java foreach lambda multiple lines

multiple lines of code in stream.forEach

stackoverflow.com › questions › 34208705 › multiple-lines-of-code-in-stream-foreach › 34209023

There are two completely different things you should ask here:

a) how do I place multiple lines of code in stream.forEach()?

b) what should I do to count the number of lines in a Stream?

The question b) is answered already by other posters; on the other hand, the general question a) has a quite different answer:

use a (possibly multi-line) lambda expression or pass a reference to multi-line method.

In this particular case, you'd either declare i a field or use a counter/wrapper object instead of i.

For example, if you want to have multiple lines in forEach() explicitly, you can use

class Counter { // wrapper class
    private int count;
    public int getCount() { return count; }
    public void increaseCount() { count++; }
}

and then

Counter counter = new Counter();
lines.stream().forEach( e -> {
    System.out.println(e);
    counter.increaseCounter(); // or i++; if you decided i is worth being a field
} );

Another way to do it, this time hiding those multiple lines in a method:

class Counter { // wrapper class
    private int count;
    public int getCount() { return count; }
    public void increaseCount( Object o ) {
        System.out.println(o);
        count++;
    }
}

and then

Counter counter = new Counter();
lines.stream().forEach( counter::increaseCount );

or even

Counter counter = new Counter();
lines.stream().forEach( e -> counter.increaseCount(e) );

The second syntax comes in handy if you need a consumer having more than one parameter; the first syntax is still the shortest and simplest though.

Answer from user719662 on Stack Overflow

Stack Overflow

stackoverflow.com › questions › 34208705 › multiple-lines-of-code-in-stream-foreach › 34209023

java - multiple lines of code in stream.forEach - Stack Overflow

Top answer

1 of 3

There are two completely different things you should ask here:

a) how do I place multiple lines of code in stream.forEach()?

b) what should I do to count the number of lines in a Stream?

The question b) is answered already by other posters; on the other hand, the general question a) has a quite different answer:

use a (possibly multi-line) lambda expression or pass a reference to multi-line method.

In this particular case, you'd either declare i a field or use a counter/wrapper object instead of i.

For example, if you want to have multiple lines in forEach() explicitly, you can use

class Counter { // wrapper class
    private int count;
    public int getCount() { return count; }
    public void increaseCount() { count++; }
}

and then

Counter counter = new Counter();
lines.stream().forEach( e -> {
    System.out.println(e);
    counter.increaseCounter(); // or i++; if you decided i is worth being a field
} );

Another way to do it, this time hiding those multiple lines in a method:

class Counter { // wrapper class
    private int count;
    public int getCount() { return count; }
    public void increaseCount( Object o ) {
        System.out.println(o);
        count++;
    }
}

and then

Counter counter = new Counter();
lines.stream().forEach( counter::increaseCount );

or even

Counter counter = new Counter();
lines.stream().forEach( e -> counter.increaseCount(e) );

The second syntax comes in handy if you need a consumer having more than one parameter; the first syntax is still the shortest and simplest though.

2 of 3

The forEach method takes an instance of any class that implements Consumer. So here is an example of using a custom Consumer implementation that keeps up with the count. Later you can call getCount() on the Consumer implementation to get the count.

import java.util.ArrayList;
import java.util.List;
import java.util.function.Consumer;

public class ConsumerDemo {
    public static void main(String[] args) {
        List<String> lines = new ArrayList<String>();
        lines.add("line 1");
        lines.add("line 2");

        MyConsumer countingConsumer = new MyConsumer();
        lines.stream().forEach(countingConsumer);
        System.out.println("Count: " + countingConsumer.getCount());
    }

    private static class MyConsumer implements Consumer<String> {
        private int count;

        @Override
        public void accept(String t) {
            System.out.println(t);
            count++;
        }

        public int getCount() {
            return count;
        }
    }
}

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stackoverflow.com › questions › 31130457 › java-8-lambda-stream-foreach-with-multiple-statements

Java 8 Lambda Stream forEach with multiple statements - Stack Overflow

Top answer

1 of 5

WARNING
As mentioned in comments, Using peek() for production code is considered bad practice
The reasson is that "According to its JavaDocs, the intermediate Stream operation java.util.Stream.peek() “exists mainly to support debugging” purposes."
As a consequence, this proposed solution SHOULD NOT be used.

Forgot to relate to the first code snippet. I wouldn't use forEach at all. Since you are collecting the elements of the Stream into a List, it would make more sense to end the Stream processing with collect. Then you would need peek in order to set the ID.

List<Entry> updatedEntries = 
    entryList.stream()
             .peek(e -> e.setTempId(tempId))
             .collect (Collectors.toList());

For the second snippet, forEach can execute multiple expressions, just like any lambda expression can :

entryList.forEach(entry -> {
  if(entry.getA() == null){
    printA();
  }
  if(entry.getB() == null){
    printB();
  }
  if(entry.getC() == null){
    printC();
  }
});

However (looking at your commented attempt), you can't use filter in this scenario, since you will only process some of the entries (for example, the entries for which entry.getA() == null) if you do.

2 of 5

List<String> items = new ArrayList<>();
items.add("A");
items.add("B");
items.add("C");
items.add("D");
items.add("E");

//lambda
//Output : A,B,C,D,E
items.forEach(item->System.out.println(item));

//Output : C
items.forEach(item->{
    System.out.println(item);
    System.out.println(item.toLowerCase());
  }
});

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