Many scenarios come in day to day Java programming when we need to convert a Double value to String or vice versa. In my earlier article we have seen how to convert String to Integer and in this article we will first see how to convert double to String and later opposite of that from String to double. One important thing to note is Autoboxing which automatically converts primitive type to Object type and only available from Java 5 onwards. These conversion example assumes code is running above Java 5 version and actually tested in JDK 1.6, which makes it enable to pass Double object when method is expecting double primitive value e.g. String.valueOf(double d) which expect a double value.
Showing posts with label core java. Show all posts
Showing posts with label core java. Show all posts
Spring Hibernate How to create read only List, Map and Set in Java – unmodifiable example
Read only List, Map and Set in Java
Read only List means a List where you can not perform modification operations like add, remove or set. You can only read from the List by using get method or by using Iterator of List, This kind of List is good for certain requirement where parameters are final and can not be changed. In Java you can use Collections.unModifiableList() method to create read only List , Collections.unmodifiableSet() for creating read-only Set like read only HashSet and similarly creating a read only Map in Java, as shown in below example. Any modification in read only List will result in java.lang.UnSupportedOperationException in Java. This read-only List example is based on Java 5 generics but also applicable to other Java version like JDK 1.4 or JDK 1.3, just remove Generics code i.e. angle bracket which is not supported prior to Java 5. One common mistake programmer makes is that assuming fixed size List and read only List as same. As shown in our 3 example of converting Array to ArrayList , we can use Arrays.asList() method to create and initialized List at same line.List implementation returned by this method is fixed size and it doesn’t allow adding or removal of element but it is not read only because you can update objects by calling set(index) method. How to make a collection read only is also a popular Java collection interview question, which makes this Java collection tutorial even more important.
Spring Hibernate Best Practices while dealing with Password in Java
While working in core Java application or enterprise web application there is always a need of working with passwords in order to authenticate user. Passwords are very sensitive information like Social Security Number(SSN) and if you are working with real human data like in online banking portal or online health portal its important to follow best practices to deal with passwords or Social security numbers. here I will list down some of the points I learned and take care while doing authentication and authorization or working with password. I recommend to read more on this topic and have a checklist of things based on your application requirement. Anyway here are few points which make sense to me:
Java J2EE Spring Difference between Wait and Sleep , Yield in Java
Difference between wait and sleep or difference between Sleep and yield in Java are popular core Java interview questions and asked on multi-threading interviews. Out of three Sleep () and Yield () methods are defined in thread class while wait() is defined in Object class, which is another interview question. In this Java tutorial we will learn what is sleep in Java, important points of sleep in java and difference between Wait and sleep in Java.
Spring Hibernate Private in Java: Why should you always keep fields and methods private?
Making members private in Java is one of best coding practice. Private members (both fields and methods) are only accessible inside the class they are declared or inside inner classes. private keyword is one of four access modifier provided by Java and its a most restrictive among all four e.g. public, default(package), protected and private. Though there is no access modifier called package, rather its a default access level provided by Java. In this Java tutorial we will see why should we always make members of class by default as private and answer to one of popular Java interview question can override private methods in Java.
This article is in continuation of my earlier post on Java e.g. 10 Object Oriented Design principles Java programmer should know and 10 best practice to follow while writing comments. If you haven’t read them already you may find them interesting and worth reading.
Spring Hibernate Core Java Interview Questions and Answers: String concatenation
It is a very common beginner level question on String concatenation. Its imperative to understand that NOT all String concatenations are bad. It depends on how you are using it. You need to have the basic understanding that in Java, String is an immutable object and StringBuilder (not thread-safe) and StringBuffer (thread-safe) are mutable. In, Java you can concatenate strings a number of ways with the "+" operator, using the append( ) in StringBuilder and StringBuffer classes, and the other methods like concat( ) in String class.
Q1. Is anything wrong with the following code?
public class StringConcat {
public static String withStringBuilder(int count) {
String s = "Hello" + " " + " peter " + count;
return s;
}
}A1. No, the compiler internally uses a StringBuilder to use two append( ) calls to append the string and converts it to a String object using the toString( ) method.
Note: you can try the javap command described below to see why.
