I am dubious
A few days ago, I have been writing the company’s official website page. I have been cutting pictures for two days. It is too difficult to live
Last article: “Effective.Java” read the code of the grass hall note 4
41. Lambda expressions are superior to anonymous classes
Lambda expressions are a syntax-sugar added to Java 1.8, and I feel a bit like ES6’s → functions, lambda expressions, and method references, which make it easier to build objects
// Anonymous classes and lambda expressions are provided for sorting
Collections.sort(words, new Comparator<String>() {
public int compare(String s1, String s2) {
returnInteger.compare(s1.length(), s2.length() ); }});// Lambda is much simpler
Collections.sort(words,(s1, s2) -> Integer.compare(s1.length(), s2.length()));
Copy the codeIn response to the section on constant-specific implementations of Effective.Java described in the previous article, lambda can be used to refactor to
public enum Operation {
PLUS("+", (x, y) -> x + y), MINUS("-", (x, y) -> x - y), TIMES("*", (x, y) -> x * y), DIVIDE("/", (x, y) -> x / y);
private final String symbol;
// DoubleBinaryOperator is a predefined function interface for java.util.function
private final DoubleBinaryOperator op;
Operation(String symbol, DoubleBinaryOperator op) {
this.symbol = symbol;
this.op = op;
}
@Override
public String toString(a) {
return symbol;
}
public double apply(double x, double y) {
return op.applyAsDouble(x, y);
}
public static void main(String[] args) {
double apply = Operation.PLUS.apply(1.0); System.out.print(apply); }}Copy the codeHowever, although lambda is good, but it is not a panacea, at least it is only suitable for functional interfaces, what is functional interfaces? The interface annotated by @functionalInterface has only one method in it, and since lambdas are modded, it is not recommended to use lambdas if calculations are not straightforward
42. Method references are superior to lambda expressions
The main advantage lambda has over anonymous classes is that it is more concise. What is simpler than lambda is method references. What are method references? A method reference is simply a more compact, readable Lambda expression. Note that a method reference is a Lambda expression with a double colon “::” as its operator.
List<String> lists = new ArrayList<String>();
lists.add("Tan");
lists.add("Jing");
lists.add("Jay");
lists.forEach(System.out::print);
Copy the codeA brief introduction to method references
The method reference operator is ::, which divides a method reference into a class name or an object reference and a method name
@FunctionalInterface
public interface Supplier<T> {
T get(a);
}
class Car {
Supplier is the JDK1.8 interface, which is used with lamda
public static Car create(final Supplier<Car> supplier) {
return supplier.get();
}
public static void collide(final Car car) {
System.out.println("Collided " + car.toString());
}
public void follow(final Car another) {
System.out.println("Following the " + another.toString());
}
public void repair(a) {
System.out.println("Repaired " + this.toString()); }}Copy the code- Object reference :: instance method name
final Car police = Car.create( Car::new );
cars.forEach( police::follow );
Copy the code- Class name :: Static method name
cars.forEach( Car::collide );
Copy the code- Class name :: Instance method name
cars.forEach( Car::repair );
Copy the code- The name of the class: : new
final Car car = Car.create( Car::new );
final List< Car > cars = Arrays.asList( car );
Copy the code- Type: [] : new
Of course method references are only available in jdk1.8, not anywhere else
43. Standard functional interfaces are preferred
Java provides several functions in the interface, in the package of Java. Util. Function. The function below
Main function interface
Function<T,R> gets what the method executes
R apply(T t);
Copy the codepublic class Student {
private String name;
private String address;
public static void main(String[] args) {
Function<Student, String> uft = u -> u.getName();
Student student = new Student();
student.setAddress("Changsha, Hunan");
student.setName("Tan Jingjie");
String apply = uft.apply(student);
System.out.print(apply);
}
public String getName(a) {
return name;
}
public void setName(String name) {
this.name = name;
}
public String getAddress(a) {
return address;
}
public void setAddress(String address) {
this.address = address; }}Copy the codeConsumer, use the Accept method to operate on the parameter
void accept(T t);
Copy the code// Use the same example above
Consumer<Student> cs = i->System.out.println(i.getAddress());
Student student = new Student();
student.setAddress("Changsha, Hunan");
student.setName("Tan Jingjie");
cs.accept(student);
Copy the codeThe Supplier,get method gets the result returned
T get(a);
Copy the codeSupplier<Student> spp = ()->new Student();
Student student = spp.get();
Copy the codePredicate returns true or false, and tests are performed using the test method and returns the test results
boolean test(T t);
Copy the codePredicate<Student> up = u -> ! u.getName().isEmpty(); up.test(new Student());
Copy the code
These functions can produce other variations, and generally we should use the functions defined by the JDK in preference to our own, but follow these points if we do define them:
- It will be widely used and can benefit from descriptive names.
