> For the complete documentation index, see [llms.txt](https://lin-4.gitbook.io/cs-notebook-java8/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://lin-4.gitbook.io/cs-notebook-java8/java-8-functional-interface.md).

# functional interface

<http://tutorials.jenkov.com/java-functional-programming/functional-interfaces.html>

<https://www.baeldung.com/java-8-functional-interfaces>

## Java Functional Interfaces

A functional interface in Java is an interface that contains only one single abstract(unimplemented) method. A functional interface can contain default and static methods which do have an implementation, in addition to the single unimplemented method.

### 1. Functional Interfaces Can Be Implemented by a Lambda Expression

### 2. Built-in Functional Interfaces in Java

Java contains a set of functional interfaces designed for commonly occuring use cases.

### 2.1 Function

### 2.2 Predicate

The Java `Predicate`interface, `java.util.function.Predicate`, represents a simple function that takes a single value as parameter, and returns true or false.

eg:

```java
public interface Predicate{
    boolean test(T t);
}

public class CheckForNull implements Predicate{
    @Override
    public boolean test(Object o){
        return o!=null;
    }
}
```

```java
Predicate predicate = (value)->value!=null;
```

### 2.3 Supplier

that represents a function that supplies a value of some sorts. The *Supplier* interface can also be thought of as a factory interface.

It does not take any arguments.

eg:

```java
Supplier<Integer> supplier = ()-> new Integer((int)(Math.random()*1000D));
```

### 2.4 Consumer

that consumes a value without returning any value. A Java **Consumer** implementation could be printing out a value, or writing it to a file, or over the network etc.

eg:

```java
Consumer<Integer> consumer = (value)->System.out.println(value);
```