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Java Lambda Expressions

This post explores Oracle's Java 8 specification for lambda expression , the syntax and use cases. Lambda expressions can be used as method arguments or to iterate, filter, and extract data from a collection.

Uday Chauhan

Functional interfaces

I'm sure you're aware that Java is not considered a functional programming language. It's considered an object oriented programming language but with

Java 8⁠

, we do have functional interfaces. With the introduction of

Java 8⁠

, Oracle added two new packages, the

java.util.function⁠

and the

java.util.stream⁠

.The lambda expressions, also introduced with

Java 8⁠

, use both of these new packages so we wanted to review some of the functional interfaces that are now available.

Definition of functional interface.

It is defined as an interface that contains only one abstract function or method.

Functional interfaces can represent abstract concepts such as functions, actions or predicates.

We will review some of the most commonly used functional interfaces in lambda notation but there are more interfaces available in the

java.util.function⁠

package. To see a list of all the functional interfaces available, check out the Javadoc for

java.util.function⁠

Some of the more commonly used interfaces include Predicate.

Predicates :

Predicates ⁠

are Boolean valued functions of one argument meaning they take in one argument, use a test method to evaluate it and return either true or false. Since this is an interface, we will have to override the test method with logic that determines what should be evaluated.

Consumer :The

Consumer ⁠

interface consumes the argument. It accepts a single argument and does not return a result.

Function :

Function ⁠

which transforms a value from one type to another. It accepts one argument and produces a result.

Supplier:

Supplier ⁠

supplies a value. It produces a result of a given type. Unlike Functions, Suppliers do not accept arguments but they do return a result.

UnaryOperator :

UnaryOperator⁠

interface takes a single argument and returns a single value and the

BinaryOperator⁠

interface takes two arguments and returns one.

Here, I’m using the

Predicates⁠

interface.

//using the test method of Predicate

Predicate<String> stringLen = (s)-> s.length() < 10;

System.out.println(stringLen.test("Apples") + " - Apples is less than 10");

As you can see, in the angle brackets after the word Predicate I’m forming the interface to expect a String. It has a name of stringLen and it’s equal to on the right hand side is where I’m actually implementing my method.

I’m used lambda notation so it might look a little different than what you’re used to. On the right hand side in parentheses, I list either no arguments, a single argument or multiple arguments. For Predicate, I need to pass in a single argument then I have the arrow which is a hyphen(-) and a greater than sign(>). Make sure those two are together. After that, I have the logic of my Predicate. So in this case, I’m saying if the string.length or the length of the string is < 10, return true. Otherwise, return false. For each of my examples, I print out the result. So calling stringLen.test() with the value “Apples” and it’ll print out true or false.

Here, I’m using the

Consumer⁠

interface.

//Consumer example uses accept method

Consumer<String> consumerStr = (s) -> System.out.println(s.toLowerCase());

consumerStr.accept("ABCDefghijklmnopQRSTuvWxyZ");

Remember, the

Consumer⁠

interface accepts a value but does not return anything. So on the right hand side, I’m passing it, the argument s but the actual method itself is just going to print out that value in all lowercase letters. It doesn’t return anything.

Now, I have a

Function⁠

interface which has two values in the angle brackets.

//Function example

Function<Integer,String> converter = (num)-> Integer.toString(num);

System.out.println("length of 26: " + converter.apply(26).length());

The

Function⁠

interface, the first value represents the value coming in for the argument and the second value represents the return value. So in this case, I’m passing in an integer and I’m going to return a string. The logic that I use to do that is on the right hand side of the arrow that says integer.toString() num where num is my argument, I’m going to pass in a value of 26 which gets converted to a string which will just be the characters 2 and 6 which will have a length of 2.

Here is my

Supplier⁠

interface.