Q2. Is anything wrong with the following code?
public class StringConcat {
public static String withStringBuilder() {
String s = "Hello" + " " + " peter " + " how are you";
return s;
}
}A2. No, the compiler is smart enough to work out that it is a static concatenation and it uses its optimization to concatenate the string during compile-time. If you verify this by using a Java decompiler like jd-gui.exe to decompile the compiled class back, you will get the source code as below.
public class StringConcat
{
public static String withStringBuilder(int count)
{
String s = "Hello peter how are you";
return s;
}
}
You can also use the javap command to dissemble the compiled class file using the following command
C:\workspaces\proj\Test\src>javap -c StringConcat
Gives the following output
Compiled from "StringConcat.java"The line 0 shows that it has been optimized
public class StringConcat extends java.lang.Object{
public StringConcat();
Code:
0: aload_0
1: invokespecial #1; //Method java/lang/Object."<init>":()V
4: return
public static java.lang.String withStringBuilder(int);
Code:
0: ldc #2; //String Hello peter how are you
2: astore_1
3: aload_1
4: areturn
}
Q3. Is anything wrong with the following code snippet
public class StringConcat {
public static String withoutStringBuilder(int count) {
String str = "";
for (int i = 0; i < count; i++) {
str += i;
}
return str;
}
}A3. Yes. it consumes more memory and can have performance implications. This is because, a String object in Java is immutable. This means, you can't modify a String. If the value of count is 100, then the above code will create 100 new StringBuilder objects, of which 99 of them will be discarded for garbage collection. Creating new objects unnecessarily is not efficient, and the garbage collector needs to clean up those unreferenced 99 objects. StringBuffer and StringBuilder: are mutable and use them when you want to modify the contents. StringBuilder was added in Java 5 and it is identical in all respects to StringBuffer except that it is not synchronized, which makes it slightly faster at the cost of not being thread-safe. The code below creates only two new objects, the StringBuilder and the final String that is returned.
public class StringConcat {
public static String withStringBuilder(int count) {
StringBuilder sb = new StringBuilder(100);
for (int i = 0; i < count; i++) {
sb.append(i);
}
return sb.toString();
}
}Now, if you want to be more pedantic as to how we know that the 100 StringBuilder objects are created, we can use the javap option to our rescue. The javap is a class file dissembler. If you compile the codebase in the question and then run the javap command with the StringConcat.class file as shown below
C:\workspaces\proj_blue\Test\src>javap -c StringConcat
You will get an output as shown below
Compiled from "StringConcat.java"The dissembled looks cryptic, but if you inspect it carefully the code within the public static java.lang.String withoutStringBuilder(int);
public class StringConcat extends java.lang.Object{
public StringConcat();
Code:
0: aload_0
1: invokespecial #1; //Method java/lang/Object."<init>":()V
4: return
public static java.lang.String withoutStringBuilder(int);
Code:
0: ldc #2; //String
2: astore_1
3: iconst_0
4: istore_2
5: iload_2
6: iload_0
7: if_icmpge 35
10: new #3; //class java/lang/StringBuilder
13: dup
14: invokespecial #4; //Method java/lang/StringBuilder."<init>":()V
17: aload_1
18: invokevirtual #5; //Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
21: iload_2
22: invokevirtual #6; //Method java/lang/StringBuilder.append:(I)Ljava/lang/StringBuilder;
25: invokevirtual #7; //Method java/lang/StringBuilder.toString:()Ljava/lang/String;
28: astore_1
29: iinc 2, 1
32: goto 5
35: aload_1
36: areturn
}
Line 5 to 32: is the code within the for loop. The "goto 5" indicates looping back.
Line 10: creates a new StringBuilder object every time
Line 18: uses the StringBuilder's append method to concatenate the String.
Line 25: uses the toString( ) method to convert the StringBuilder back to the existing String reference via toString( ) method.
If you run the improved code snippet in the answer through javap, you get the following output
Compiled from "StringConcat.java"As you could see
public class StringConcat extends java.lang.Object{
public StringConcat();
Code:
0: aload_0
1: invokespecial #1; //Method java/lang/Object."<init>":()V
4: return
public static java.lang.String withStringBuilder(int);
Code:
0: new #2; //class java/lang/StringBuilder
3: dup
4: bipush 100
6: invokespecial #3; //Method java/lang/StringBuilder."<init>":(I)V
9: astore_1
10: iconst_0
11: istore_2
12: iload_2
13: iload_0
14: if_icmpge 29
17: aload_1
18: iload_2
19: invokevirtual #4; //Method java/lang/StringBuilder.append:(I)Ljava/lang/StringBuilder;
22: pop
23: iinc 2, 1
26: goto 12
29: aload_1
30: invokevirtual #5; //Method java/lang/StringBuilder.toString:()Ljava/lang/String;
33: areturn
}
Line 0 to 6: initializes one StringBuilder object outside the for loop.
Line 12 to 26: is the for loop.
Line 19: indicates that since the StringBuilder is mutable, the string is appended via the append method.
