Lambda expressions
are a new and important feature included in Java SE 8. They provide a clear and
concise way to represent one method interface using an expression. Lambda
expressions also improve the Collection libraries making it easier to iterate
through, filter, and extract data from a Collection.
A lambda
expression can be understood as a concise representation of
an anonymous function that can be passed around: it doesn’t have a name, but it
has a list of parameters, a body, a return type, and also possibly a list of
exceptions that can be thrown.
o
Anonymous—
Anonymous because
it doesn’t have an explicit name like a method would normally have: less to
write and think about!
o
Function—
Function because
a lambda isn’t associated with a particular class like a method is. But like a
method, a lambda has a list of parameters, a body, a return type, and a
possible list of exceptions that can be thrown.
o
Passed around—
A lambda expression can be passed as argument to
a method or stored in a variable.
o
Concise—
You don’t need to write a lot of boilerplate like
you do for anonymous classes.
Lambdas
technically let you do anything that you could do prior to Java 8. But you no
longer have to write clumsy code using anonymous classes.
The result is that your code will be clearer
and more flexible. For example, using a lambda expression you can create a
custom Comparator object in a more concise way.
Before:
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Comparator<Apple> byWeight = new Comparator<Apple>() {
public int compare(Apple a1, Apple a2){
return a1.getWeight().compareTo(a2.getWeight());
}
};
|
After (with
lambda expressions):
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Comparator<Apple> byWeight =
(Apple a1, Apple a2) -> a1.getWeight().compareTo(a2.getWeight());
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Lambda Expression Syntax
Lambda expressions address the bulkiness of anonymous inner. A lambda
expression is composed of three parts.
A
lambda expression is composed of parameters, an arrow,
and
a body.
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Argument List
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Arrow Token
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Body
|
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(int x, int y)
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->
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x + y
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The body can be either a single expression or a statement block. In the
expression form, the body is simply evaluated and returned. In the block form,
the body is evaluated like a method body and a return statement returns control
to the caller of the anonymous method. The break and continue keywords
are illegal at the top level, but are permitted within loops. If the body
produces a result, every control path must return something or throw an
exception.
Take a look at these examples:
(int x, int y) -> x + y
() -> 42
(String s) -> {
System.out.println(s); }
The first expression takes two integer arguments, named x and y,
and uses the expression form to return x+y.
The second expression takes no arguments and uses the expression form to
return an integer 42.
The third expression takes a string and uses the block form to print the
string to the console, and returns nothing.
Lambda Examples
Runnable Lambda
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public class RunnableTest
{
public static void main(String[]
args) {
System.out.println("===
RunnableTest ===");
// Anonymous Runnable
Runnable r1 = new Runnable(){
@Override
public void run(){
System.out.println("Hello
world one!");
}
};
// Lambda Runnable
Runnable r2 = () ->
System.out.println("Hello
world two!");
// Run them
r1.run();
r2.run();
}
}
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Comparator Lambda
In Java, the Comparator class is used for sorting collections.
In the following example, an ArrayList consisting of Person objects is sorted based on surName. The following are the fields
included in the Person class.
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public class Person {
private String givenName;
private String surName;
private int age;
private String eMail;
private String phone;
private String address;
}
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The following code applies a Comparator by using an anonymous inner class and
a couple lambda expressions.
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public class
ComparatorTest {
public static void main(String[]
args) {
// Create List of Person
List<Person> personList1 = null;
// Sort with Inner Class
Collections.sort(personList1, new
Comparator<Person>(){
public int
compare(Person p1, Person p2){
return p1.getSurName().compareTo(p2.getSurName());
}
});
System.out.println("===
Sorted Asc SurName ===");
for(Person p:personList1){
System.out.println(
"Name: " + p.getGivenName()
+ "
" + p.getSurName());
}
// Use Lambda instead
// Print Asc
System.out.println("===
Sorted Asc SurName ===");
Collections.sort(personList1, (Person p1, Person p2) -> p1.getSurName().compareTo(p2.getSurName()));
for(Person p:personList1){
System.out.println(
"Name: " + p.getGivenName()
+ "
" + p.getSurName());
}
// Print Desc
System.out.println("===
Sorted Desc SurName ===");
Collections.sort(personList1, (p1, p2) -> p2.getSurName().compareTo(p1.getSurName()));
for(Person p:personList1){
System.out.println(
"Name: " + p.getGivenName()
+ "
" + p.getSurName());
}
}
}
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