CODEWITHSUNDEEP
javalambdaanonymous-functionjava-8
alinsoar
alinsoar

Reputation: 15793

syntax of lambda functions in java 8

I am new to java, and I do not know how to write a simple lambda function. I tried to read some articles, like this one, but I do not manage to compile, as I get error of syntax.

I wish to replace the function F is in this code with a λ-function

class Test {
  private int N;
  public Test (int n) {
      N = n;
  }

  private int f (int x) {                   /* <== replace this */
      return 2*x;
  }

  public void print_f () {
      for (int i = 0; i < this.N; i++)
          System.out.println (f(i));        /* <== with lambda here*/
  }

  public static void main (String[] args) {
      int n = 10;
      if (args.length == 1)
          n = Integer.parseInt(args[0]);
      Test t = new Test (n);
      t.print_f ();
  }
}

EDIT: My question concerns only the syntax of λ-functions in Java. Not the syntax of anonymous classes.

Upvotes: 4

Views: 2395

Answers (2)

alinsoar
alinsoar

Reputation: 15793

To answer my own question , using the answer provided by Marko Topolnik, here is a complete file Test.java, which does exactly what I asked, using the principle Keep It Simple Stupid.

In this case, I generalized from the function λ(int)->int to λ(int,int)->int.

All possible type of functions that can be defined are found here:

http://download.java.net/jdk8/docs/api/java/util/function/package-summary.html

import java.util.function.BiFunction;

class Test {
  public static void main (String[] args) {
      int n = 10;
      if (args.length == 1) n = Integer.parseInt(args[0]);
      for (int i=0; i <= n; i++)
          System.out.println (((BiFunction<Integer, Integer, Integer>)
                               (x,y)->2*x+y).apply(i,1));
  }
}

Many more examples can be found here:

http://www.oracle.com/webfolder/technetwork/tutorials/obe/java/Lambda-QuickStart/index.html?cid=7180&ssid=105274749521607

Upvotes: 1

Marko Topolnik
Marko Topolnik

Reputation: 200148

The first recommendation would be to use NetBeans, which will teach you how to transform your code into lambdas in many cases. In your specific code you want to transform a for (int i = 0;...) kind of loop. In the lambda world, you must express this as a list comprehension and more specifically for Java, as a stream transformation. So the first step is to acquire a stream of integers:

IntStream.range(0, this.N)

and then apply a lambda function to each member:

IntStream.range(0, this.N).forEach(i->System.out.println(f(i)));

A complete method which replaces your print_f would look as follows:

public void print_f() {
    IntStream.range(0, this.N).forEach(i->System.out.println(f(i)));
}

However, in the world of lambdas it would be more natural to fashion print_f as a higher-order function:

public void print_f(IntConsumer action) {
    IntStream.range(0, this.N).forEach(action);
}

And now your complete program would look like this:

import java.util.function.IntConsumer;
import java.util.stream.IntStream;

class Testing {
    private int N;
    public Testing (int n) {
        N = n;
    }

    private static int f (int x) {
        return 2*x;
    }

    public void print_f(IntConsumer action) {
        IntStream.range(0, this.N).forEach(action);
    }

    public static void main (String[] args) {
        int n = 10;
        if (args.length == 1)
            n = Integer.parseInt(args[0]);
        Testing t = new Testing (n);
        t.print_f(i->System.out.println(f(i)));
    }
}

... well, except that a print_f method should really do the printing and accept only the transformation function, which turns your code into the following:

public void print_f(IntFunction f) {
    IntStream.range(0, this.N).forEach(i->System.out.println(f.apply(i)));
}

public static void main (String[] args) {
    int n = 10;
    if (args.length == 1)
        n = Integer.parseInt(args[0]);
    Testing t = new Testing (n);
    t.print_f(Testing::f);
}

... or, eliminating the f method altogether,

t.print_f(i->2*i);

Upvotes: 5

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