Java 8 Learn Notes

Main reference:

[1] http://winterbe.com/posts/2014/03/16/java-8-tutorial/

[2] https://plus.google.com/107771575694787223844/posts

1. Add default methods for Interfaces

In java 8, Interface could have concrete methods using keyword 'default'. 

interface People {
    void kick(int a);

    default String eat(int a) {
        return "Done";
    }
}

2. Lambda Expressions

Before Java 8:

List<String> ages = Arrays.asList(20, 30, 40, 50);

Collections.sort(names, new Comparator<String>() {
    @Override
    public int compare(int a, int b) {
        return b.compareTo(a);
    }
});

Use Lambda:

Collections.sort(names, (a, b) -> b.compareTo(a));

3. Functional Interfaces

Each lambda corresponds to a given type, specified by an interface. A so called functional interface must contain exactly one abstract method declaration.

@FunctionalInterface
interface Converter<F, T> {
    T convert(F from);
}

The code is also valid if the @FunctionalInterface annotation would be ommited.

Converter<String, Integer> converter = (from) -> Integer.valueOf(from);
Integer converted = converter.convert("123");
System.out.println(converted);    // 123

4. Method and Constructor References

The above example code can be further simplified by utilizing static method references:

Converter<String, Integer> converter = Integer::valueOf;
Integer converted = converter.convert("123");
System.out.println(converted);   // 123

Java 8 enables you to pass references of methods or constructors via the :: keyword. The above example shows how to reference a static method. But we can also reference object methods: 

class Something {
    String startsWith(String s) {
        return String.valueOf(s.charAt(0));
    }
}

Something something = new Something();
Converter<String, String> converter = something::startsWith;
String converted = converter.convert("Java");
System.out.println(converted);    // "J"

Let's see how the :: keyword works for constructors. First we define an example bean with different constructors: 

class Person {
    String firstName;
    String lastName;

    Person() {}

    Person(String firstName, String lastName) {
        this.firstName = firstName;
        this.lastName = lastName;
    }
}

Next we specify a person factory interface to be used for creating new persons:

interface PersonFactory<P extends Person> {
    P create(String firstName, String lastName);
}

Instead of implementing the factory manually, we glue everything together via constructor references:

PersonFactory<Person> personFactory = Person::new;   //http://www.byteslounge.com/tutorials/java-8-method-references
Person person = personFactory.create("Peter", "Parker");

We create a reference to the Person constructor via Person::new. The Java compiler automatically chooses the right constructor by matching the signature of PersonFactory.create.

5. Lambda Scopes

Accessing outer scope variables from lambda expressions is very similar to anonymous objects. You can access final variables from the local outer scope as well as instance fields and static variables.

a. Accessing Local Variables:

final int num = 1; //should be explicitly or implicitly final
Converter<Integer, String> stringConverter =
        (from) -> String.valueOf(from + num);  //(parameter) -> return codes, this is a method

stringConverter.convert(2);     // 3

Writing to num from within the lambda expression is also prohibited.

b. Accessing Fields and Static Variables

In constrast to local variables we have both read and write access to instance fields and static variables from within lambda expressions. This behaviour is well known from anonymous objects.

class Lambda4 {
    static int outerStaticNum;
    int outerNum;

    void testScopes() {
        Converter<Integer, String> stringConverter1 = (from) -> {
            outerNum = 23;
            return String.valueOf(from);
        };

        Converter<Integer, String> stringConverter2 = (from) -> {
            outerStaticNum = 72;
            return String.valueOf(from);
        };
    }
}

c. Accessing Default Interface Methods

The "default" keyword in Inteface does not work for lambda Interfaces. Default methods cannot be accessed from within lambda expressions.

6. Built-in Functional Interfaces

The JDK 1.8 API contains many built-in functional interfaces. Some of them are well known from older versions of Java like Comparator or Runnable. Those existing interfaces are extended to enable Lambda support via the @FunctionalInterface annotation.

But the Java 8 API is also full of new functional interfaces to make your life easier. Some of those new interfaces are well known from the Google Guava library. Even if you're familiar with this library you should keep a close eye on how those interfaces are extended by some useful method extensions.

