With the introduction of Java 8, Oracle added two new packages: java.util.function, and java.util.streams. The lambda expressions use both of these new packages, so I wanted to review some of the functional interfaces that are now available.
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- [Instructor] I'm sure you're aware that Java is not considered a functional programming language. It's considered an object oriented programming language but with Java 8, we do have functional interfaces. With the introduction of Java 8, Oracle added two new packages, the java.util.function and the java.util.stream. The lambda expressions, also introduced with Java 8, use both of these new packages so we wanted to review some of the functional interfaces that are now available.
Let's start with the definition of functional interface. It is defined as an interface that contains only one abstract function or method. Functional interfaces can represent abstract concepts such as functions, actions or predicates. In this movie, I will review some of the most commonly used functional interfaces in lambda notation but there are more interfaces available in the java.util.function package. To see a list of all the functional interfaces available, check out the Javadoc for java.util.function.
Some of the more commonly used interfaces inlcude Predicate. Predicates are Boolean valued functions of one argument meaning they take in one argument, use a test method to evaluate it and return either true or false. Since this is an interface, we will have to override the test method with logic that determines what should be evaluated. Next, we have Consumer. The Consumer interface consumes the argument. It accepts a single argument and does not return a result.
Then we have Function which transforms a value from one type to another. It accepts one argument and produces a result. Supplier supplies a value. It produces a result of a given type. Unlike Functions, Suppliers do not accept arguments but they do return a result. UnaryOperator interface takes a single argument and returns a single value and the BinaryOperator interface takes two arguments and returns one.
I'd like to switch now to NetBeans so I can show you an example of each of these functional interfaces in action. This exercise file is available in the Exercise Files folder under Chapter 01 0101. Starting on line 17, I'm using the Predicate interface. As you can see, in the angle brackets after the word Predicate I'm forming the interface to expect a string. It has a name of stringLen and it's equal to on the right hand side is where I'm actually implementing my method.
I'm used lambda notation so it might look a little different than what you're used to. On the right hand side in parentheses, I list either no arguments, a single argument or multiple arguments. For Predicate, I need to pass in a single argument then I have the arrow which is a hyphen and a greater than sign. Make sure those two are together. If you put a space between, the compiler will not know what you mean. After that, I have the logic of my Predicate. So in this case, I'm saying if the string.length or the length of the string is less than 10, return true.
Otherwise, return false. For each of my examples, I print out the result. So line 18 is calling stringLen.test with the value Apples and it'll print out true or false. Line 21, I'm using the Consumer interface. Remember, the Consumer interface accepts a value but does not return anything. So on the right hand side, I'm passing it, the argument s but the actual method itself is just going to print out that value in all lowercase letters. It doesn't return anything.
On line 25, I have a Function interface which has two values in the angle brackets. The Function interface, the first value represents the value coming in for the argument and the second value represents the return value. So in this case, I'm passing in an integer and I'm going to return a string. The logic that I use to do that is on the right hand side of the arrow that says integer.toString num where num is my argument. On line 26, I'm going to pass in a value of 26 which gets converted to a string which will just be the characters two and six which will have a length of two.
Line 29 is my Supplier interface. Remember, the Supplier does not contain any arguments. That's why I have the open and closed parentheses with nothing inside. It's important to make sure that you understand you have to include the empty argument list. Otherwise, the compiler will think you made a mistake. So we have an empty argument list, our arrow and on the right hand side, I have a string that says Java is fun. Line 30 uses the get method which is the functional method for Supplier to get the value of the string and I have that inside of a print line statement so it'll print it out.
Let me scroll down a little bit. The last two, the BinaryOperator and the UnaryOperator. The BinaryOperator on line 33 is going to return an integer. On the right hand side, you can see it expects two values, a and b. It's going to then add them together and return those values. Now it is so far that none of my arguments have their data types explicitly mentioned. That's because when we're using lambda notation, it is inferred. So since the BinaryOperator is returning an integer, it knows that the two values coming in as arguments, a and b, must also be integers.
On line 34, I'm using the apply method of the BinaryOperator to add 10 and 25. Line 37, the UnaryOperator is going to return a string. It takes in a string, msg, the message, and it converts it to uppercase. On line 38, I'm going to print that out using the UnaryOperator method apply. Let's run the program so you can see how it works. In the Output window, the first was the Predicate which returned true that the string Apples is less than 10 characters.
Next, I used the Consume interface to take in a series of letters representing the alphabet in all upper and lowercase letters mixed. It used the two lower to print it back out in all lowercase followed by the Function interface which took in an integer, 26, converted it to a string of two characters, two and six and then found the length of that string which was two. The Supplier actually supplied the string, Java is fun. So you can see in line 30 right above, I'm using the s.get.
It gets the value, Java is fun and prints it to the command line followed by the BinaryOperator which took 10 and 25, added them together and gave us 35 and the last one was the UnaryOperator that took in a message that was in mixed case and returned that message in all uppercase. So these are a few of the more commonly used functional interfaces available with Java 8. Remember, there are a lot more that you can use so don't forget to check out that Javadoc on java.util.function.
- Creating new functional interfaces
- Lambda syntax
- Methods as lambdas
- Collections, maps, and streams