9 min read

In this article, you will learn how to use lambda expressions in Java 11.
This article is an excerpt from a book written by Nick Samoylov and Mohamed Sanaulla titled Java 11 Cookbook – Second Edition. In this book, you will learn how to build graphical user interfaces using JavaFX.

Getting ready

Creating and using lambda expressions is actually much simpler than writing a method. One just needs to list the input parameters, if any, and the code that does what has to be done.

Let’s see an implementation of standard functional interfaces rewritten using lambda expressions. Here’s how we have implemented the four main functional interfaces using anonymous classes:

Function ourFunc = new Function(){
    public Double apply(Integer i){
        return i * 10.0;
    }
};
System.out.println(ourFunc.apply(1));       //prints: 10.0
Consumer consumer = new Consumer() {
    public void accept(String s) {
        System.out.println("The " + s + " is consumed.");
    }
};
consumer.accept("Hello!"); //prints: The Hello! is consumed.
Supplier supplier = new Supplier() {
    public String get() {
        String res = "Success";
        //Do something and return result—Success or Error.
        return res;
    }
};
System.out.println(supplier.get());      //prints: Success
Predicate pred = new Predicate() {
    public boolean test(Double num) {
       System.out.println("Test if " + num + " is smaller than 20");
       return num 

And here’s how they look with lambda expressions:


Function ourFunc = i -> i * 10.0;
System.out.println(ourFunc.apply(1)); //prints: 10.0

Consumer consumer = 
            s -> System.out.println("The " + s + " is consumed.");
consumer.accept("Hello!");       //prints: The Hello! is consumed.

Supplier supplier = () - > {
        String res = "Success";
        //Do something and return result—Success or Error.
        return res;
    };
System.out.println(supplier.get());  //prints: Success

Predicate pred = num -> {
   System.out.println("Test if " + num + " is smaller than 20");
   return num 

The examples of specialized functional interfaces we have presented are as follows:

IntFunction ifunc = new IntFunction() {
    public String apply(int i) {
        return String.valueOf(i * 10);
    }
};
System.out.println(ifunc.apply(1));   //prints: 10
BiFunction bifunc =
        new BiFunction() {
            public Double apply(String s, Integer i) {
                return (s.length() * 10d) / i;
            }
        };

System.out.println(bifunc.apply("abc",2));     //prints: 15.0
BinaryOperator binfunc = new BinaryOperator(){
    public Integer apply(Integer i, Integer j) {
        return i >= j ? i : j;
    }
};
System.out.println(binfunc.apply(1,2));  //prints: 2
IntBinaryOperator intBiFunc = new IntBinaryOperator(){
    public int applyAsInt(int i, int j) {
        return i >= j ? i : j;
    }
};
System.out.println(intBiFunc.applyAsInt(1,2)); //prints: 2

And here’s how they look with lambda expressions:

IntFunction ifunc = i -> String.valueOf(i * 10);
System.out.println(ifunc.apply(1));             //prints: 10

BiFunction bifunc = 
                            (s,i) -> (s.length() * 10d) / i;
System.out.println(bifunc.apply("abc",2));      //prints: 15.0

BinaryOperator binfunc = (i,j) -> i >= j ? i : j;
System.out.println(binfunc.apply(1,2));         //prints: 2

IntBinaryOperator intBiFunc = (i,j) -> i >= j ? i : j;
System.out.println(intBiFunc.applyAsInt(1,2));  //prints: 2

As you can see, the code is less cluttered and more readable.

How to form lambda expressions

Those who have some traditional code-writing experience, when starting functional programming, equate functions with methods.  They try to create functions first because that was how we all used to write traditional code—by creating methods. Yet, functions are just smaller pieces of functionality that modify some aspects of the behavior of the methods or provide the business logic for the otherwise non-business-specific code. In functional programming, as in traditional programming, methods continue to provide the code structure, while functions are the nice and helpful additions to it. So, in functional programming, creating a method comes first, before the functions are defined. Let’s demonstrate this.

The following are the basic steps of code writing. First, we identify the well-focused block of code that can be implemented as a method. Then, after we know what the new method is going to do, we can convert some pieces of its functionality into functions:

  1. Create the calculate() method:
void calculate(){
    int i = 42;        //get a number from some source
    double res = 42.0; //process the above number 
    if(res 

The preceding pseudocode outlines the idea of the calculate() method’s functionality. It can be implemented in a traditional style—by using methods, as follows:

int getInput(){
   int result;
   //getting value for result variable here
   return result;
}
double process(int i){
    double result;
    //process input i and assign value to result variable
}
boolean checkResult(double res){
    boolean result = false;
    //use some criteria to validate res value
    //and assign value to result
    return result;
}
void processSuccess(double res){
     //do something with res value
}
void processFailure(double res){
     //do something else with res value
}
void calculate(){
    int i = getInput();
    double res = process(i); 
    if(checkResult(res)){     
        processSuccess(res);
    } else {
        processFailure(res);
    }
}