Predicates

//http://howtodoinjava.com/2014/04/04/how-to-use-predicate-in-java-8/
Predicate<String> predicate = (s) -> s.length() > 0;

predicate.test("foo");              // true
predicate.negate().test("foo");     // false

Predicate<Boolean> nonNull = Objects::nonNull;
Predicate<Boolean> isNull = Objects::isNull;

Predicate<String> isEmpty = String::isEmpty;
Predicate<String> isNotEmpty = isEmpty.negate();

Functions

Functions accept one argument and produce a result. Default methods can be used to chain multiple functions together (compose, andThen).

Function<String, Integer> toInteger = Integer::valueOf;
Function<String, String> backToString = toInteger.andThen(String::valueOf);

backToString.apply("123");     // "123"

Suppliers & Consumers

Suppliers produce a result of a given generic type. Unlike Functions, Suppliers don't accept arguments.

Supplier<Person> personSupplier = Person::new;  //http://www.byteslounge.com/tutorials/java-8-consumer-and-supplier
personSupplier.get();   // new Person

Consumers represents operations to be performed on a single input argument.

Consumer<Person> greeter = (p) -> System.out.println("Hello, " + p.firstName);
greeter.accept(new Person("Luke", "Skywalker"));

Comparators

Comparators are well known from older versions of Java. Java 8 adds various default methods to the interface.

Comparator<Person> comparator = (p1, p2) -> p1.firstName.compareTo(p2.firstName);

Person p1 = new Person("John", "Doe");
Person p2 = new Person("Alice", "Wonderland");

comparator.compare(p1, p2);             // > 0
comparator.reversed().compare(p1, p2);  // < 0

Optionals

Optionals are not functional interfaces, instead it's a nifty utility to prevent NullPointerException. Optional is a simple container for a value which may be null or non-null. Think of a method which may return a non-null result but sometimes return nothing. Instead of returning nullyou return an Optional in Java 8.

Optional<String> optional = Optional.of("bam");

optional.isPresent();           // true
optional.get();                 // "bam"
optional.orElse("fallback");    // "bam"

optional.ifPresent((s) -> System.out.println(s.charAt(0)));     // "b"

7. Streams

A java.util.Stream represents a sequence of elements on which one or more operations can be performed. Stream operations are either intermediate or terminal. While terminal operations return a result of a certain type, intermediate operations return the stream itself so you can chain multiple method calls in a row. Streams are created on a source, e.g. ajava.util.Collection like lists or sets (maps are not supported). Stream operations can either be executed sequential or parallel.

List<String> stringCollection = new ArrayList<>();
stringCollection.add("ddd2");
stringCollection.add("aaa2");
stringCollection.add("bbb1");
stringCollection.add("aaa1");
stringCollection.add("bbb3");
stringCollection.add("ccc");
stringCollection.add("bbb2");
stringCollection.add("ddd1");

Filter

Filter accepts a predicate to filter all elements of the stream. This operation is intermediatewhich enables us to call another stream operation (forEach) on the result. ForEach accepts a consumer to be executed for each element in the filtered stream. ForEach is a terminal operation. It's void, so we cannot call another stream operation.

stringCollection
    .stream()
    .filter((s) -> s.startsWith("a"))
    .forEach(System.out::println);

// "aaa2", "aaa1"

Sorted

Sorted is an intermediate operation which returns a sorted view of the stream. The elements are sorted in natural order unless you pass a custom Comparator. 

stringCollection
    .stream()
    .sorted()
    .filter((s) -> s.startsWith("a"))
    .forEach(System.out::println);

// "aaa1", "aaa2"

Keep in mind that sorted does only create a sorted view of the stream without manipulating the ordering of the backed collection. The ordering of stringCollection is untouched:

System.out.println(stringCollection);
// ddd2, aaa2, bbb1, aaa1, bbb3, ccc, bbb2, ddd1

Map

The intermediate operation map converts each element into another object via the given function. The following example converts each string into an upper-cased string. But you can also use map to transform each object into another type. The generic type of the resulting stream depends on the generic type of the function you pass to map.

