Lambda Expressions

Lambda Expressions
An Introduction with Examples in Java

Prof. David Bernstein
James Madison University

Computer Science Department

bernstdh@jmu.edu

Motivation

Review:
- Local classes allow for the inclusion of classes that are tightlty coupled to a black of code
- Anonymous classes allow for the inclusion of classes that are only used once and, hence, needn't be named
Lambda Expressions:
- Provide a compact syntax for the equivalent of an anonymous class with only one method

Syntax

Formal Parameters:
- A comma-separated list enclosed in parentheses (if there is more than one), with or without type declarations
The Arrow Token:
- ->
The Body:
- A single experssion or block statement

Details About the Body

A Single Expression:
- The expression is evaluated and the result is returned
A Block with a return Statement:
- Since the return statement is not an expression, it must be within enclosing curly brackets (i.e., in a block)
A Block without a return:
- Is appropriate for void invocations

Common Uses

Compact Versions of Anonymous Classes:
- An anonymous class would work but seems needlessly verbose
Method Passing:
- You want to pass code as data (as in the command pattern and strategy pattern)

Access

Members in the Enclosing Class:
- The local class can access all members
Access to Local Variables:
- The local class can access all local variables that are declared to be final or (since Java v8) are effectively final

An Example with a Traditional Interface

javaexamples/nested/ComparatorExample.java (Fragment: lambda)

An Example with a Nested Interface

The Utility Class and Nested Interface javaexamples/nested/VectorMath.java

An Example with a Nested Interface (cont.)

Using the Utility Class with One Lambda Expression javaexamples/nested/VectorMathDriver.java (Fragment: rectilinear)

An Example with a Nested Interface (cont.)

Using the Utility Class with Another Lambda Expression javaexamples/nested/VectorMathDriver.java (Fragment: Euclidean)

A JUnit Example

@Test public void sqrt() { Throwable exception = assertThrows(IllegalArgumentException.class, () -> { RealMath.sqrt(-1.0); }); assertEquals("The result is not real", exception.getMessage()); }

A GUI Example

JButton close = new JButton("Close"); // The most complete version close.addActionListener( (ActionEvent event) -> {f.dispose();} );

JButton close = new JButton("Close"); // Because the method is void it needn't be in a block close.addActionListener( (ActionEvent event) -> f.dispose() );

JButton close = new JButton("Close"); // The type of the parameter can be omitted close.addActionListener( (event) -> f.dispose() );

JButton close = new JButton("Close"); // Because there is one parameter the parentheses can be omitted close.addActionListener( event -> f.dispose() );

There's Always More to Learn