MethodReferenceExamples

LambdasAndMethodReferences_.idea_misc.xml

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LambdasAndMethodReferences_.idea_modules.xml

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LambdasAndMethodReferences_LambdasAndMethodReferences.iml

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LambdasAndMethodReferences_src_BoundedMethodReferenceExample.java

import java.util.Arrays;
import java.util.List;
import java.util.stream.Collectors;

/**
 * Bounded instance method reference example.
 * Static method reference example.
 * Bounded instance method references are similar to static method references.
 */

public class BoundedMethodReferenceExample {
    public static void main(String[] args) {
        // Program - Parse list of integers in string into a list of integer
        List<String> intStringList = Arrays.asList("1","2","3","4","5","6");

        // static method reference example
        // No. of arguments to the method reference (Integer.parseInt(arg)) are same as
        // the num of arguments to function object (Function.apply(arg))
        List<Integer> intList = intStringList.stream().map(Integer::parseInt).collect(Collectors.toList());

        // bounded instance method reference example
        // Num of arguments to the method referenced(println(arg)) are same as
        // the num of arguments to the function object (Consumer.accept(arg))
        intStringList.forEach(System.out::println);

        // static method reference example
        // Num of arguments to the method referenced(sum(arg1,arg2)) are same as
        // the num of arguments to the function object (BiFunction.apply(arg1, arg2))
        Integer sum = intList.stream().reduce(Integer::sum).get();
    }
}

LambdasAndMethodReferences_src_ConstructorReferenceExample.java

import java.util.HashMap;
import java.util.HashSet;
import java.util.function.Supplier;

/**
 * Constructor reference example.
 * It covers 2 types of constructor reference
 * 1) Class Constructor
 * 2) Array Constructor
 */
public class ConstructorReferenceExample {
    public static void main(String[] args) {
        // Class Constructor reference

        // Be mindful that Class::new will resolve to a functional interface and do not
        // misunderstand this with Class ref = Class::new; This won't compile.

        Supplier<HashMap<String, String>> mapSupplier = HashMap::new;
        Supplier<HashSet<String>> setSupplier = HashSet::new;

        // Array Constructor reference
        ArrayConstruction<String> stringArrayConstruction = String[]::new;
        ArrayConstruction<Integer> integerArrayConstruction = Integer[]::new;
    }
}

@FunctionalInterface
interface ArrayConstruction<T> {
    T[] accept(int l);
}

LambdasAndMethodReferences_src_ConstructorReferenceWithMoreArgs.java

import java.util.function.BiFunction;
import java.util.function.Function;

/**
 * Use Constructor Reference to create an instance of a class
 * who has parametrized constructor.
 */
public class ConstructorReferenceWithMoreArgs {
    public static void main(String[] args) {
        Function<String, Student> studentFactory1 = Student::new;
        Function<String, Student> studentFactory1Equivalent = fname -> new Student(fname);

        BiFunction<String, String, Student> studentFactory2 = Student::new;
        BiFunction<String, String, Student> studentFactory2Equivalent = (fname, lname) -> new Student(fname, lname);
    }
}

class Student {
    public final  String fname;
    public final String lname;

    public Student(String name) {
        System.out.println("Using one arg constructor.");
        this.fname = name;
        this.lname = "last name";
    }

    public Student(String fname, String lname) {
        System.out.println("Using two args constructor.");
        this.fname = fname;
        this.lname = lname;
    }

}

LambdasAndMethodReferences_src_Example1.java

import java.util.Arrays;
import java.util.Collections;
import java.util.Comparator;
import java.util.List;

/**
 * Sort list of String in order of string's length
 */
public class Example1 {
    public static void main(String[] args) {
        List<String> strings = Arrays.asList("thane","mulund","bhandup","cst","parel","sion");
        System.out.println("Before sorting\n" + strings);

        Collections.sort(strings);
        System.out.println("After sorting\n" + strings);

        //-------------------------------------------------------

        // Anonymous class instance as a "function object"
        // Comparator represents an abstract strategy for sorting and function object provides
        // concrete strategy for sorting. (Strategy Design Pattern)

//        Collections.sort(strings, new Comparator<String>() {
//            @Override
//            public int compare(String o1, String o2) {
//                return Integer.compare(o1.length(), o2.length());
//            }
//        });
//
//        System.out.println("After sorting using function object\n" + strings);

        //--------------------------------------------------------

        // Using Lambda expression as a function object (Replaces anonymous class)

//        Collections.sort(strings, (String s1, String s2) -> Integer.compare(s1.length(), s2.length()));
//        Collections.sort(strings, (s1, s2) -> Integer.compare(s1.length(), s2.length()));

        // Unbounded instance method reference String::length
        // The return type of
        // String.length()
        // matches the return of
        // ToIntFunction.applyAsInt(...)
//        Collections.sort(strings, Comparator.comparingInt(String::length));
//        strings.sort(Comparator.comparingInt(String::length));
//
//        System.out.println("After sorting using lambda expression\n" + strings);
    }
}

LambdasAndMethodReferences_src_Example2.java

import java.util.Arrays;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

/**
 * Count occurrences of each word in list
 */

public class Example2 {
    public static void main(String[] args) {
        List<String> words = Arrays.asList("word2","word3","word2","word3","word1","word3");

        Map<String, Integer> resultMap = new HashMap<>();
        words.forEach(word -> resultMap.merge(word,1, (count, incr) -> count + incr));

        // FYI - merge(...) method was added to the Map interface in Java 8

        System.out.println("Result1 = \n" + resultMap);

        // The code above looks clean but still there is some boilerplate.
        // The words count and incr does not add much value and take a lot of horizontal space.
        // All we know is that the lambda returns the sum of 2 arguments supplied.
        // As of Java 8 all the boxed numerical primitive classes are having static sum method which does exactly the  same
        // thing. We will leverage this method below.

        // With less visual clutter
        Map<String, Integer> resultMap2 = new HashMap<>();
        words.forEach(word -> resultMap2.merge(word, 1, Integer::sum));
        System.out.println("Result2 = \n" + resultMap2);

        // Integer::sum -> Takes 2 arguments are returns an Integer
        // which is same as
        // lambda expression which takes 2 arguments are returns an Integer.
    }
}

LambdasAndMethodReferences_src_Example3LongClassName.java

public class Example3LongClassName {
    public static void main(String[] args) {
        // calling the method of the same class using
        // a method reference
        Thread ref1 = new Thread(Example3LongClassName::action);

        // calling the method of the same class using
        // a lambda expression
        Thread ref2 = new Thread(() -> action());

        ref1.start();
        ref2.start();

        // The latter approach using lambda is more succinct than method reference.
    }

    static void action() {
        System.out.println("This is an action");
    }
}

LambdasAndMethodReferences_src_UnboundInstanceMethodReferenceExample.java

import java.util.Arrays;
import java.util.List;
import java.util.stream.Collectors;

/**
 * Unbounded instance method reference example
 */
public class UnboundInstanceMethodReferenceExample {
    public static void main(String[] args) {
        // 1) Convert a list of String from lowercase strings to uppercase strings
        List<String> stringsLowerCase = Arrays.asList("java","python","html","perl","react","haskell");
        System.out.println("String Lower Case = " + stringsLowerCase);

        List<String> stringsUpperCase =  stringsLowerCase.stream().map(str -> str.toUpperCase()).collect(Collectors.toList());
        // The above statement can be rewritten using an unbounded instance method reference like below
        stringsUpperCase =  stringsLowerCase.stream().map(String::toUpperCase).collect(Collectors.toList());
        System.out.println("String Upper Case = " + stringsUpperCase);


    }
}