Java 8 : Frequently Asked Questions for Interview - 2

What is Lamdba Expression

Lambda Expressions are at the heart of Java SE 8. A “Lambda Expression” is a block of code that you can pass around so it can be executed later, once or multiple times.

Lambda Expressions are Closures or anonymous methods that provide developers with a simple and compact means for representing behavior as data.

• The use or lambda expression improves readability, performance and reusability
• The use of lambda expressions is all about abstracting complexity away into libraries
• The use of lambda expressions improves developer productivity  

For example:  If one wants to model a workflow such as ` do A before you start, do B for each file in the file group, do C upon error and do D after you finish`, than one needs to break the phases of the workflow & the client code has to be directly involved in each phase.

=> Java (before java8) lacks in tools which can express the behavior from A to D. It also affects the APIs design.

Things to Remember in Lambda Expressions :

• A lambda expression is a block of code with parameters.

BinaryOperator add = (long x, long y) -> {
    x+y
}

• Use a lambda expression whenever you want a block of code executed at a later point in time.

Runnable myRunnable = () -> { 
    System.out.Println("Executing the Run method inside the Runnable!!!");
    DateFormat df = new SimpleDateFormat("yyyy/MM/dd HH:mm:ss");
    Date date = new Date();
    System.out.println("Running at a deferred time > "+df.format(date));
}

• Lambda expressions can be converted to functional interfaces.
• Lambda expressions can access effectively final or final variables from the enclosing scope.

public void enclosingMethod() {

    final String IamFinalVariable = "This is an immutable final variable";
    Button button = new Button("click me");
    button.addActionListener( event -> System.out.println(IamFinalVariable);

    String IamEffectivelyFinalVariable = "This is effectively final variable. Its value should not change in this scope.";
    IamEffectivelyFinalVariable = "Do not try to change me. Otherwise compiler will error out";

    JRadioButton finalRadioButton = new JRadioButton("Final Var Selected");
    finalRadioButton.setActionCommand(IamFinalVariable);
    JRadioButton effectiveFinalRadioButton = new JRadioButton("Effective Final Var Selected");
    effectiveFinalRadioButton.setActionCommand(IamEffectivelyFinalVariable);

    ButtonGroup group = new ButtonGroup();
    group.add(finalRadioButton);
    group.add(effectiveFinalRadioButton);

    ActionListener selectOrDeselect = event -> {
        System.out.println("You are talking to >"+ event.getActionCommand(    ));
   }
}

• Method and constructor references refer to methods or constructors without invoking them.
• You can now add default and static methods to interfaces that provide concrete implementations.
• You must resolve any conflicts between default methods from multiple interfaces