- It has a strong contract.
- It will benefit from custom default methods.
Finally, you should annotate an @functional Interface annotation
44. Use Stream wisely and judiciously
Stream represents a Stream. It is a collection of elements that can be aggregated sequentially and in parallel
List<Integer> nums = Lists.newArrayList(1.null.3.4.null.6);
// Get the number of non-null elements in a Listnums.stream().filter(num -> num ! =null).count();
Copy the codeStream sources can include collections, arrays, files, regular expressions, etc. Elements in a Stream can be object references or primitive types
The Stream API is general-enough that virtually any computation can be performed using Stream, but just because you can, doesn’t mean you should. When used properly, streams can make programs shorter and cleaner; If used incorrectly, they can make the program difficult to read and maintain.
Some usage scenarios
- Unify transform element sequences
- Filter element sequence
- Combine sequences of elements using a single operation (such as add, join, or compute a minimum)
- Accumulate sequences of elements into a collection, possibly grouping them by some common attributes
- Searches for elements in a sequence of elements that meet certain criteria
45. Give priority to functions in the stream that have no side effects
A function (in Java is method) in the call for many times, as long as the input parameter values are the same, the output value must be the same, and in this function does not change the procedure in the implementation process of any external state (such as global variables, object properties, belong to the external state), also does not depend on any external state of the program.
What is called side effect, simple and crude explanation is like a calculator, you input 1+1, no matter how is the wind or rain he still output 2, can not say today output 2, tomorrow changed 1, that calculator manufacturer had to close down
Note: I like to use forEach because I’m used to it, but 📕 says that the forEach operation is one of the least powerful terminal operations and the least stream-friendly. It is explicitly iterative and therefore not suitable for parallelization. ForEach operations should only be used to report the results of flow calculations, not to perform calculations.
Here’s an example of code with and without side effects
// No side effects
public class NoSideEffect {
public static int add(int a, int b) {
return a+b;
}
public boolean isEmpty(String s) {
if (s == null || s.trim().length() == 0) {
return true;
}
return false; }}// As a side effect, the return value of getCount depends on the external baseValue
public class HasSideEffect {
public int baseValue;
public int getCount(int addtion) {
returnbaseValue + addtion; }}Copy the codeTo use streams properly, you must understand the collector.
Based on Lambda expressions, the collect() interface is called to accumulate the elements in the flow into a summary result using methods provided by the Collector class
Streams support two types of operations, intermediate operations (converting one stream to another) and terminal operations (streaming produces a result that consumes the stream)
// Count the number of cars
List<MemberCar> lists = new ArrayList<MemberCar>();
MemberCar memberCar = new MemberCar();
memberCar.setCarId(1);
lists.add(memberCar);
lists.stream().collect(Collectors.counting());
Copy the codeSeveral built-in collectors
| The collector | behavior |
|---|---|
toList() |
Collect elements into oneListIn the. |
toSet() |
Collect elements into oneSetIn the. |
toCollection(Supplier<Collection>) |
To collect elements to a specific typeCollectionIn the. |
toMap(Function<T, K>, Function<T, V>) |
Collect elements into oneMapConvert elements to key values based on the provided mapping function. |
summingInt(ToIntFunction<T>) |
The calculation will be providedintThe value mapping function applies to each element (as welllong 和 doubleVersion). |
summarizingInt(ToIntFunction<T>) |
The calculation will be providedintThe value mapping function applies to each element (as welllong 和 doubleVersion) of the resultssum,min,max,count 和 average. |
reducing() |
Apply reduction to elements (often used as a downstream collector, such as forgroupingBy) (various versions). |
partitioningBy(Predicate<T>) |
Elements are divided into two groups: groups for which the provided expectations are preserved and groups for which the expectations are not preserved. |
partitioningBy(Predicate<T>, Collector) |
Partitions the elements, using the specified downstream collector to process each partition. |
groupingBy(Function<T,U>) |
Group elements into oneMap, where the key is the provided function applied to the flow element and the value is the list of elements that share the key. |
groupingBy(Function<T,U>, Collector) |
Groups the elements, using the specified downstream collector to process the values associated with each group. |
minBy(BinaryOperator<T>) |
Computes the minimum value of an element (andmaxBy()The same). |
mapping(Function<T,U>, Collector) |
The provided mapping function is applied to each element using the specified downstream collector (usually used as the downstream collector itself, such as forgroupingBy) for processing. |
joining() |
Suppose the element isStringType to join these elements into a string (perhaps using delimiters, prefixes, and suffixes). |
counting() |
Count elements. (Usually used as a downstream collector.) |
46. Use Collection rather than Stream as the return type of a method
A Collection is a single-column Collection. Collectors are Collectors, like Collectors. The common Collectors are described below: java.util.