Important: The creation of extra strings is not limited to the overloaded mathematical operator "+", but there are several other methods like concat( ), trim( ), substring( ), and replace( ) in the String class that generate new string instances. So use StringBuffer or StringBuilder for computation intensive operations to get better performance. Experiment with javap for the String methods like concat( ), trim( ), substring( ), and replace( ) .
Note: So, javap and jd-gui.exe are handy tools for debugging your application for certain issues. For example, the java decompiler is handy for debugging generics to see how the java source code with generics is converted after compilation by decompiling the .class file back to source code.
Java J2EE Spring Step By Step guide to Read XML file in Java Using SAX Parser Example
Reading XML file in java using SAX Parser is little different than reading xml file in Java with DOM parser which we had discussed in last article of this series. This tutorial is can be useful for those who are new to the java world and got the requirement for read an xml file in java in their project or assignment, key feature of java is it provides built in class and object to handle everything which makes our task very easy. Basically this process of handling XML file is known as parsing means break down the whole string into small pieces using the special tokens.
Parsing can be done using two ways:
Java J2EE Spring java.lang.UnsupportedClassVersionError: Bad version number in .class files Cause and Solution
How to fix Bad version number in .class file
"java.lang.UnsupportedClassVersionError: Bad version number in .class file" is common error in Java programming language which comes when you try to run a Java class file. In our last article we discussed that how to resolve Java.lang.UnSupportedClassVersionError and found that it comes when major and minor version of class is not supported by Java virtual machine or JRE running the program. Though "java.lang.UnsupportedClassVersionError: Bad version number in .class file" is little different than that in its manifestation and Cause. UnsupportedClassVersionError is not as difficult as Java.lang.OutOfMemoryError and neither its solution is too complex but what is hard is thinking in right direction because cause of different types of UnsupportedClassVersionError is different.
Java J2EE Spring Difference between DOM and SAX Parsers in Java
Difference between SAX and DOM Parser is very popular Java interview and often asked when interviewed on Java and XML. Both DOM and SAX parser are extensively used to read and parse XML file in java and have there own set of advantage and disadvantage which we will cover in this article. Though there is another way of reading xml file using xpath in Java which is more selective approach like SQL statements people tend to stick with XML parsers. DOM Parser vs SAX parsers are also often viewed in terms of speed, memory consumption and there ability to process large xml files.
Java J2EE Spring How to read and write Images in java using ImageIO Utility
Writing an Image file in Java is very common scenario and in this article we will see a new way to write images into file in Java. javax.imageio.ImageIO is a utility class which provides lots of utility method related to images processing in Java. Most common of them is reading form image file and writing images to file in java. You can write any of .jpg, .png, .bmp or .gif images to file in Java. Just like writing, reading is also seamless with ImageIO and you can read BufferedImage directly from URL. Reading Images are little different than reading text or binary file in Java as they they are associated with different format. Though you can still use getClass().getResourceAsStream() approach for loading images.
Spring Hibernate Difference between EnumMap and HashMap in Java
HashMap vs EnumMap in Java
What is difference between EnumMap and HashMap in Java is the latest Java collection interview question which has been asked to couple of my friends. This is one of the tricky Java question, specially if you are not very much familiar with EnumMap in Java, which is not uncommon, given you can use it with only Enum keys. Main difference between EnumMap and HashMap is that EnumMap is a specialized Map implementation exclusively for Enum as key. Using Enum as key, allows to do some implementation level optimization for high performance which is generally not possible with other object's as key. We have seen lot of interview questions on HashMap in our article How HashMap works in Java but what we missed there is this question which is recently asked to some of my friend. Unlike HashMap, EnumMap is not applicable for every case but its best suited when you have Enum as key. We have already covered basics of EnumMap and some EnumMap example in my last article What is EnumMap in Java and In this post we will focus on key differences between HashMap and EnumMap in Java.
Spring Hibernate ThreadLocal in Java - Example Program and Tutorial
ThreadLocal in Java is another way to achieve thread-safety apart from writing immutable classes. If you have been writing multi-threaded or concurrent code in Java then you must be familiar with cost of synchronization or locking which can greatly affect Scalability of application, but there is no choice other than synchronize if you are sharing objects between multiple threads. ThreadLocal in Java is a different way to achieve thread-safety, it doesn't address synchronization requirement, instead it eliminates sharing by providing explicitly copy of Object to each thread. Since Object is no more shared there is no requirement of Synchronization which can improve scalability and performance of application. In this Java ThreadLocal tutorial we will see important points about ThreadLocal in Java, when to use ThreadLocal in Java and a simple Example of ThreadLocal in Java program.
