Lambda Expressions - Beginning Java 8 Language Features - page 131

The target type of the lambda expression in this context is T . The compiler uses the following rules to determine
whether the <LambdaExpression> is assignment compatible with its target type T :
•

T must be a functional interface type.
The lambda expression has the same number and type of parameters as the abstract method

•

of T . For an implicit lambda expression, the compiler will infer the types of parameters from
the abstract method of T .
The type of the returned value from the body of the lambda expression is assignment

•

compatible to the return type of the abstract method of T .
If the body of the lambda expression throws any checked exceptions, those exceptions must •

be compatible with the declared throws clause of the abstract method of T . It is a compile-time
error to throw checked exceptions from the body of a lambda expression, if its target type's
method does not contain a throws clause.
Let's look at few examples of target typing. Consider two functional interfaces, Adder and Joiner , as shown in
Listing 5-1 and Listing 5-2, respectively.

Listing 5-1. A Functional Interface Named Adder

// Adder.java
package com.jdojo.lambda;

@FunctionalInterface
public interface Adder {
double add(double n1, double n2);
}

Listing 5-2. A Functional Interface Named Joiner

// Joiner.java
package com.jdojo.lambda;

@FunctionalInterface
public interface Joiner {
String join(String s1, String s2);
}

The add() method of the Adder interface adds two numbers. The join() method of the Joiner interface
concatenates two strings. Both interfaces are used for trivial purposes; however, they will serve the purpose of
demonstrating the target typing for lambda expressions very well.
Consider the following assignment statement:

Adder adder = (x, y) -> x + y;

The type of the adder variable is Adder . The lambda expression is assigned to the variable adder , and therefore,
the target type of the lambda expression is Adder . The compiler verifies that Adder is a functional interface. The
lambda expression is an implicit lambda expression. The compiler finds that the Adder interface contains a double
add(double, double) abstract method. It infers the types for x and y parameters as double and double , respectively.
At this point, the compiler treats the statement as shown:

Adder adder = (double x, double y) -> x + y;