Collection is a top-level interface for collections and collections, and many familiar Collection classes implement it, such as ArrayList
public class ArrayList<E> extends AbstractList<E> implements List<E>, RandomAccess.Cloneable.java.io.Serializable{}
public abstract class AbstractList<E> extends AbstractCollection<E> implements List<E> {}
public abstract class AbstractCollection<E> implements Collection<E> {}
// It's like this
ArrayList
-- AbstractList
-- AbstractCollection
-- Collection
Copy the codeSince the Collection implements the Iterable
interface, it is an Iterable object, and there are many methods in it. Here is the source code, and you can see it posted
public interface Collection<E> extends 可迭代<E> {
// Query Operations
/**
* Returns the number of elements in this collection. If this collection
* contains more than <tt>Integer.MAX_VALUE</tt> elements, returns
* <tt>Integer.MAX_VALUE</tt>.
*
* @return the number of elements in this collection
*/
int size(a);
/**
* Returns <tt>true</tt> if this collection contains no elements.
*
* @return <tt>true</tt> if this collection contains no elements
*/
boolean isEmpty(a);
/** * Returns <tt>true</tt> if this collection contains the specified element. * More formally, returns <tt>true</tt> if and only if this collection * contains at least one element <tt>e</tt> such that * <tt>(o==null ? e==null : o.equals(e))</tt>. * *@param o element whose presence in this collection is to be tested
* @return <tt>true</tt> if this collection contains the specified
* element
* @throws ClassCastException if the type of the specified element
* is incompatible with this collection
* (<a href="#optional-restrictions">optional</a>)
* @throws NullPointerException if the specified element is null and this
* collection does not permit null elements
* (<a href="#optional-restrictions">optional</a>)
*/
boolean contains(Object o);
/**
* Returns an iterator over the elements in this collection. There are no
* guarantees concerning the order in which the elements are returned
* (unless this collection is an instance of some class that provides a
* guarantee).
*
* @return an <tt>Iterator</tt> over the elements in this collection
*/
Iterator<E> iterator(a);
/** * Returns an array containing all of the elements in this collection. * If this collection makes any guarantees as to what order its elements * are returned by its iterator, this method must return the elements in * the same order. * * <p>The returned array will be "safe" in that no references to it are * maintained by this collection. (In other words, this method must * allocate a new array even if this collection is backed by an array). * The caller is thus free to modify the returned array. * * <p>This method acts as bridge between array-based and collection-based * APIs. * *@return an array containing all of the elements in this collection
*/
Object[] toArray();
/** * Returns an array containing all of the elements in this collection; * the runtime type of the returned array is that of the specified array. * If the collection fits in the specified array, it is returned therein. * Otherwise, a new array is allocated with the runtime type of the * specified array and the size of this collection. * * <p>If this collection fits in the specified array with room to spare * (i.e., the array has more elements than this collection), the element * in the array immediately following the end of the collection is set to * <tt>null</tt>. (This is useful in determining the length of this * collection <i>only</i> if the caller knows that this collection does * not contain any <tt>null</tt> elements.) * * <p>If this collection makes any guarantees as to what order its elements * are returned by its iterator, this method must return the elements in * the same order. * * <p>Like the {@link#toArray()} method, this method acts as bridge between * array-based and collection-based APIs. Further, this method allows * precise control over the runtime type of the output array, and may, * under certain circumstances, be used to save allocation costs. * * <p>Suppose <tt>x</tt> is a collection known to contain only strings. * The following code can be used to dump the collection into a newly * allocated array of <tt>String</tt>: * * <pre> * String[] y = x.toArray(new String[0]); </pre> * * Note that <tt>toArray(new Object[0])</tt> is identical in function to * <tt>toArray()</tt>. * *@param <T> the runtime type of the array to contain the collection
* @param a the array into which the elements of this collection are to be
* stored, if it is big enough; otherwise, a new array of the same
* runtime type is allocated for this purpose.
