Lambda Expression Target Type Error

Why This Happens

Lambda expressions in Java require a target type that is a functional interface, which is an interface with exactly one abstract method. You cannot assign a lambda to Object, a class type, or an interface with multiple abstract methods.

The Problem

Object fn = () -> System.out.println("hello"); // Error: Object is not a functional interface

The Fix

Runnable fn = () -> System.out.println("hello"); // Runnable is a functional interface
// Or use a specific functional interface:
Consumer<String> printer = s -> System.out.println(s);

Step-by-Step Fix

1. 1

   Identify the target type

   Check what type the lambda is being assigned to or what parameter type it is being passed as.

2. 2

   Verify it is a functional interface

   The target type must be an interface with exactly one abstract method. Check if it has @FunctionalInterface annotation.

3. 3

   Use the correct functional interface

   Use Runnable, Supplier<T>, Consumer<T>, Function<T,R>, Predicate<T>, or define your own @FunctionalInterface.