Spring Framework How to Check if Integer Number is Power of Two in Java - 3 examples
How to check if an integer number is power of 2 in Java is one of the popular programming interview question and has been asked in many interviews. Surprisingly, this problem which looks simple enough to answer, doesn't turn out that simple if for many developers. Many Java programmers, both freshers and less experienced, struggle to write code for a function, which can check if number is power of 2 or not. There could be many different reasons for that, but it’s expected to at least come up with brute force solution. For those who are familiar with bitwise operators in Java , how positive and negative numbers are represented in binary format, this exercise is quite easy. Since negative numbers are represented as 2's complement value in Java, you can easily find if any number is power of 2 or not by looking at its bit pattern. Remember checking for power of two is different than checking if number is even or odd, that’s another thing to note. A number can be even, but it’s not necessary to be a power of two, e.g. 6 is even but it’s not a power of two.
Spring Framework How to Find Runtime of a Process in UNIX and Linux
So you checked your process is running in Linux operating system and it's running find, by using ps command. But now you want to know, from how long process is running, What is start date of that process etc. Unfortunately ps command in Linux or any UNIX based operating system doesn't provide that information. But as said, UNIX has commands for everything, there is a nice UNIX tip, which you can use to check from how long a process is running. It’s been a long time, I have posted any UNIX or Linux command tutorial, after sharing some UNIX productivity tips . So I thought to share this nice little tip about finding runtime of a process in UNIX based systems e.g. Linux and Solaris. In this UNIX command tutorial, we will see step by step guide to find, since when a particular process is running in a server.
Java J2EE Spring How to Convert Map to List in Java Example
Map and List are two common data-structures available in Java and in this article we will see how can we convert Map values or Map keys into List in Java. Primary difference between Map (HashMap, ConcurrentHashMap or TreeMap) and List is that Map holds two object key and value while List just holds one object which itself is a value. Key in hashmap is just an addon to find values, so you can just pick the values from Map and create a List out of it. Map in Java allows duplicate value which is fine with List which also allows duplicates but Map doesn't allow duplicate key.
Spring Framework How to compare Arrays in Java – Equals vs deepEquals Example
java.util.Arrays class provides equals() and deepEquals() method to compare two Arrays in Java. Both of these are overloaded method to compare primitive arrays e.g. int, long, float, double and Object arrays e.g. Arrays.equals(Object[] , Object[]). Arrays.equals() returns true if both Arrays which it is comparing are null, If both array pointing to same Array Object or they must be of same length and contains same element in each index. In all other cases it returns false. Arrays.equals() calls equals() method of each Object while comparing Object arrays. One of the tricky question in Java related to Array comparison is Difference between Arrays.equals() and Arrays.deepEquals() method. Since both equals and deepEquals is used for array comparison, what is difference between them becomes important. Short answer of this questions is that, Array's equals() method does not perform deep comparison and fails logical comparison in case of nested Array, on other hand deepEquals() perform deep comparison and returns logical comparison in case of nested array.
Java J2EE Spring How to reverse String in Java using Iteration and Recursion
How to reverse string in java is popular core java interview question and asked on all levels from junior to senior java programming job. since Java has rich API most java programmer answer this question by using StringBuffer reverse() method which easily reverses an String in Java and its right way if you are programming in Java but most interview doesn't stop there and they ask interviewee to reverse String in Java without using StringBuffer or they will ask you to write an iterative reverse function which reverses string in Java. In this tutorial we will see how to reverse string in both iterative and recursive fashion. This example will help you to prepare better for using recursion in java which is often a weak area of Java programmer and exposed during a programming interview.
Java J2EE Spring How to Stop Thread in Java Code Example
Thread is one of important Class in Java and multi-threading is most widely used feature,but there is no clear way to stop Thread in Java. Earlier there was a stop method exists in Thread Class but Java deprecated that method citing some safety reason. By default a Thread stops when execution of run() method finish either normally or due to any Exception.In this article we will How to Stop Thread in Java by using a boolean State variable or flag. Using flag to stop Thread is very popular way of stopping thread and its also safe, because it doesn't do anything special rather than helping run() method to finish it self.
