Lambda expression in java not working

Question

I'm tryig to make a lambda expresion for an ActionListener, but it's giving me an IllegalStart of expression, what i'm trying to run so far looks like this:

    JFrame frame = new JFrame();
    JButton boton = new JButton("Lambda Button");
    boton.addActionListener(event -> System.out.println("Hello World!"));
    frame.add(boton);

    frame.setVisible(true);

On the other hand, when i use this code instead:

JFrame frame = new JFrame();
    JButton boton = new JButton("Lambda Button");
    boton.addActionListener( new ActionListener() {
                                public void actionPerformed(ActionEvent e)
                                {
                                    System.out.println("Hello World!");
                                }
                                }   );
    frame.add(boton);
    frame.setVisible(true);

It works perfectly fine,

Initially i tought the issue could be the version of java i'm running, but i just updated and keeps doing the same, when i do a java -version i gives me the following:

java -version java version "1.8.0_45" Java(TM) SE Runtime Environment (build 1.8.0_45-b14) Java HotSpot(TM) Client VM (build 25.45-b02, mixed mode)

So, as far as i know it have a version compatible with lambda expression but not succed on making them work, any ideas or suggestion of what could he happening ?

EDIT: When i try to compile i get this:

Prueba.java:57: error: illegal start of expression
            boton.addActionListener(event -> System.out.println("Hello World !"));
                                           ^1 error

EDIT2: I'm not using any IDE, im compiling from the command line

What IDE are you using? Are you sure the JDK for your project is defined to Java 8 JDK home? — mtyurt, Apr 15 '15 at 15:38
In your IDE, are you compiling your code as 1.8? (in Eclipse: project properties > java compiler > JDK compliance). Regardless of which JRE you are using to run it, you still need to compile it as 1.8 too. — David Lavender, Apr 15 '15 at 15:38
Im not using any IDE, im running it from the command line... — Jose Elias Vargas Lebreault, Apr 15 '15 at 15:51
@user4792574 You run the command line from the same terminal as `java -version` ? — Jean-François Savard, Apr 15 '15 at 15:54
also, verify that "event" is not the name of an already defined local variable or current method parameter. Lambda parameter names are defined in the same scope as their enclosing environment. — Olivier Croisier, Apr 15 '15 at 15:59
@OlivierCroisier The error would be "variable event is already defined". — Jean-François Savard, Apr 15 '15 at 16:02
java -version java version "1.8.0_45" Java(TM) SE Runtime Environment (build 1.8.0_45-b14) Java HotSpot(TM) Client VM (build 25.45-b02, mixed mode) javac -version: javac 1.7.0_13 Could be because the compiler doesn't have the same version as the JRE ? — Jose Elias Vargas Lebreault, Apr 15 '15 at 16:10
@user4792574 You are compiling with java 7, this is why it does not work. As I mentionned in my edit. — Jean-François Savard, Apr 15 '15 at 16:11
@flo what do you mean by "not necessarily" ? How could someone compile java 8 code with a java 7 compiler ? — Jean-François Savard, Apr 15 '15 at 16:32
ok, perfect, the issue was with the version of javac, installed the last version now and runs perfectly, thank you very much guys, you've all been of a great help. — Jose Elias Vargas Lebreault, Apr 15 '15 at 17:35
@Jean-FrançoisSavard I meant a Java 8 JRE does not necessarily mean a JAVA 8 javac. — flo, Apr 15 '15 at 20:47
@flo Oh I misunderstood your comment, make more sense now :) — Jean-François Savard, Apr 15 '15 at 22:17

score 10 · Accepted Answer · edited May 23 '17 at 10:29

Edit

As per your comments, you are not using any IDE and your javac version is 1.7. You need to compile with java 8, else the lambda won't be recognized.

I will keep this part in my answer as it may solve the problem for someone else, even if this was not the solution in this case.

Your lambda is perfectly fine. The error is probably due to a wrong compiler compliance level setting.

If you use eclipse, refer to this post for how to change the compiler compliance level :

How to change JDK version for an Eclipse project

If you use Netbeans, remember (this is something many people forget) that you have to set both the source and libraries target to 8 if you want it to function properly.

Source enter image description here

Libraries

enter image description here

Calling java -version from the command-line does not mean that the proper library is set in the IDE, so you have to verify this.

If you use any other IDE then google "How to change jdk version on YourIDE".

score 5 · Answer 2 · answered Apr 15 '15 at 15:51

5

And if you are using intellij by any chance, please make sure Project language level is set to 8 - Lambdas..

enter image description here

answered Apr 15 '15 at 15:51

Mukesh Kumar

783
1
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score 2 · Answer 3 · edited Oct 27 '22 at 09:35

2

boton.addActionListener(event -> System.out.println("Hello World!")); - Lambda Expression is perfectly fine. You are passing event to actionPerformed method of ActionListener functional interface which will be mapped while compilation of code using java 8 compiler.

Please check your java version.

edited Oct 27 '22 at 09:35

Michu93

5,058
7
47
80

answered Jan 23 '17 at 17:03

kavita

139
9

Lambda expression in java not working

3 Answers3