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Tips and tricks for working with Java.

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README.md

You learn a lot of Java here at Lehigh, but when it comes down to it, you’re just scratching the surface.

IDE stands for Integrated Development Environment. It’s basically an advanced code editor that provides extra features on top of editing text, like file management, automating builds, and testing. Dr. Java, the editor used in CSE 2, is not an IDE. Netbeans, often used in CSE 17, is. However, not many professionals use Netbeans. Instead, most of them use Eclipse and IntelliJ.

Free to use. You can just grab the standard version. Download

There’s a professional version you have to pay for, but they also have a free Community Edition that will satisfy all of your needs. This project includes IntelliJ project files to make getting started easy. Download

Compiling a Project from the Command Line

If you stick with Java, at some point in your life, you’ll run into a situation where you can’t have an IDE compile for you. For example, for small things, I personally don’t like to use IDEs at all. I use my favorite text editor, Sublime Text, which doesn’t compile Java out of the box. Thankfully, doing simple Java compilations are a piece of cake (although they can quickly get quite complex).

1. Open your terminal of choice.
2. Navigate to the folder of your .java file.
3. To compile your program, first run the command javac YourFile.java . This will create the file YourFile.class . If there were any compilation errors, it’ll tell you.
4. To actually run your program, use the command java YourFile . Note that we didn’t put any extension after YourFile . If YourFile.class exists, Java will automatically grab and run it.
5. That’s it! Pretty simple, right? You could write Java programs in Notepad this way (although I wouldn’t recommend it. ).

About

Tips and tricks for working with Java.

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Essential Java Tips and Tricks for Programmers

For String manipulations: Use StringBuffer for string manipulations, as String in java is immutable. Using strings carefully — One should be clear with the concept of strings, and how they work when different symbols are used with it, whether it is a single string or an array of string.

Essential Java Tips and Tricks for Programmers

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Understand the difference between an array and an arraylist clearly- Data structures play an important role since they decide how data will be stored, how it will be accessed, manipulated, and finally shown to the user.

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Java Tips and Tricks | Java Tutorial for Beginners

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Java tricks for competitive programming (for Java 8)

Although the practice is the only way that ensures increased performance in programming contests but having some tricks up your sleeve ensures an upper edge and fast debugging.
1) Checking if the number is even or odd without using the % operator:
Although this trick is not much better than using a % operator but is sometimes efficient (with large numbers).

Use & operator:

num = 5 Binary: "101 & 1" will be 001, so false num = 4 Binary: "100 & 1" will be 000, so true.

2) Fast Multiplication or Division by 2

Multiplying by 2 means shifting all the bits to left and dividing by 2 means shifting to the right.

Example: 2 (Binary 10): shifting left 4 (Binary 100) and right 1 (Binary 1)

3) Swapping of 2 numbers using XOR:

This method is fast and doesn’t require the use of the 3rd variable.

// A quick way to swap a and b a ^= b; b ^= a; a ^= b;

4) Faster I/O:

Refer here for Fast I/O in java

5) For String manipulations:

Use StringBuffer for string manipulations, as String in java is immutable. Refer here.

6) Calculating the most significant digit:

To calculate the most significant digit of any number log can be directly used to calculate it.

Suppose the number is N then Let double K = Math.log10(N); now K = K - Math.floor(K); int X = (int)Math.pow(10, K); X will be the most significant digit.

7) Calculating the number of digits directly:

To calculate the number of digits in a number, instead of looping we can efficiently use log :

No. of digits in N = Math.floor(Math.log10(N)) + 1;

8) Inbuilt GCD Method:

Java has inbuilt GCD method in BigInteger class. It returns a BigInteger whose value is the greatest common divisor of abs(this) and abs(val). Returns 0 if this==0 && val==0.

public BigInteger gcd(BigInteger val) Parameters : val - value with which the GCD is to be computed. Returns : GCD(abs(this), abs(val))

Below is the implementation of GCD:

Java

9) checking for a prime number:

Java has an inbuilt isprobableprime() method in BigInteger class. It returns true if this BigInteger is probably prime(with some certainty), false if it’s definitely composite.

BigInteger.valueOf(1235).isProbablePrime(1)

10) Efficient trick to know if a number is a power of 2 :

The normal technique of division the complexity comes out to be O(logN), but it can be solved using O(v) where v is the number of digits of the number in binary form.

