Java Program to Iterate over ArrayList using Lambda Expression

Introduction

The use of lambda expressions in Java simplifies the process of iterating over collections such as ArrayLists. Introduced in Java 8, lambda expressions provide a clear and concise way to represent one-method interfaces using an expression. They are particularly useful for implementing functional programming concepts in Java, making tasks like iterating over collections, filtering data, and applying operations on each element much simpler.

In this article, you will learn how to effectively leverage lambda expressions to iterate over an ArrayList in Java. Discover techniques and examples that enhance readability and efficiency in handling collections, ranging from simple print operations to more complex data manipulations.

Basic Iteration with Lambda Expressions

Using lambda expressions for iterating over ArrayList elements introduces a more functional style of writing Java code. Here's how you can achieve this:

Print Elements of an ArrayList

1. First, create an ArrayList and add some elements to it.

2. Use the forEach method with a lambda expression to print each element.

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   import java.util.ArrayList;
   
   ArrayList<String> fruits = new ArrayList<>();
   fruits.add("Apple");
   fruits.add("Banana");
   fruits.add("Cherry");
   fruits.forEach(item -> System.out.println(item));

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   In this code, fruits.forEach(item -> System.out.println(item)) iterates over each element of the fruits ArrayList and prints it. The lambda expression item -> System.out.println(item) specifies the action to be performed on each element.

Modify Elements Before Printing

1. Start with an ArrayList of Integer.

2. Use forEach to modify and print elements, such as printing their squares.

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   import java.util.ArrayList;
   
   ArrayList<Integer> numbers = new ArrayList<>();
   numbers.add(1);
   numbers.add(2);
   numbers.add(3);
   numbers.forEach(number -> System.out.println(number * number));

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   Here, each number in numbers is squared and printed. The lambda expression number -> System.out.println(number * number) modifies each element before printing.

Using Lambda Expressions for Condition-Based Actions

Lambda expressions paired with conditional logic offer powerful solutions for custom iterations over ArrayList.

Print Elements Based on Condition

1. Create an ArrayList with elements.

2. Use forEach along with a conditional statement inside the lambda expression to selectively print elements.

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   import java.util.ArrayList;
   
   ArrayList<Integer> ages = new ArrayList<>();
   ages.add(18);
   ages.add(22);
   ages.add(16);
   ages.forEach(age -> {
       if (age >= 18) {
           System.out.println(age + " is eligible.");
       } else {
           System.out.println(age + " is not eligible.");
       }
   });

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   In this example, each age is checked for eligibility (being 18 or older). The lambda expression includes a multi-line block, allowing for more complex operations like conditional logic.

Advanced Usage of Lambda in Collection Operations

Lambda expressions can not only iterate but also seamlessly integrate with other Java Stream API operations for more sophisticated manipulations.

Filter and Execute Actions

1. Prepare an ArrayList with mixed elements.

2. Stream the elements, filter them, and perform an action on the filtered set.

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   import java.util.ArrayList;
   
   ArrayList<String> names = new ArrayList<>();
   names.add("John");
   names.add("Sarah");
   names.add("Sue");
   names.stream()
        .filter(name -> name.startsWith("S"))
        .forEach(name -> System.out.println(name + " starts with S"));

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   This code uses stream() coupled with filter() and forEach(). It filters out names that start with the letter "S" and then prints them. The lambda in filter() defines the criteria, while the lambda in forEach() defines the action.

Conclusion

Lambda expressions in Java provide a compact and expressive way to handle operations on collections, especially for iterating over ArrayList. They not only reduce the boilerplate code but also enhance the readability and maintainability of your Java applications. Experimenting with lambda expressions in various scenarios like simple iterations, condition-based actions, and integrated stream operations unlocks extensive possibilities in functional programming in Java. By mastering these techniques, you ensure your Java code is both efficient and modern.