In this article, we will learn about Java lambda expression and the use of lambda expression with functional interfaces, generic functional interface, and stream API with the help of examples.<\/p>\n\n\n\n
The lambda expression was introduced first time in Java 8. Its main objective to increase the expressive power of the language.<\/p>\n\n\n\n
But, before getting into lambdas, we first need to understand functional interfaces.<\/p>\n\n\n\n
If a Java interface contains one and only one abstract method then it is termed as functional interface. This only one method specifies the intended purpose of the interface.<\/p>\n\n\n\n
For example, the In the above example, the interface MyInterface has only one abstract method getValue(). Hence, it is a functional interface.<\/p>\n\n\n\n Here, we have used the annotation In Java 7, functional interfaces were considered as\u00a0Single Abstract Methods or\u00a0SAM<\/strong>\u00a0type. SAMs were commonly implemented with Anonymous Classes in Java 7.<\/p>\n\n\n\n Output<\/strong>:<\/p>\n\n\n\n Here, we can pass an anonymous class to a method. This helps to write programs with fewer codes in Java 7. However, the syntax was still difficult and a lot of extra lines of code were required.<\/p>\n\n\n\n Java 8 extended the power of a SAMs by going a step further. Since we know that a functional interface has just one method, there should be no need to define the name of that method when passing it as an argument. Lambda expression allows us to do exactly that.<\/p>\n\n\n\n Lambda expression is, essentially, an anonymous or unnamed method. The lambda expression does not execute on its own. Instead, it is used to implement a method defined by a functional interface.<\/p>\n\n\n\n Here is how we can define lambda expression in Java.<\/p>\n\n\n\n The new operator ( Suppose, we have a method like this:<\/p>\n\n\n\n We can write this method using lambda expression as:<\/p>\n\n\n\n Here, the method does not have any parameters. Hence, the left side of the operator includes an empty parameter. The right side is the lambda body that specifies the action of the lambda expression. In this case, it returns the value 3.1415.<\/p>\n\n\n\n In Java, the lambda body is of two types.<\/p>\n\n\n\n 1. A body with a single expression<\/strong><\/p>\n\n\n\n This type of lambda body is known as the expression body.<\/p>\n\n\n\n 2. A body that consists of a block of code.<\/strong><\/p>\n\n\n\n This type of the lambda body is known as a block body. The block body allows the lambda body to include multiple statements. These statements are enclosed inside the braces and you have to add a semi-colon after the braces.<\/p>\n\n\n\n Note<\/strong>: For the block body, you can have a return statement if the body returns a value. However, the expression body does not require a return statement.<\/p>\n\n\n\n Let’s write a Java program that returns the value of Pi using the lambda expression.<\/p>\n\n\n\n As mentioned earlier, a lambda expression is not executed on its own. Rather, it forms the implementation of the abstract method defined by the functional interface.<\/p>\n\n\n\n So, we need to define a functional interface first.<\/p>\n\n\n\n Output<\/strong>:<\/p>\n\n\n\n In the above example,<\/p>\n\n\n\n Till now we have created lambda expressions without any parameters. However, similar to methods, lambda expressions can also have parameters. For example,<\/p>\n\n\n\n Here, the variable n inside the parenthesis is a parameter passed to the lambda expression. The lambda body takes the parameter and checks if it is even or odd.<\/p>\n\n\n\n Output<\/strong>:<\/p>\n\n\n\n Till now we have used the functional interface that accepts only one type of value. For example,<\/p>\n\n\n\n The above functional interface only accepts\u00a0 Output<\/strong>:<\/p>\n\n\n\n In the above example, we have created a generic functional interface named GenericInterface<\/var>. It contains a generic method named Here, inside the Main class,<\/p>\n\n\n\n The new\u00a0java.util.stream\u00a0package has been added to JDK8 which allows java developers to perform operations like search, filter, map, reduce, or manipulate collections like\u00a0 For example, we have a stream of data (in our case a For this, we can perform bulk operations in the stream by the combination of Stream API and Lambda expression.