Java 9 : Collections factory methods

As of Java 8, there’s  no simple or single statement way of creating collections with a small number of elements. In scenarios like unit tests where a Set of Strings are to be created as mock data,  it is needed to create a new Set and start repeatedly adding the elements which is highly verbose. Java 9 simplifies it through varargs based factory methods to create collections with a small number of elements. List, Set and Map interfaces have of method that creates the respective immutable Collection/Map. These convenience methods are implemented based on varargs

Creating a List

Prior to Java 9, immutable list is created using

List<String> immutableList = Arrays.asList("Cersei", "Hound", "Ilyn Payne", "Melissandre", "Beric Dondarrion");

if we add an element to the immutableList,

immutableList.add("Walder Frey");

an UnsupportedOperationException is thrown. This is OK as we expect the list to be immutable. But the List returned by Arrays.asList is vulnerable to set operation

immutableList.set(2, "Walder Frey");

after this operation, Ilyn Payne is replaced by Walder Frey.

in Java 9 an immutable list can be created using

List<String> immuatbleListJ9 = List.of("Cersei", "Hound", "Ilyn Payne", "Melissandre", "Beric Dondarrion");

if we add an element to the immutableListJ9

immutableListJ9.add("Walder Frey");

an UnsupportedOperationException is thrown as expected.

if we try set operation

immutableListJ9.set(2, "Walder Frey");

an UnsupportedOperationExceptionon is thrown.

Creating a Set

Prior to Java 9, there’s no easy way to create a Set
One way is to create a new Set and add elements one by one

Set<String> theSet = new HashSet&lt;&gt;();
theSet.add("Cersei");
theSet.add("Hound");
theSet.add("Ilyn Payne");

other way is to use

Set<String> theSet = new HashSet<>(Arrays.asList("Cersei", "Hound", "Ilyn Payne", "Melissandre", "Beric Dondarrion")); 

The Set created in above ways is mutable, we can keep adding or removing elements to or from it. This Set can be converted to immutable with the help of Collections.unmodifiableSet

in Java 9, the Set can be created using

Set<String> theSetJ9 = Set.of("Cersei", "Hound", "Ilyn Payne", "Melissandre", "Beric Dondarrion");

The Set created using factory method is an immutable set, it con’t be modified. If try to add or remove an element to or from this set, an UnsupportedOperationException is thrown.

Creating a Map

Prior to Java 9, Map also doesn’t have a easy way to create

Map<Integer, String> theMap = new HashMap&gt;();
theMap.put(1, "Cersei");
theMap.put(2, "Hound");
theMap.put(3, "Ilyn Payne");

The Map created in above way is mutable, we can keep adding or removing entries to or from it. This map can be converted to immutable with the help of Collections.unmodifiableMap

in Java 9, the Map can be created using

Map<Integer, String> theMapJ9 = Map.of(1, "Cersei", 2, "Hound", 3, "Ilyn Payne", 4, "Melissandre",
5, "Beric Dondarrion");

Map.of() method expects the arguments in key1, value, key2, value2 …. keyn, valuen order. As the map needs two different types of elements, Map.of method doesn’t take varags instead it has over loaded versions that takes upto 10 key-value pairs. Map has another version that takes the varags of Map.Entry<K,V>; Map.ofEntries(Map.Entry<K,V> ....). To facilitate the easy creation of Map.Entry objects, another convenience method, Map.entry() is added to the Map interface. Using these two convenience methods a Map object can be created

Map<Integer, String> theMapJ9 = Map.ofEntries(entry(1, "Cersei"), entry(2, "Hound"), entry(3, "Ilyn Payne"),
entry(4, "Melissandre"), entry(5, "Beric Dondarrion")); 

If we perform a put operation on this map, an UnsupportedOperationException is thrown.

The convenience factory methods of List, Set and Map interfaces provides an easy way to create an instance of respective Collection or Map with a small number of elements. The Objects returned by these methods are immutable, any operation trying to modify them will throw an UnsupportedOperationException.

JShell : A Primer


JShell is the REPL (Read-Eval-Print Loop) for java introduced as part of JDK 9. It is an interactive command line interface that enables the users to execute a single statement at a time, JShell evaluates and prints the result of the statement. The statement can be any valid java statement ranging from declaration of a variable to a basic arithmetic operation ….. to defining a class.

JShell is a handy tool for the language learners  and for experienced java programmers as well to try out something quickly. We all know the pain we went through when we started learning Java; to run a simple System.out.println, we had to create a Class with main method, compile it and run it. Even the worse when we want to do it in IDE, have to create a Project, then package and class. It’s a kind of turn off for someone who’s experienced in Scala or Python and trying to learn Java. Now with the JShell, it’s a more seamless learning experience for beginners of the Java, you are not obligated to create a class and a main method and even liberated from the semicolon at the end of every statement.

JShell can be found in bin directory of the JDK installation, can be launched from the command line interface of the operating system. I have used the windows 10. Below screen shot shows the command to launch JShell.

launch.jpg
Launch JShell

What can we do with JShell

  1. Perform simple arithmetic operations such as 1+2 or 5%2, Jshell evaluates and prints the resultarithmetic.jpg
  2. Print a string or any value by directly typing System.out.print. We can declare a variable and then we can print it. The below image shows printing of String directly typed in System.out.print and declaring a starting variable and then printing it. Once a variable is declared, JShell prints the variable name and value PrintString.jpg
  3. Execute a loop statement, print values from 0 to 9 using for looploop.jpg
  4. Declare a method and call that method. In the example shown in the image, I have declared a method printString that takes a String argument and prints it, then I called that method with the String value “Calling a method from JShell”; the method is executed and the string value is printed on the console. Method.jpgif we type any semantically wrong code, an error is shown. I have tried inserting an extra double quote inside the println after the variable name and I saw the error as belowFunctionError.jpg
  5. View the history of the statements executed with /list command.list.jpg
  6. View the variables and the methods declared with /v and /m commandsVariablesNMethods.jpg$1 is the variable that is created as a result of the arithmetic operation 1+2. When user performs an operation without assigning the result to a variable, JShell creates a variable automatically, $1 and $2 are such variables and str is the variable for the String literal created.
  7. JShell evaluates the method dependencies,  shows a warning if a method definition is not found. If we declare a method, method1 and if it calls an undefined method, method2, JShell shows warning as shown in the image belowmethod_dep.jpgif we list the methods now with /m command, its lists the methods printString and method1 as well it shows the warning for the missing method2. After creating the method2, /m shows the methods printString, method1 and method2 without warnings. method_dep2.jpg
  8. Update the value of variable by assigning a new value to it. We can not change the type of a variable without declaring it again with the new type. Below image shows an example of updating an int variable and creating a new variable with the same name and different type; the int variable is removed in this caseupdate_var.jpg
  9. Edit the history of the statements with /edit command. When /edit is performed,  JShell Edit Pad (a pop up) comes with the history of the statements executed. We can edit one or more statements or can add/delete a statement. In the example shown below I have added a statement to include a new String variable, blogedit.jpgNow the list of variables shows the newly added String variable, blogedit_result.png
  10. Save all the statements executed to a file using /save file_name command . File extension can be anything. The saved file can be opened using /open filen_name
  11. Create a Class,  instantiate a class, call a method on the instance. Class.jpg

Jshell is a quick means to test code snippets. With the JShell java programmers are no more obliged to declare variables or methods inside a Class. JShell acts like a Unix shell for Java statements. 

Hope this article helped in giving you a good understanding of how to use JShell.