//Supplier example

Supplier<String> s = ()-> "Java is fun";

System.out.println(s.get());

Remember, the

Supplier⁠

does not contain any arguments. That’s why I have the open and closed parentheses with nothing inside. It’s important to make sure that you understand you have to include the empty argument list. Otherwise, the compiler will think you made a mistake. So we have an empty argument list, our arrow and on the right hand side, I have a string that says “Java is fun”. It uses the get() method which is the functional method for Supplier to get the value of the string and I have that inside of a print line statement so it’ll print it out.

//Binary Operator example

BinaryOperator<Integer> add = (a, b) -> a + b;

System.out.println("add 10 + 25: " + add.apply(10, 25));

The

BinaryOperator⁠

is going to return an integer. On the right hand side, you can see it expects two values, a and b. It’s going to then add them together and return those values. Now it is so far that none of my arguments have their data types explicitly mentioned. That’s because when we’re using lambda notation, it is inferred. So since the

BinaryOperator⁠

is returning an integer, it knows that the two values coming in as arguments, a and b, must also be integers. I’m using the apply method of the

BinaryOperator⁠

to add 10 and 25.

//Unary Operator example

UnaryOperator<String> str = (msg)-> msg.toUpperCase();

System.out.println(str.apply("This is my message in upper case"));

The

UnaryOperator⁠

is going to return a string. It takes in a string( msg) the message, and it converts it to uppercase. On next line, I’m going to print that out using the

UnaryOperator⁠

method .apply().

Here is the full class :

⁠

Here is the output: In the Output window,

⁠

the first was the

Predicates⁠

which returned true that the string Apples is less than 10 characters.

Next, I used the

Consumer⁠

interface to take in a series of letters representing the alphabet in all upper and lowercase letters mixed. It used the two lower to print it back out in all lowercase followed by the Function interface which took in an integer, 26, converted it to a string of two characters, two and six and then found the length of that string which was two.

The

Supplier⁠

actually supplied the string, Java is fun. So you can see in line 31 right above, I’m using the s.get. It gets the value, “Java is fun” and prints it to the command line followed by the

BinaryOperator⁠

which took 10 and 25, added them together and gave us 35 and the last one was the

UnaryOperator⁠

that took in a message that was in mixed case and returned that message in all uppercase.

So these are a few of the more commonly used functional interfaces available with

Java 8⁠

. Remember, there are a lot more that you can use so don’t forget to check out that Javadoc on

java.util.function⁠

Lambda syntax

If you’ve ever written programs that involve threads, you’re familiar with the concept of a

Runnable interface⁠

The

Runnable interface⁠

does not take any arguments, and we can use lambda notation to help us write the inner class needed to implement this interface.

// Anonymous Inner Class: Runnable

Runnable r1 = new Runnable() {

@Override

public void run() {

System.out.println("run 1");

}

};

Here, I’m using the old style of creating an anonymous inner class.

// Lambda version of Runnable (no arguments)

Runnable r2 = () -> System.out.println("run 2");

It is the new version of implementing that inner class using Runnable r2 equals. Open and close parentheses to indicate no arguments, and we’re printing out run 2.

// Start both threads

r1.run();

r2.run();

Now, we start r1 and r2 by using r1.run(), r2.run(), and that’ll initiate those methods.

I have some additional examples so you can see some more lambda syntax.

//using an existing functional interface BiFunction

BiFunction<String, String, String> concat = (a, b) -> a + b;

String sentence = concat.apply("Today is ", "a great day");

System.out.println(sentence);

Here is the

BiFunction interface⁠

, which has angle brackets with three string values inside. This indicate the two values that are coming in are going to be strings, and the return value will be a string. On the right-hand side is what we call our lambda notation: parentheses around the two arguments, (a, b) , our arrow notation (->), and then a + b. When you use a + b and you have strings, it puts them together. I use the concat.apply() to put together “Today is a great day”.