<\/p>\n\n\n\n Output<\/strong>:<\/p>\n\n\n\n In the above example, notice the statement,<\/p>\n\n\n\n Here, we are using the methods like We can also define our own expressions based on the syntax we learned above. This allows us to reduce the lines of code drastically as we saw in the above example.<\/p>\n","protected":false},"excerpt":{"rendered":" In this article, we will learn about Java lambda expression and the use of lambda expression with functional interfaces, generic functional interface, and stream API with the help of examples. The lambda expression was introduced first time in Java 8. Its main objective to increase the expressive power of the language. But, before getting into […]<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[597],"tags":[],"_links":{"self":[{"href":"https:\/\/mudassirbackup.infinitycodestudio.com\/index.php\/wp-json\/wp\/v2\/posts\/1310"}],"collection":[{"href":"https:\/\/mudassirbackup.infinitycodestudio.com\/index.php\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/mudassirbackup.infinitycodestudio.com\/index.php\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/mudassirbackup.infinitycodestudio.com\/index.php\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/mudassirbackup.infinitycodestudio.com\/index.php\/wp-json\/wp\/v2\/comments?post=1310"}],"version-history":[{"count":0,"href":"https:\/\/mudassirbackup.infinitycodestudio.com\/index.php\/wp-json\/wp\/v2\/posts\/1310\/revisions"}],"wp:attachment":[{"href":"https:\/\/mudassirbackup.infinitycodestudio.com\/index.php\/wp-json\/wp\/v2\/media?parent=1310"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/mudassirbackup.infinitycodestudio.com\/index.php\/wp-json\/wp\/v2\/categories?post=1310"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/mudassirbackup.infinitycodestudio.com\/index.php\/wp-json\/wp\/v2\/tags?post=1310"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}Runnable<\/code> interface from package java.lang<\/code>; is a functional interface because it constitutes only one method i.e. run()<\/code>.<\/p>\n\n\n\nExample 1: Define a Functional Interface in java<\/h3>\n\n\n\n
import java.lang.FunctionalInterface;\n@FunctionalInterface\npublic interface MyInterface{\n \/\/ the single abstract method\n double getValue();\n}<\/code><\/pre>\n\n\n\n@FunctionalInterface<\/code>. The annotation forces the Java compiler to indicate that the interface is a functional interface. Hence, does not allow to have more than one abstract method. However, it is not compulsory though.<\/p>\n\n\n\nExample 2: Implement SAM with anonymous classes in java<\/h3>\n\n\n\n
public class FunctionInterfaceTest {\n public static void main(String[] args) {\n\n \/\/ anonymous class\n new Thread(new Runnable() {\n @Override\n public void run() {\n System.out.println(\"I just implemented the Runnable Functional Interface.\");\n }\n }).start();\n }\n}<\/code><\/pre>\n\n\n\nI just implemented the Runnable Functional Interface.<\/samp><\/pre>\n\n\n\n
\n\n\n\nIntroduction to lambda expressions<\/h2>\n\n\n\n
How to define lambda expression in Java?<\/h3>\n\n\n\n
(parameter list) -> lambda body<\/code><\/pre>\n\n\n\n-><\/code>) used is known as an arrow operator or a lambda operator. The syntax might not be clear at the moment. Let’s explore some examples,<\/p>\n\n\n\ndouble getPiValue() {\n return 3.1415;\n}<\/code><\/pre>\n\n\n\n() -> 3.1415<\/code><\/pre>\n\n\n\n
\n\n\n\nTypes of Lambda Body<\/h3>\n\n\n\n
() -> System.out.println(\"Lambdas are great\");<\/code><\/pre>\n\n\n\n() -> {\n double pi = 3.1415;\n return pi;\n};<\/code><\/pre>\n\n\n\n
\n\n\n\nExample 3: Lambda Expression<\/h2>\n\n\n\n
import java.lang.FunctionalInterface;\n\n\/\/ this is functional interface\n@FunctionalInterface\ninterface MyInterface{\n\n \/\/ abstract method\n double getPiValue();\n}\n\npublic class Main {\n\n public static void main( String[] args ) {\n\n \/\/ declare a reference to MyInterface\n MyInterface ref;\n \n \/\/ lambda expression\n ref = () -> 3.1415;\n \n System.out.println(\"Value of Pi = \" + ref.getPiValue());\n } \n}<\/code><\/pre>\n\n\n\nValue of Pi = 3.1415<\/samp><\/code><\/pre>\n\n\n\ngetPiValue()<\/code><\/li>
\/\/ it will throw an error MyInterface ref = new myInterface(); \/\/ it is valid MyInterface ref;<\/code><\/li>ref = () -> 3.1415;<\/code><\/li>getPiValue()<\/code> using the reference interface. When
