using clone() method?This type of questions generally comes some time by asking how to break singleton or when Singleton is not Singleton in Java.
Answer: Preferred way is not to implement Clonnable interface as why should one wants to create clone() of Singleton and if you do just throw Exception from clone() method as  “Can not create clone of Singleton class”. 

How many ways you can write Singleton Class in Java?
Answer:  I know at least four ways to implement Singleton pattern in Java
1) Singleton by synchronizing getInstance() method
2) Singleton with public static final field initialized during class loading.
3) Singleton generated by static nested class, also referred as Singleton holder pattern.
4) From Java 5 on-wards using Enums

What is lazy and early loading of Singleton and how will you implement it?This is another great Singleton interview question in terms of understanding of concept of loading and cost associated with class loading in Java. Many of which I have interviewed not really familiar with this but its good to know concept.

Answer: As there are many ways to implement Singleton like using double checked locking or Singleton class with static final instance initialized during class loading. Former is called lazy loading because Singleton instance is created only when client calls getInstance() method while later is called early loading because Singleton instance is created when class is loaded into memory.


Is it better to make whole getInstance() method synchronized or just critical section is enough? Which one you will prefer?This is really nice question and I mostly asked to just quickly check whether candidate is aware of performance trade off of unnecessary locking or not. Since locking only make sense when we need to create instance and rest of the time its just read only access so locking of critical section is always better option. read more about synchronization on How Synchronization works in Java
Answer: This is again related to double checked locking pattern, well synchronization is costly and when you apply this on whole method than call to getInstance() will be synchronized and contented. Since synchronization is only needed during initialization on singleton instance, to prevent creating another instance of Singleton,  It’s better to only synchronize critical section and not whole method. Singleton pattern is also closely related to factory design pattern where getInstance() serves as static factory method.

Data Structures and Algorithm Interview Questions

Question 1 : How to find middle element of linked list in one pass?

One of the most popular question from data structures and algorithm, mostly asked on telephonic interview. Since many programmer know that, in order to find length of linked list we need to first traverse through linkedlist till we find last node, which is pointing to null, and then in second pass we can find middle element by traversing only half of length. They get confused when interviewer ask him to do same job in one pass. In order to find middle element of linked list in one pass you need to maintain two pointer, one increment at each node while other increments after two nodes at a time, by having this arrangement, when first pointer reaches end, second pointer will point to middle element of linked list. See this trick to find middle element of linked list in single pass for more details.

Question 2 : How to find if linked list has loop ?

This question has bit of similarity with earlier algorithm and data structure interview question. I mean we can use two pointer approach to solve this problem. If we maintain two pointers, and we increment one pointer after processing two nodes and other after processing every node, we are likely to find a situation where both the pointers will be pointing to same node. This will only happen if linked list has loop.

 

Question 3 : In an integer array, there is 1 to 100 number, out of one is duplicate, how to find ?

This is a rather simple data structures question, especially for this kind of. In this case you can simply add all numbers stored in array, and total sum should be equal to n(n+1)/2. Now just subtract actual sum to expected sum, and that is your duplicate number. Of course there is a brute force way of checking each number against all other numbers, but that will result in performance of O(n^2) which is not good. By the way this trick will not work if array have multiple duplicates or its not numbers forming arithmetic progression.

Difference Between Stack and Heap in Java

In general both stack and heap are part of memory, a program is allocated and used for different purposes. Java program runs inside JVM which is launched as a process by "java" command. Java also uses both stack and heap memory for different needs

 

1) Main difference between heap and stack is that stack memory is used to store local variables and function call, while heap memory is used to store objects in Java. No matter, where object is created in code e.g. as member variable, local variable or class variable,  they are always created inside heap space in Java.

2) Each Thread in Java has there own stack which can be specified using -Xss JVM parameter, similarly you can also specify heap size of Java program using JVM option -Xms and -Xmx where -Xms is starting size of heap and -Xmx is maximum size of java heap.

 

3) If there is no memory left in stack for storing function call or local variable, JVM will throw java.lang.StackOverFlowError, while if there is no more heap space for creating object, JVM will throw java.lang.OutOfMemoryError: Java Heap Space.

 

4) If you are using Recursion, on which method calls itself, You can quickly fill up stack memory. Another difference between stack and heap is that size of stack memory is lot lesser than size of  heap memory in Java.

5) Variables stored in stacks are only visible to the owner Thread, while objects created in heap are visible to all thread. In other words stack memory is kind of private memory of Java Threads, while heap memory is shared among all threads.

JAVA HEAP MEMORY

What is Heap space in Java?

When a Java program started Java Virtual Machine gets some memory from Operating System. Java Virtual Machine or JVM uses this memory for all its need and part of this memory is call java heap memory. Heap in Java generally located at bottom of address space and move upwards. whenever we create object using new operator or by any another means object is allocated memory from Heap and When object dies or garbage collected ,memory goes back to Heap space.

 

 

How to increase size of Java Heap

Default size of Heap space  in Java is 128MB on most of 32 bit Sun's JVM but its highly varies from JVM to JVM.

 

you can increase size of java heap space based on your application need and I always recommend this to avoid using default JVM heap values. if your application is large and lots of object created you can change size of heap space by using JVM options -Xms and -Xmx.  Xms denotes starting size of Heap while -Xmx denotes maximum size of Heap in Java. There is another parameter called -Xmn which denotes Size of new generation of Java Heap Space. Only thing is you can not change the size of Heap in Java dynamically, you can only provide Java Heap Size parameter while starting JVM.

 

OutOfMemoryError in Java Heap

When JVM starts JVM heap space is equal to the initial size of Heap specified by -Xms parameter, as application progress more objects get created and heap space is expanded to accommodate new objects. JVM also run garbage collector periodically to reclaim memory back from dead objects. JVM expands Heap in Java some where near to Maximum Heap Size specified by -Xmx and if there is no more memory left for creating new object in java heap , JVM throws  java.lang.OutOfMemoryError and  your application dies. Before throwing OutOfMemoryError No Space in Java Heap, JVM tries to run garbage collector to free any available space but even after that not much space available on Heap in Java it results into OutOfMemoryError.

 

10 Points about Java Heap Space

1. Java Heap Memory is part of memory allocated to JVM by Operating System.

2. Whenever we create objects they are created inside Heap in Java.

3. Java Heap space is divided into three regions or generation for sake of garbage collection called New Generation, Old or tenured Generation or Perm Space. Permanent generation is garbage collected during full gc in hotspot JVM.

4. You can increase or change size of Java Heap space by using JVM command line option -Xms, -Xmx and -Xmn. don't forget to add word "M" or "G" after specifying size to indicate Mega or Gig. for example you can set java heap size to 258MB by executing following command java -Xmx256m HelloWord. 

5. You can use either JConsole or Runtime.maxMemory(), Runtime.totalMemory(), Runtime.freeMemory() to query about Heap size programmatic in Java.

6. You can use command "jmap" to take Heap dump in Java and "jhat" to analyze that heap dump.

7. Java Heap space is different than Stack which is used to store call hierarchy and local variables.

8. Java Garbage collector is responsible for reclaiming memory from dead object and returning to Java Heap space

9. Don’t panic when you get java.lang.OutOfMemoryError, sometimes its just matter of increasing heap size but if it’s recurrent then look for memory leak in Java.

10. Use Profiler and Heap dump Analyzer tool to understand Java Heap space and how much memory is allocated to each object.