New in Java 8 : Consumers and Predicates : a simple introduction

The java.util.function package is new in Java 8 and contains interfaces that are used for lambda expressions and functional references. In this blog, I give a brief introduction of  2 interfaces of this package :

  • Consumer
  • Predicate

For the examples in this blog, we have a list of invoices with name, amount and category :

public class Invoice {
   private String name,amount,category;
   public Invoice(String name,String amount,String category) {
   public String getName() {
   public String getAmount() {
     return this.amount;
   public String getCategory() {
     return this.category;

To generate a list of invoices, we’ll use the following method:

public static List<Invoice> generateInvoices()  {
   List<Invoice> list = new ArrayList<Invoice>();
   list.add(new Invoice("Oracle","1000","SOFTWARE"));
   list.add(new Invoice("Microsof","30000","HARDWARE"));
   list.add(new Invoice("Apple","5000","SOFTWARE"));


A Consumer is an interface that ‘consumes’ an object. It takes an argument and does something with it. It does not return a result.

The Consumer interface has 2 methods :

  • void accept(T t) : contains the code that is executed on t
  • default Consumer<T> andThen(Consumer<? super T> after) : This method returns a consumer that is executed after the previous one and enables you to execute a chain of consumers.

For this demo, we are using the (new in Java8) method of the Collection API :

Collection.forEach(Consumer<? super T> action)

This method executes the consumer ‘action’ on every item of the collection.

First we create 2 methods that each return a Consumer object. The first will print the name of the invoice, the second prints the amount.

Finally we use these 2 methods in a Collection.foreach method.

public static Consumer<Invoice> printName() {
    return new Consumer<Invoice>() {
         public void accept(Invoice invoice) {

public static Consumer<Invoice> printAmount() {
    return new Consumer<Invoice>() {
         public void accept(Invoice invoice) {


As you can see in the last line, first the printName() is executed, and then the printAmount(). This line will print the following :

When an error occurs in the foreach method, an exception is thrown, and further processing of the List stops.


A Predicate is an interface that is used to assign lambda expressions. It has a functional method :

boolean Test(T t )

Predicates are used as stream operations. Stream operations can be executed on Collections in order to execute complex data processing queries. But in this blog we’ll keep it simple, we just want to select all the invoices with category=’HARDWARE’, and put them in a new List.

Using a predicate in combination with the new Streams API, will simplify and shorten our code and make it more readable.

First we define our predicate, and then we’ll use it on our List of invoices. The stream method will filter the List using our predicate, and then collect the items that fulfill the predicate in a new List.

public static Predicate<Invoice> isHardware() {
     return i -> i.getCategory().equals("HARDWARE");

List<Invoice> listHardware = generateInvoices().stream.filter(isHardware()).collect(Collectors.<Invoice>toList());

Our new list will now contain 1 invoice, the one from Microsoft which has ‘HARDWARE’ as category.
As you can see, Predicate is a class that contains a function that we can pass to other classes. Actually it is just a reference to a function, AKA ‘a function reference’.
With Streams, you also sort and map data, before collecting, but that’s for another blog.

So that’s it for now. I hope this blog has shown that, by using Consumers and Predicates, our code will become shorter, cleaner and more readable.


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About Chris Noë