But some of these methods may be very small, so the code becomes fragmented and less readable with so many additional indirections. This disadvantage becomes especially glaring in the case when the methods come from outside the class where the calculate() method is implemented:

void calculate(){
    SomeClass1 sc1 = new SomeClass1();
    int i = sc1.getInput();
    SomeClass2 sc2 = new SomeClass2();
    double res = sc2.process(i); 
    SomeClass3 sc3 = new SomeClass3();
    SomeClass4 sc4 = new SomeClass4();
    if(sc3.checkResult(res)){     
        sc4.processSuccess(res);
    } else {
        sc4.processFailure(res);
    }
}

As you can see, in the case where each of the external methods is small, the amount of plumbing code may substantially exceed the payload it supports. Besides, the preceding implementation creates many tight dependencies between classes.

  1. Let’s look at how we can implement the same functionality using functions. The advantage is that the functions can be as small as they need to be, but the plumbing code will never exceed the payload because there is no plumbing code. Another reason to use functions is when we need the flexibility to change sections of the functionality on the fly, for the algorithm’s research purpose. And if these pieces of functionality have to come from outside the class, we do not need to build other classes just for the sake of passing a method into calculate(). We can pass them as functions:
void calculate(Supplier souc e, Function process, Predicate condition,
      Consumer success, Consumer failure){
    int i = source.get();
    double res = process.apply(i);
    if(condition.test(res)){
        success.accept(res);
    } else {
        failure.accept(res);
    }
}
  1. Here’s how the functions may look:
Supplier source = () -> 4;
Function before = i -> i * 10.0;
Function after = d -> d + 10.0;
Function process = before.andThen(after);
Predicate condition = num -> num  success = 
                  d -> System.out.println("Success: "+ d);
Consumer failure = 
                  d -> System.out.println("Failure: "+ d);
calculate(source, process, condition, success, failure);

The result of the preceding code is going to be as follows:

Success: 50.0

How lambda expressions work

The lambda expression acts as a regular method, except when you think about testing each function separately. How to do it?

There are two ways to address this issue. First, since the functions are typically small, there is often no need to test them separately, and they are tested indirectly when the code that uses them is tested. Second, if you still think the function has to be tested, it is always possible to wrap it in the method that returns the function, so you can test that method like any other method. Here is an example of how it can be done:

public class Demo {
  Supplier source(){ return () -> 4;}
  Function after(){ return d -> d + 10.0; }
  Function before(){return i -> i * 10.0; }
  Function process(){return before().andThen(after());}
  Predicate condition(){ return num -> num  success(){ 
     return d -> System.out.println("Failure: " + d); }
  Consumer failure(){ 
     return d-> System.out.println("Failure: " + d); }
  void calculate(Supplier souce, Function process, Predicate condition,
       Consumer success, Consumer failure){
    int i = source.get();
    double res = process.apply(i);
    if(condition.test(res)){
        success.accept(res);
    } else {
        failure.accept(res);
    }
}
void someOtherMethod() {
   calculate(source(), process(), 
                       condition(), success(), failure());
}

Now we can write the function unit tests as follows:

public class DemoTest {

    @Test
    public void source() {
        int i = new Demo().source().get();
        assertEquals(4, i);
    }
    @Test
    public void after() {
        double d = new Demo().after().apply(1.);
        assertEquals(11., d, 0.01);
    }
    @Test
    public void before() {
        double d = new Demo().before().apply(10);
        assertEquals(100., d, 0.01);
    }
    @Test
    public void process() {
        double d = new Demo().process().apply(1);
        assertEquals(20., d, 0.01);
    }
    @Test
    public void condition() {
        boolean b = new Demo().condition().test(10.);
        assertTrue(b);
    }
}

Typically, lambda expressions (and functions in general) are used for specializing otherwise generic functionalities—by adding business logic to a method. A good example is stream operations. The library authors have created them to be able to work in parallel, which required a lot of expertise. And now the library users can specialize the operations by passing into them the lambda expressions (functions) that provide the application’s business logic.

Function inlining

Since, as we have mentioned already, functions are often simple one-liners, they are often inlined when passed in as parameters, for example:

Consumer success = d -> System.out.println("Success: " + d);
Consumer failure = d-> System.out.println("Failure: " + d);
calculate(() -> 4, i -> i * 10.0 + 10, n -> n 

But one should not push it too far, as such inlining may decrease code readability.

In this tutorial, we learned how to use lambda expressions in Java 11. To learn more Java 11 recipes, check out the book Java 11 Cookbook – Second Edition.

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