Lambda Expressions with Predicates

Table of Contents

• Introduction
• What is a Predicate in Java?
• Lambda Expressions with Predicates
• Step-by-Step Guide to Implementing Predicates Using Lambda
• Code Walkthrough
• Conclusion

1. Introduction

Java 8 introduced both Lambda expressions and the Predicate functional interface as part of its functional programming support.
Predicates are used to test conditions in code and return a boolean result, which makes them ideal for decision-making processes.
This article will guide you through using Predicates with Lambda expressions, highlighting how you can create clean and efficient code by combining the two.

2. What is a Predicate in Java?

The Predicate interface is a part of the java.util.function package and represents a boolean-valued function. A Predicate can take an argument and return a boolean result.
It is typically used for filtering, testing conditions, and decision-making in functional programming.

Key Features of a Predicate:

• Represents a condition or test on a single input.
• Can be used with higher-order functions such as filter() and map().
• Supports logical operations like and(), or(), and negate() to combine multiple conditions.

Pros of Using Predicate:

• Simplifies condition-based logic in functional programming.
• Works well with stream processing and filtering.

Cons of Using Predicate:

• Overuse of predicates can reduce code readability.
• Understanding the combined logical operators (and(), or(), etc.) can be tricky for beginners.

Comparison between Conditional Statements and Predicates:

Approach	Readability	Flexibility	Code Size
Conditional Statements	High	Low	Larger
Predicates	Medium	High	Smaller

When to Use:

• Use predicates in functional programming contexts like filtering and decision-making logic.
• Particularly useful in stream processing or lambda expressions.

3. Lambda Expressions with Predicates

The Predicate interface works well with Lambda expressions because it is a functional interface with a single abstract method test().
The Lambda expression can be used to implement the test() method and define the condition to evaluate.

Example:

Let’s look at an example of how a Predicate is used with a Lambda expression.

In this example:

• lessThan18 is a Predicate that checks if a value is less than 18.
• moreThan18 is another Predicate that checks if a value is greater than 18.
• The or() method combines the two predicates with an OR condition.
• The negate() method negates the combined result.

4. Step-by-Step Guide to Implementing Predicates Using Lambda

Step 1: Define the Predicate

First, define a Predicate using a Lambda expression. In this example, we are using an IntPredicate, which is a specialized Predicate for integer values:

This Lambda expression evaluates whether a value is less than 18.

Step 2: Combine Predicates

Predicates can be combined using logical operations like and(), or(), and negate():

In this case, we define a second predicate that checks if a value is greater than 18.

Step 3: Use the Predicate

The predicates can be combined with logical operators. For instance, we can negate the result of the combined conditions:

In this example, the test() method evaluates whether the value 10 satisfies the negated condition (i.e., the value is neither less than nor greater than 18).

5. Code Walkthrough

Here is the complete code provided in the project file:

Explanation:

• Predicate Definition: The predicates lessThan18 and moreThan18 check if a value is less than or greater than 18, respectively.
• Logical Combination: The predicates are combined using the or() method, and then negated using the negate() method.
• Evaluation: The test(10) method checks whether the value 10 satisfies the negated condition (i.e., it is neither less than nor greater than 18).

Output:

The output is false because 10 is less than 18, so the negated condition is not satisfied.

6. Conclusion

Lambda expressions and the Predicate interface work hand-in-hand to create concise and readable code for evaluating conditions. Predicates simplify condition-based logic and provide a powerful tool for functional programming in Java.

By using predicates with Lambda expressions, developers can write more expressive code, especially in scenarios that involve filtering or decision-making.