Lambda expressions are usually passed as parameters to a function:
Example
Use a lamba expression in the ArrayList's forEach() method to print every item in the list:
import java.util.ArrayList;
public class Main {
public static void main(String[] args) {
ArrayList<Integer> numbers = new ArrayList<Integer>();
numbers.add(5);
numbers.add(9);
numbers.add(8);
numbers.add(1);
numbers.forEach( (n) -> { System.out.println(n); } );
}
}
Lambda expressions can be stored in variables if the variable's type is an interface which has only one method. The lambda expression should have the same number of parameters and the same return type as that method. Java has many of these kinds of interfaces built in, such as the Consumer interface (found in the java.util package) used by lists.
Example
Use Java's Consumer interface to store a lambda expression in a variable:
import java.util.ArrayList;
import java.util.function.Consumer;
public class Main {
public static void main(String[] args) {
ArrayList<Integer> numbers = new ArrayList<Integer>();
numbers.add(5);
numbers.add(9);
numbers.add(8);
numbers.add(1);
Consumer<Integer> method = (n) -> { System.out.println(n); };
numbers.forEach( method );
}
}
To use a lambda expression in a method, the method should have a parameter with a single-method interface as its type. Calling the interface's method will run the lambda expression:
Example
Create a method which takes a lambda expression as a parameter:
interface StringFunction { String run(String str);}public class Main { public static void main(String[] args) { StringFunction exclaim = (s) -> s + "!"; StringFunction ask = (s) -> s + "?"; printFormatted("Hello", exclaim); printFormatted("Hello", ask);}public static void printFormatted(String str, StringFunction format) { String result = format.run(str); System.out.println(result);}}
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