//example of the Consumer functional interface

Consumer<String> hello = name -> System.out.println("Hello, " + name);

for (String name : Arrays.asList("Duke", "Mickey", "Minnie")) {

hello.accept(name);

}

Here, I’m showing a

consumer interface⁠

. In this case, we’re passing in a name and printing out hello with that name. I have it set up so that string name is going to take each name in the Arrays.asList() with Duke, Mickey, and Minnie, and then it’ll invoke the hello.accept(name), which’ll print

Hello, Duke

Hello, Mickey

Hello, Minnie

And finally

//example of passing one value

GreetingFunction greeting = message ->

System.out.println("Java Programming " + message);

greeting.sayMessage("Rocks with lambda expressions");

}

//custom functional interface

@FunctionalInterface

interface GreetingFunction {

void sayMessage(String message);

}

Now, we have an example of passing one value. We have GreetingFunction greeting = message. message is the one parameter that’s getting passed. Notice that, we’re actually creating the functional interface called GreetingFunction. I wanted to show you how you can create your own interface, as well as use the existing ones in the java.util.function.

Here the class:

⁠

Now if you run this program, you'll see the first two threads print out as:

⁠

I hope these examples are starting to make sense as far as the syntax of a lambda notation.

Methods as lambdas

Another feature of

Java 8⁠

is we can now take any type of method and convert it into a lambda including static method, instance methods, and even constructors.

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Loading…⁠

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Do you notice any syntax that looks a little different? If you've programmed in C++ before you might recognize this symbol that I'm referring to. Notice on the right hand side where the lambda notation would normally be, we have integer::ToString. This is the structure that creates a lambda from a method. It's called a method reference and it enables us to pass references of methods or constructors via the :: syntax.

IntFunction<String> intToString = num -> Integer.toString(num);

System.out.println("expected value 3, actual value: " +

intToString.apply(123).length());

we have the interface int function which is going to return a string and take in an integer. It's a special function in function, we don't have to specify the data type of our argument num because the interface provides enough information to the compiler for it to infer that num is going to be an integer. On the right hand side of our arrow we have integer.toString which takes num, converts it to a string.

Next we're printing out the value, intTwoString.apply where apply is the functional method of the int function interface. It'll convert the number, 123, to a string and get the length.

//static method reference using ::

IntFunction<String> intToString2 = Integer::toString;

System.out.println("expected value 4, actual value: " +

intToString2.apply(4567).length());

And as I stated we're using this new method reference, the ::. So this time we don't even specify the variable name or the argument on the right hand side. We just have integer::toString. Now the compiler is smart enough to know that it should expect an integer and return a string. I'm actually going to use the 4567 which is an integer which will be converted to a string.

There's a few more examples.

//lambdas made using a constructor

Function<String,BigInteger> newBigInt = BigInteger::new;

System.out.println("expected value: 123456789, actual value: "+

newBigInt.apply("123456789"));

I'm using the function interface which has two values in the angle brackets. It's saying I'm going to give you a string and I want you to return a big integer.

On the right hand side I have an example of how we can use the colon colon with a constructor. So it's saying that whatever string is coming in I'm going to create a new big integer. So I'm going to convert the string value to a big integer. We use the .apply for the function interface and I'm giving it a string that has the values 123456789 and it will convert that to an integer.

//example of a lambda made from an instance method

Consumer<String> print = System.out::println;

print.accept("Coming to you directly from a lambda...");

This is the consumer interface. Remember the consumer interface consumes data but does not return anything. So I have print.accept(). That is the method name of the functional method for the consumer interface. And I'm giving it the string coming to you directly from a lambda. Now on next line in the right hand side notice the print line is an instance method of system.out. It will automatically take in the string, provide it on next line and print it to the console.

//these two are the same using the static method concat

UnaryOperator<String> greeting = x -> "Hello, ".concat(x);

System.out.println(greeting.apply("World"));

UnaryOperator<String> makeGreeting = "Hello, "::concat;

System.out.println(makeGreeting.apply("Peggy"));

The last example is the unary operator which is going to operate on a string. It has the value hello::concat. The concat is the method that I'm actually going to be using, and again I'm not specifying the fact that I'm going to provide a second string. It can infer that.

Let's run the program and take a look at the output.