System.out.println(\"Value of Pi = \" + ref.getPiValue());<\/code><\/li><\/ul>\n\n\n\n
\n\n\n\nLambda Expressions with parameters<\/h2>\n\n\n\n
(n) -> (n%2)==0<\/code><\/pre>\n\n\n\nExample 4: Using lambda expression with parameters<\/h3>\n\n\n\n
@FunctionalInterface\ninterface MyInterface {\n\n \/\/ abstract method\n String reverse(String n);\n}\n\npublic class Main {\n\n public static void main( String[] args ) {\n\n \/\/ declare a reference to MyInterface\n \/\/ assign a lambda expression to the reference\n MyInterface ref = (str) -> {\n\n String result = \"\";\n for (int i = str.length()-1; i >= 0 ; i--)\n result += str.charAt(i);\n return result;\n };\n\n \/\/ call the method of the interface\n System.out.println(\"Lambda reversed = \" + ref.reverse(\"Lambda\"));\n }\n\n}<\/code><\/pre>\n\n\n\nambda reversed = adbmaL<\/samp><\/code><\/pre>\n\n\n\n
\n\n\n\nGeneric Functional Interface<\/h2>\n\n\n\n
@FunctionalInterface\ninterface MyInterface {\n String reverseString(String n);\n}<\/code><\/pre>\n\n\n\nString<\/code>\u00a0and returns\u00a0String<\/code>. However, we can make the functional interface generic, so that any data type is accepted. If you are not sure about generics, visit\u00a0Java Generics.<\/p>\n\n\n\nExample 5: Generic Functional Interface and Lambda Expressions<\/h3>\n\n\n\n
\/\/ GenericInterface.java\n@FunctionalInterface\ninterface GenericInterface<T> {\n\n \/\/ generic method\n T func(T t);\n}\n\n\/\/ GenericLambda.java\npublic class Main {\n\n public static void main( String[] args ) {\n\n \/\/ declare a reference to GenericInterface\n \/\/ the GenericInterface operates on String data\n \/\/ assign a lambda expression to it\n GenericInterface<String> reverse = (str) -> {\n\n String result = \"\";\n for (int i = str.length()-1; i >= 0 ; i--)\n result += str.charAt(i);\n return result;\n };\n\n System.out.println(\"Lambda reversed = \" + reverse.func(\"Lambda\"));\n\n \/\/ declare another reference to GenericInterface\n \/\/ the GenericInterface operates on Integer data\n \/\/ assign a lambda expression to it\n GenericInterface<Integer> factorial = (n) -> {\n\n int result = 1;\n for (int i = 1; i <= n; i++)\n result = i * result;\n return result;\n };\n\n System.out.println(\"factorial of 5 = \" + factorial.func(5));\n }\n}<\/code><\/pre>\n\n\n\nLambda reversed = adbmaL\nfactorial of 5 = 120<\/samp><\/code><\/pre>\n\n\n\nfunc()<\/code>.<\/p>\n\n\n\nGenericInterface<String> reverse<\/code> – creates a reference to the interface. The interface now operates on String<\/code> type of data.<\/li>GenericInterface<Integer> factorial<\/code> – creates a reference to the interface. The interface, in this case, operates on the Integer<\/code> type of data.<\/li><\/ul>\n\n\n\n
\n\n\n\nLambda Expression and Stream API<\/h2>\n\n\n\n
Lists<\/code>.<\/p>\n\n\n\nList<\/code> of String<\/code>) where each string is a combination of country name and place of the country. Now, we can process this stream of data and retrieve only the places from Nepal.<\/p>\n\n\n\nExample 6: Demonstration of using lambdas with the Stream API<\/h3>\n\n\n\n
import java.util.ArrayList;\nimport java.util.List;\n\npublic class StreamMain {\n\n \/\/ create an object of list using ArrayList\n static List<String> places = new ArrayList<>();\n\n \/\/ preparing our data\n public static List getPlaces(){\n\n \/\/ add places and country to the list\n places.add(\"Nepal, Kathmandu\");\n places.add(\"Nepal, Pokhara\");\n places.add(\"India, Delhi\");\n places.add(\"USA, New York\");\n places.add(\"Africa, Nigeria\");\n\n return places;\n }\n\n public static void main( String[] args ) {\n\n List<String> myPlaces = getPlaces();\n System.out.println(\"Places from Nepal:\");\n \n \/\/ Filter places from Nepal\n myPlaces.stream()\n .filter((p) -> p.startsWith(\"Nepal\"))\n .map((p) -> p.toUpperCase())\n .sorted()\n .forEach((p) -> System.out.println(p));\n }\n\n}<\/code><\/pre>\n\n\n\nPlaces from Nepal:\nNEPAL, KATHMANDU\nNEPAL, POKHARA<\/samp><\/code><\/pre>\n\n\n\nmyPlaces.stream()\n .filter((p) -> p.startsWith(\"Nepal\"))\n .map((p) -> p.toUpperCase())\n .sorted()\n .forEach((p) -> System.out.println(p));<\/code><\/pre>\n\n\n\nfilter()<\/code>, map()<\/code> and forEach()<\/code> of the Stream API. These methods can take a lambda expression as input.<\/p>\n\n\n\n