Tag: recap

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In the last post I reviewed Java lambda expressions. They represent a concise syntax to implement functional interfaces.

Enter Java method references. They represent a concise syntax to implement functional interface using existing methods. Like with lambda expressions, referenced methods are not allowed to throw checked exceptions.

Syntax

It’s simply “class-or-instance name” “::” “method name”, like

Function<String, Integer> string2Int = Integer::valueOf;

Types of method references

Reference to a static method

Static methods are referenced using the class name like in the example above.

Reference to an instance method of a particular object

Methods of a particular object are referenced using the variable name of that object:

Map<Integer, String> aMap = new HashMap<>();
Function<Integer, String> getRef = aMap::get;
// call it
String s = getRef.apply(42);

Reference to an instance method of an arbitary object of a particular type

Instead of using an already existing object you can just state the class and a non-static method. Then the instance is an additional parameter. In the following example toURI is a method with no arguments that returns a String. The function of this method reference takes a File (the object) and returns a String:

Function<File, URI> file2Uri = File::toURI;

Reference to a constructor

Constructors are references using its type and “new”:

Function<String, StringBuffer> bufferFromString = StringBuffer::new;

Here the constructor of StringBuffer with String parameter is referenced. Return type is the type of the constructor, parameters of the function are the parameters of the constructors.

 

 

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Lambda expressions in Java represent “functions”, something that take a number of parameters and produce at most one return value.

This could be expressed with anonymous classes but lambda expressions offer a more concise syntax.

Syntax

Lambda expression consist of a parameter list, an “arrow” and a body.

(String s1, String s2) -> s1 + "|" + s2

The parameter list is enclosed in round brackets. Types are optional. When the expression has exactly one parameter, the brackets can be omitted.

s -> s!=null && s.length>0

The body can either be an expression (that returns a value) or a block. A block is a sequence of statements, enclosed in curly braces.

n -> { if (n<10) System.out.println(n); }

Lambda expressions and types

In the Java type system, lambda expressions are instances of “functional interfaces”. A functional interface is an interface with exactly one abstract method.

Functional interfaces in java.util.function

The package java.util.function in the JDK contains a number of functional interfaces:

  • Function<T,U>  represents a function with one parameter of type T and return type U
  • Consumer<T>  represents a function with one parameter of type T and return type void
  • Supplier<T>  represents a function with no parameter and return type T
  • Predicate<T>  represents a function with one parameter of type T and return type boolean

Plus, variants with “Bi” prefix exists that have two parameters, like BiPredicate . More variants exists for using primitive types like DoubleToIntFunction .

User defined function interfaces

Any interface with exactly one abstract method can be used as type of a lambda expression. You mark this interface with @FunctionInterface .

@FunctionalInterface
interface SomeInterface {
  int someBehaviour(String a, String b);
}

SomeInterface geo = (x,y) -> x.length + y.length;

Benefits

For me, the benefits of lambda expression are

  • concise syntax for anonymous classes that represent functional code
  • improved readability
  • encouragement of a more functional programming style