Java J2EE Spring How to use string in switch case in Jdk 7 with example
Have you ever feel that String should be used in switch cases as like int and char? JDK 7 has made an important enhancement in there support of String, now you can use String in switch and case statement, No doubt String is most widely used type in Java and in my opinion they should have made this enhancement long back when they provided support for enum in java and allowed enum to be used in switch statement. In this java tutorial we will see how we can use String inside switch and case statement in JDK 7. This article is in continuation of my earlier post on JDK7 feature improved exception handling using multi cache block in Java
many a times we want to switch based upon string output received from user or other part of program but before JDK7 we can't do it directly instead either we need to map those String to final integer constant or char constant to use them inside switch and case or you need to fallback on if-else statement which gets clumsy once number of cases getting increased. But now with jdk7 you can directly use String inside switch and case statement. Though it’s pretty straight forward feature let see an example of how to use String inside switch and case statement in JDK7.
}
Overall allowing String into switch and case statement is not a wow feature but in my opinion very useful one and definitely makes coding easier and make code more readable by either removing clumsy if-else statement. So I definitely vote plus one to JDK 7 String in switch feature and thanks to guys involved in project coin for making life easier of java developers.
You also need to ensure that your JRE must have source 1.7 otherwise if you try to run string in switch in JRE less than 7 you will get following errro
Exception in thread "main" java.lang.RuntimeException: Uncompilable source code - strings in switch are not supported in -source 1.6
(use -source 7 or higher to enable strings in switch)
at jdk7demo.JDK7Demo.tradingOptionChooser(JDK7Demo.java:34)
at jdk7demo.JDK7Demo.main(JDK7Demo.java:25)
That’s all from me on String and Switch on JDK7, Please share how you are using this new feature of java 7.
many a times we want to switch based upon string output received from user or other part of program but before JDK7 we can't do it directly instead either we need to map those String to final integer constant or char constant to use them inside switch and case or you need to fallback on if-else statement which gets clumsy once number of cases getting increased. But now with jdk7 you can directly use String inside switch and case statement. Though it’s pretty straight forward feature let see an example of how to use String inside switch and case statement in JDK7.Example of String in Switch in JDK7
public static void tradingOptionChooser(String trading) {
switch (trading) {
case "Stock Trading":
System.out.println("Trader has selected Stock Trading option");
break;
case "Electronic Trading":
System.out.println("Trader has selected Electronic Trading option");
break;
case "Algorithmic Trading":
System.out.println("Trader has selected Algorithmic Trading option");
break;
case "Foreign exchange trading":
System.out.println("Trader has selected Foreign exchange Trading option");
break;
case "commodity trading":
System.out.println("Trader has selected commodity trading option");
break;
default:
throw new IllegalArgumentException();
}}
Example of String in Switch prior JDK7
Now let's see how it can be done prior to JDK 7 using if-else statement:public static void tradingOptions(String trading) {
if (trading.equals("Stock Trading")) {
System.out.println("Trader has selected Stock Trading option");
} else if (trading.equals("Electronic Trading")) {
System.out.println("Trader has selected Electronic Trading option");
} else if (trading.equals("Algorithmic Trading")) {
System.out.println("Trader has selected Algorithmic Trading option");
} else if (trading.equals("Foreign exchange trading")) {
System.out.println("Trader has selected Foreign exchange Trading option");
} else if (trading.equals("commodity trading")) {
System.out.println("Trader has selected commodity trading option");
} else {
throw new IllegalArgumentException();
}
}Overall allowing String into switch and case statement is not a wow feature but in my opinion very useful one and definitely makes coding easier and make code more readable by either removing clumsy if-else statement. So I definitely vote plus one to JDK 7 String in switch feature and thanks to guys involved in project coin for making life easier of java developers.
You also need to ensure that your JRE must have source 1.7 otherwise if you try to run string in switch in JRE less than 7 you will get following errro
Exception in thread "main" java.lang.RuntimeException: Uncompilable source code - strings in switch are not supported in -source 1.6
(use -source 7 or higher to enable strings in switch)
at jdk7demo.JDK7Demo.tradingOptionChooser(JDK7Demo.java:34)
at jdk7demo.JDK7Demo.main(JDK7Demo.java:25)
That’s all from me on String and Switch on JDK7, Please share how you are using this new feature of java 7.
Here are some of my other post on Java you may find interesting:
Spring Hibernate Builder Design pattern in Java - Example Tutorial
Builder design pattern in Java is a creational pattern i.e. used to create objects, similar to factory method design pattern which is also creational design pattern. Before learning any design pattern I suggest find out the problem a particular design pattern solves. Its been well said necessity is mother on invention. learning design pattern without facing problem is not that effective, Instead if you have already faced issues than its much easier to understand design pattern and learn how its solve the issue. In this Java design pattern tutorial we will first see what problem Builder design pattern solves which will give some insight on when to use builder design pattern in Java, which is also a popular design pattern interview question and then we will see example of Builder design pattern and pros and cons of using Builder pattern in Java.
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