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Lambda expression is an argument list follow by an arrow token and a body or code expression.

Argument List	Arrow Token	Body
(int x, int y)	->	x + y

Basic Lambda examples

(Integer x, Integer y) -> x + y
(String s) -> s.contains("word")

Let’s consider the following code:

public interface StringAnalyzer {
  public Boolean analyze(String text, String keyword);
}

public class ContainsAnalyzer implements StringAnalyzer {

  public Boolean analyze(String text, String keyword){
    return text.contains(keyword);
  }

}

public class MainAnalyzer {

  public static void main(String[] args){
    StringAnalyzer analyzer = new ContainsAnalyzer();
    assert analyzer.analyze("In the end, it's not the years in your life that count. It's the life in your years", "life");
  }

}

Since we are defining a interface StringAnalyzer we can use a lambda expression to implement this interface, that’s it, you define or use a functional interface that accepts and returns exactly what you want. In this case we are replacing ContainsAnalyzer by a lambda expression.

public class MainAnalyzer {

  public static void main(String[] args){
    StringAnalyzer analyzer = (String text, String keyword) -> text.contains(keyword);
    assert analyzer.analyze("In the end, it's not the years in your life that count. It's the life in your years", "life");
  }

}

Lambda expressions also can be treated like a variables, it could be assigned, it could be pass as parameter, so therefore the code is easily reuse. Let’s create a new class so you can see how we can use lambda as parameters:

public class AnalyzerTool {

  public Boolean analyze(String text, String keyword, StringAnalyzer analizer){
    return analizer.analyze(text,keyword);
  }

}

So now we can pass an StringAnalyzer as parameter which could be a lambda expression

public class MainAnalyzer {

  public static void main(String[] args){
    StringAnalyzer analyzerContains = (String text, String keyword) -> text.contains(keyword);
    StringAnalyzer analyzerEndsWith = (String text, String keyword) -> text.endsWith(keyword);

    assert analyzerContains.analyze("In the end, it's not the years in your life that count. It's the life in your years", "life");
    assert analyzerEndsWith.analyze("In the end, it's not the years in your life that count. It's the life in your years", "years");

  }
}

To download the code:

git clone https://github.com/josdem/java-topics.git
cd lambda-expressions/string-analyzer

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Lambda Expressions