Java Lambda Expressions

Invoke lambda expressions, creating instances of Functions, Suppliers and Consumers.
Lambdas. We use this syntax are used to create function objects. We can specify methods inside other methods—and even pass methods as arguments to other methods.
A lambda has a shape, one determined by its parameters and return values (if any) and their types. Classes like Function, Supplier, Consumer, accept lambdas with specific shapes.
Example expression. This program creates a Function object from a lambda expression. The lambda expression accepts one argument, an Integer, and returns another Integer.

Left side: On the left of a lambda expression, we have the parameters. Two or more parameters can be surrounded by "(" and ")" chars.

Right side: This is the return expression—it is evaluated using the parameters. It is executed and, when required, returned.

Apply: In this program, we call apply() on the Function object. This executes and returns the expression—10 is changed to 20.

Java program that uses lambda expression import java.util.function.*; public class Program { public static void main(String[] args) { // Create a Function from a lambda expression. // ... It returns the argument multiplied by two. Function<Integer, Integer> func = x -> x * 2; // Apply the function to an argument of 10. int result = func.apply(10); System.out.println(result); } } Output 20
Supplier, lambda arguments. A Supplier object receives no arguments. We use an empty argument list to specify a lambda expression with no arguments.

Tip: A Supplier provides values. We call get() on it to retrieve its value—it may return different values when called more than once.

Java program that uses Supplier, lambdas import java.util.function.*; public class Program { static void display(Supplier<Integer> arg) { System.out.println(arg.get()); } public static void main(String[] args) { // Pass lambdas to the display method. // ... These conform to the Supplier class. // ... Each returns an Integer. display(() -> 10); display(() -> 100); display(() -> (int) (Math.random() * 100)); } } Output 10 100 21
Predicate Lambda, ArrayList. The term predicate is used in computer science to mean a boolean-returning method. A Predicate object receives one value and returns true or false.

RemoveIf: This method on ArrayList receives a Predicate. Here, we remove all elements starting with the letter "c."

Java program that uses removeIf, Predicate lambda import java.util.ArrayList; public class Program { public static void main(String[] args) { // Create ArrayList and add four String elements. ArrayList<String> list = new ArrayList<>(); list.add("cat"); list.add("dog"); list.add("cheetah"); list.add("deer"); // Remove elements that start with c. list.removeIf(element -> element.startsWith("c")); System.out.println(list.toString()); } } Output [dog, deer]
Consumer. Opposite a Supplier, a Consumer acts upon a value but returns nothing. It means a void method. We can use a consumer to call println or other void methods.

Also: A Consumer can be used to mutate data, as in an array, ArrayList or even just a class field.

Java program that uses Consumer import java.util.function.*; public class Program { static void display(int value) { switch (value) { case 1: System.out.println("There is 1 value"); return; default: System.out.println("There are " + Integer.toString(value) + " values"); return; } } public static void main(String[] args) { // This consumer calls a void method with the value. Consumer<Integer> consumer = x -> display(x - 1); // Use the consumer with three numbers. consumer.accept(1); consumer.accept(2); consumer.accept(3); } } Output There are 0 values There is 1 value There are 2 values
UnaryOperator. This functional object receives a value of a certain type (like Integer) and returns a same-typed value. So it operates on, and returns, a value.
Java program that uses UnaryOperator import java.util.function.*; public class Program { public static void main(String[] args) { // This returns one value of the same type as its one parameter. // ... It means the same as the Function below. UnaryOperator<Integer> operator = v -> v * 100; // This is a generalized form of UnaryOperator. Function<Integer, Integer> function = v -> v * 100; System.out.println(operator.apply(5)); System.out.println(function.apply(6)); } } Output 500 600
UnaryOperator, ArrayList. This example uses a lambda expression as a UnaryOperator argument to the ArrayList's replaceAll method. It adds ten to all elements.ArrayList add, insert

Note: The forEach method on ArrayList does not change element values. ReplaceAll allows this action.

Java program that uses replaceAll, UnaryOperator import java.util.ArrayList; public class Program { public static void main(String[] args) { // Add ten to each element in the ArrayList. ArrayList<Integer> list = new ArrayList<>(); list.add(5); list.add(6); list.add(7); list.replaceAll(element -> element + 10); // ... Display the results. System.out.println(list); } } Output [15, 16, 17]
BiConsumer, HashMap. A BiConsumer is a functional object that receives two parameters. Here we use a BiConsumer in the forEach method on HashMap.HashMap

Note: The forEach lambda here, which is a valid BiConsumer, prints out all keys, values, and the keys' lengths.

Java program that uses BiConsumer, HashMap forEach import java.util.HashMap; public class Program { public static void main(String[] args) { HashMap<String, String> hash = new HashMap<>(); hash.put("cat", "orange"); hash.put("dog", "black"); hash.put("snake", "green"); // Use lambda expression that matches BiConsumer to display HashMap. hash.forEach((string1, string2) -> System.out.println(string1 + "..." + string2 + ", " + string1.length())); } } Output, 3, 5, 3
Identifiers. These do not matter in a lambda expression. The identifiers do not impact external parts of the program, but can be accessed on both sides of the lambda.

Note: As with variables, there is no reason to name the lambda expression variable a specific thing. Here we use the word "carrot."

Java program that uses unusual lambda identifier import java.util.function.Consumer; public class Program { public static void main(String[] args) { // The identifier in the lambda expression can be anything. Consumer<Integer> consumer = carrot -> System.out.println(carrot); consumer.accept(1989); } } Output 1989
Benchmark, apply. Here we benchmark a Function object, which we invoke with apply(), against a static method. Both code blocks do the same thing.

Version 1: This version of the code uses a lambda expression and calls the apply() method to execute it many times.

Version 2: Here we invoke a method directly, using traditional Java syntax. This version is much faster.

Result: If a method can be called with no loss of code clarity, this may result in better performance over a lambda or functional object.

Java program that times Function apply, method call import java.util.function.*; public class Program { static int method(int element) { return element + 1; } public static void main(String[] args) { Function<Integer, Integer> function = element -> element + 1; long t1 = System.currentTimeMillis(); // Version 1: apply a function specified as a lambda expression. for (int i = 0; i < 10000000; i++) { int result = function.apply(i); if (result == -1) { System.out.println(false); } } long t2 = System.currentTimeMillis(); // Version 2: call a static method. for (int i = 0; i < 10000000; i++) { int result = method(i); if (result == -1) { System.out.println(false); } } long t3 = System.currentTimeMillis(); // ... Benchmark results. System.out.println(t2 - t1); System.out.println(t3 - t2); } } Output 93 ms, Function apply() 6 ms, method call
Filter. This method works on streams like IntStream. It returns a modified stream. And we can use methods like findFirst to access elements from filtered streams.Filter
Sum. With a lambda expression, IntStream and the reduce() method we can sum an array. This approach has better parallel potential. But it is slow in simple cases.Sum
A review. At first, functional object names in Java are confusing. What is a Supplier? What is a Consumer? What is supplying what, and who is consuming?
With practice, and some effort, the functional object system in this language is powerful and expressive. It is beautiful. It condenses syntax for complex logical forms.
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