⁠

As you can see in

Java 8⁠

it's starting to make things a little simpler as far as the amount of code that we need to write. In this particular example we were making methods into lambdas whether those methods were static methods, instant methods or even constructors.

Create new functional interfaces

Although Java 8 has included a new package, java.util.function, that includes many different functional interfaces, sometimes we just need to create our own functional interface.

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Loading…⁠

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We can create a new functional interface by creating a new file or we can include the functional interface inside of our class file. Either way when creating our own functional interfaces it's helpful to add annotation that says @FunctionalInterface to help ensure this is a functional interface.

As you can see

@FunctionalInterface

interface Calculate {

int calc(int x, int y);

}

I have the at functional interface. Now this particular interface has only one method header. A functional interface is an interface that has only one method.

The only code that I need in here is the name of the interface and then the method header or the method signature. In this case i, int calc is the name of my method and it takes two parameters that are both integers. I do need to specify the data types for each value here. The return type and the two arguments.

Calculate add =(a,b)-> a + b;

Calculate difference = (a,b) -> Math.abs(a-b);

Calculate divide =(a,b)-> (b != 0 ? a/b : 0);

This is really nice because my interface is very simple. It simply has the interface name and the one method signature. And then using the lambdas I can define that method signature in different ways. Add, difference, divide and multiply.

Collections

Since collections are used so heavily in

Java 8 lambda notation⁠

, I wanted to do a quick review. The

Collections⁠

API was introduced with

Java 7⁠

A collection is an object that groups multiple elements into a single unit.

Collections are used to store, retrieve, manipulate, and communicate aggregate data.

The collections API provides the following interfaces:

set : which is a collection that does not contain duplicates.

list : which is just an ordered collection based on the way the user entered the data.

map : IT is an object that maps keys to values.

The collection interface contains methods that perform basic operations such as

int size() : which gets the size of the collection.

boolean isEmpty() returns true if it's empty, false if it's not.

boolean contains() : checks to see if an element is inside the collection, returning true if it is and false if not

add() : an element returns true if it was able to add the element, or false if there was an error.

boolean remove() : which tries to remove an element. If the element is not found, it returns false. If it successfully removes it, it'll return true.

iterator() : it returns an iterator over the elements in this collection.

Now let's review some examples of collections.

I have a program that lists some collections. I'm going to start by using collections and not using the lambda notation, then I'm going to show how the lambda notations can simplify our code.

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I create a list of names.

List<String> names = Arrays.asList("Paul", "Jane", "Michaela", "Sam");

//way to sort prior to Java 8 lambdas

Collections.sort(names, new Comparator<String>() {

@Override

public int compare(String a, String b) {

return b.compareTo(a);

}

});

I have four names in my list. In order to sort those, prior to using the Java 8 lambdas. we could say Collections.sort() and provide the variable names and new Comperator, and then we'd have to override the collections interface, the method compare, to tell it how to do the compare.

//first iteration with lambda

Collections.sort(names, (String a, String b) -> {

return b.compareTo(a);

});

Now I have the next iteration which uses lambda, but it's still not quite the most concise.

//now remove the return statement

Collections.sort(names, (String a, String b) -> b.compareTo(a));

This is the best version of this code using lambda notation. Remember we don't have to specify the data types in the lambda notation because it can be inferred since there is only one method inside the Collections interface.

//now remove the data types and allow the compile to infer the type

Collections.sort(names, (a, b) -> b.compareTo(a));

In this case the sort method takes in a list called names, it takes in two values, a and b, and compares b to a, returning them in the right order.

All right, next I wanted to show you how we could do a book collection. So I've created three random books here.

//total pages in your book collection

Book book1 = new Book("Miss Peregrine's Home for Peculiar Children",

"Ranson", "Riggs", 382);

Book book2 = new Book("Harry Potter and The Sorcerers Stone",

"JK", "Rowling", 411);

Book book3 = new Book("The Cat in the Hat",

"Dr", "Seuss", 45);

I do have a Book class, so let's take a quick look at that.