stringCollection
    .stream()
    .map(String::toUpperCase)
    .sorted((a, b) -> b.compareTo(a))
    .forEach(System.out::println);

// "DDD2", "DDD1", "CCC", "BBB3", "BBB2", "AAA2", "AAA1"

Match

Various matching operations can be used to check whether a certain predicate matches the stream. All of those operations are terminal and return a boolean result.

boolean anyStartsWithA =
    stringCollection
        .stream()
        .anyMatch((s) -> s.startsWith("a"));

System.out.println(anyStartsWithA);      // true

boolean allStartsWithA =
    stringCollection
        .stream()
        .allMatch((s) -> s.startsWith("a"));

System.out.println(allStartsWithA);      // false

boolean noneStartsWithZ =
    stringCollection
        .stream()
        .noneMatch((s) -> s.startsWith("z"));

System.out.println(noneStartsWithZ);      // true

Count

Count is a terminal operation returning the number of elements in the stream as a long.

long startsWithB =
    stringCollection
        .stream()
        .filter((s) -> s.startsWith("b"))
        .count();

System.out.println(startsWithB);    // 3

Reduce

This terminal operation performs a reduction on the elements of the stream with the given function. The result is an Optional holding the reduced value.

Optional<String> reduced =
    stringCollection
        .stream()
        .sorted()
        .reduce((s1, s2) -> s1 + "#" + s2);

reduced.ifPresent(System.out::println);
// "aaa1#aaa2#bbb1#bbb2#bbb3#ccc#ddd1#ddd2"

8. Parallel Streams

As mentioned above streams can be either sequential or parallel. Operations on sequential streams are performed on a single thread while operations on parallel streams are performed concurrent on multiple threads.

The following example demonstrates how easy it is to increase the performance by using parallel streams.

First we create a large list of unique elements:

int max = 1000000;
List<String> values = new ArrayList<>(max);
for (int i = 0; i < max; i++) {
    UUID uuid = UUID.randomUUID();
    values.add(uuid.toString());
}

For Sequential Sort

long t0 = System.nanoTime();

long count = values.stream().sorted().count();
System.out.println(count);

long t1 = System.nanoTime();

long millis = TimeUnit.NANOSECONDS.toMillis(t1 - t0);
System.out.println(String.format("sequential sort took: %d ms", millis));

// sequential sort took: 899 ms

For Parallel Sort

long t0 = System.nanoTime();

long count = values.parallelStream().sorted().count();
System.out.println(count);

long t1 = System.nanoTime();

long millis = TimeUnit.NANOSECONDS.toMillis(t1 - t0);
System.out.println(String.format("parallel sort took: %d ms", millis));

// parallel sort took: 472 ms

9. Map

As already mentioned maps don't support streams. Instead maps now support various new and useful methods for doing common tasks.

Map<Integer, String> map = new HashMap<>();

for (int i = 0; i < 10; i++) {
    map.putIfAbsent(i, "val" + i);
}

map.forEach((id, val) -> System.out.println(val));

The above code should be self-explaining: putIfAbsent prevents us from writing additional if null checks; forEach accepts a consumer to perform operations for each value of the map.

This example shows how to compute code on the map by utilizing functions:

map.computeIfPresent(3, (num, val) -> val + num);
map.get(3);             // val33

map.computeIfPresent(9, (num, val) -> null);
map.containsKey(9);     // false

map.computeIfAbsent(23, num -> "val" + num);
map.containsKey(23);    // true

map.computeIfAbsent(3, num -> "bam");
map.get(3);             // val33

Next, we learn how to remove entries for a a given key, only if it's currently mapped to a given value:

map.remove(3, "val3");
map.get(3);             // val33

map.remove(3, "val33");
map.get(3);             // null

Another helpful method:

map.getOrDefault(42, "not found");  // not found

Merging entries of a map is quite easy:

map.merge(9, "val9", (value, newValue) -> value.concat(newValue));
map.get(9);             // val9

map.merge(9, "concat", (value, newValue) -> value.concat(newValue));
map.get(9);             // val9concat

Merge either put the key/value into the map if no entry for the key exists, or the merging function will be called to change the existing value.