Java

11) Sorting Algorithm:

1. Arrays.sort() used to sort elements of an array.
2. Collections.sort() used to sort elements of a collection.

For primitives, Arrays.sort() uses Dual pivot Quicksort algorithms.

12) Searching Algorithm:

1. Arrays.binarySearch()(SET 1 | SET2) used to apply binary search on a sorted array.
2. Collections.binarySearch() used to apply binary search on a collection based on comparators.

13) Copy Algorithm:

1. Arrays.copyOf() and copyOfRange() copy the specified array.
2. Collections.copy() copies specified collection.

14) Rotation and Frequency

We can use Collections.rotate() to rotate a collection or an array by a specified distance. You can also use Collections.frequency() method to get the frequency of a specified element in a collection or an array.

15) Most data structures are already implemented in the Collections Framework .

For example Stack, LinkedList, HashSet, HashMaps, Heaps etc.

16) Use Wrapper class functions for getting radix conversions of a number Sometimes you require radix conversion of a number. For this, you can use wrapper classes.

Java

Binary representation of A : 1000001101 Binary representation of B : 1011100110011101111100110101101101 Octal representation of A : 1015 Octal representation of B : 134635746555

17) NullPointerException(Why ?) Refer here and here to avoid it.

This article is contributed by Gaurav Miglani . If you like GeeksforGeeks and would like to contribute, you can also write an article using write.geeksforgeeks.org or mail your article to review-team@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.
Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above.

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Tips and tricks when working with Java Strings, for performance, security

I find my self constantly changing my utility methods to improve string handling in my Java code. For example I have changed bunch of my search and replace code to use commons StringUtils.replace method. Or upgrading 1.4 to 1.6 java to type safe code.

I would like to ask you to post practices you employ in order to have your string operations run smoothly, securely, fast and the re-usability of your code is fairly simple and elegant.

And if you have pattern to it, even better.

Also if you now about any new 1.7 java features that are worth the while please post them here.

What to do when working with very large strings? Break them up?

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Are there libraries that have similar functions as grep, ack, diff | spell check | filter curse words (any words) .

When possible you should use StringBuilder , especially for concatenating strings. The performance improvement can be very large.

StringBuilder sb = new StringBuilder("Mat"); sb.append(" "); sb.append("Bank"); // oops int i = sb.indexOf("k"); sb.insert(i, 'i'); // character String mb = sb.toString(); // result = "Mat Banik" 

In a large program the use of s1+s2 is one of the worst and simplest performance hits to cure.

StringBuilder has almost all the features of String. You can extract substrings without copying. When you need a String (e.g. for Pattern/Matcher) you can convert with toString().

Security-wise, make sure to escape properly when you combine strings of different content types, for example, when concatenating a plain text string with a string of HTML to produce a string of HTML.

http://commons.apache.org/lang/api-2.5/org/apache/commons/lang/StringEscapeUtils.html has a bunch of useful escaping functions and their inverses.

For example, to avoid XSS attacks, you can encode output like:

void doGet(HttpServletRequest req, HttpServletResponse resp) < String message = req.getParameter("message"); // Unless I check, in code, that an input is of some other content type, // I need to conservatively assume it's plain text. . resp.setContentType("text/html;charset=UTF-8"); . // Since resp is a channel with content-type text/html, // I need to only write HTML to it. resp.getWriter().write( "
" // This is already HTML. + StringEscapeUtils.escapeHtml(message) // plain text -> innocuous HTML + "
" // Also already HTML. ); . > 

Remember that String.substring() only creates a new «view» into the original char array. So, when taking a small substring of a large String and storing it, it’s a good idea to create a new String of just the substring. This avoids a hard to track down memory leak where your substrings really hold the entire original string.

Beware though that IBM’s and Sun’s JVMs String constructor implementations behave differently. Sun’s does what you expect — it creates a new char array. IBM’s however does not, you have to get the char array first. To avoid this, I had to do the following ugly bit of code:

private static final boolean IS_IBM_JVM = System.getProperty("java.vm.vendor").startsWith("IBM"); . if (IS_IBM_JVM) < substring = new String(substring.toCharArray()); >else

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