* @return an array containing all of the elements in this collection
* @throwsArrayStoreException if the runtime type of the specified array * is not a supertype of the runtime type of every element in * this collection *@throws NullPointerException if the specified array is null
*/
<T> T[] toArray(T[] a);
// Modification Operations
/** * Ensures that this collection contains the specified element (optional * operation). Returns <tt>true</tt> if this collection changed as a * result of the call. (Returns <tt>false</tt> if this collection does * not permit duplicates and already contains the specified element.)<p> * * Collections that support this operation may place limitations on what * elements may be added to this collection. In particular, some * collections will refuse to add <tt>null</tt> elements, and others will * impose restrictions on the type of elements that may be added. * Collection classes should clearly specify in their documentation any * restrictions on what elements may be added.<p> * * If a collection refuses to add a particular element for any reason * other than that it already contains the element, it <i>must</i> throw * an exception (rather than returning <tt>false</tt>). This preserves * the invariant that a collection always contains the specified element * after this call returns. * *@param e element whose presence in this collection is to be ensured
* @return <tt>true</tt> if this collection changed as a result of the
* call
* @throws UnsupportedOperationException if the <tt>add</tt> operation
* is not supported by this collection
* @throws ClassCastException if the class of the specified element
* prevents it from being added to this collection
* @throws NullPointerException if the specified element is null and this
* collection does not permit null elements
* @throws IllegalArgumentException if some property of the element
* prevents it from being added to this collection
* @throws IllegalStateException if the element cannot be added at this
* time due to insertion restrictions
*/
boolean add(E e);
/** * Removes a single instance of the specified element from this * collection, if it is present (optional operation). More formally, * removes an element <tt>e</tt> such that * <tt>(o==null ? e==null : o.equals(e))</tt>, if * this collection contains one or more such elements. Returns * <tt>true</tt> if this collection contained the specified element (or * equivalently, if this collection changed as a result of the call). * *@param o element to be removed from this collection, if present
* @return <tt>true</tt> if an element was removed as a result of this call
* @throws ClassCastException if the type of the specified element
* is incompatible with this collection
* (<a href="#optional-restrictions">optional</a>)
* @throws NullPointerException if the specified element is null and this
* collection does not permit null elements
* (<a href="#optional-restrictions">optional</a>)
* @throws UnsupportedOperationException if the <tt>remove</tt> operation
* is not supported by this collection
*/
boolean remove(Object o);
// Bulk Operations
/**
* Returns <tt>true</tt> if this collection contains all of the elements
* in the specified collection.
*
* @param c collection to be checked for containment in this collection
* @return <tt>true</tt> if this collection contains all of the elements
* in the specified collection
* @throws ClassCastException if the types of one or more elements
* in the specified collection are incompatible with this
* collection
* (<a href="#optional-restrictions">optional</a>)
* @throws NullPointerException if the specified collection contains one
* or more null elements and this collection does not permit null
* elements
* (<a href="#optional-restrictions">optional</a>),
* or if the specified collection is null.
* @see #contains(Object)
*/
boolean containsAll(Collection
c);
/**
* Adds all of the elements in the specified collection to this collection
* (optional operation). The behavior of this operation is undefined if
* the specified collection is modified while the operation is in progress.
* (This implies that the behavior of this call is undefined if the
* specified collection is this collection, and this collection is
* nonempty.)
*
* @param c collection containing elements to be added to this collection
* @return <tt>true</tt> if this collection changed as a result of the call
* @throws UnsupportedOperationException if the <tt>addAll</tt> operation
* is not supported by this collection
* @throws ClassCastException if the class of an element of the specified
* collection prevents it from being added to this collection
* @throws NullPointerException if the specified collection contains a
* null element and this collection does not permit null elements,
* or if the specified collection is null
* @throws IllegalArgumentException if some property of an element of the
* specified collection prevents it from being added to this
* collection
* @throws IllegalStateException if not all the elements can be added at
* this time due to insertion restrictions
* @see #add(Object)
*/
boolean addAll(Collection<? extends E> c);
/**
* Removes all of this collection's elements that are also contained in the
* specified collection (optional operation). After this call returns,
* this collection will contain no elements in common with the specified
* collection.
*
* @param c collection containing elements to be removed from this collection
* @return <tt>true</tt> if this collection changed as a result of the
* call
* @throws UnsupportedOperationException if the <tt>removeAll</tt> method
* is not supported by this collection
* @throws ClassCastException if the types of one or more elements
* in this collection are incompatible with the specified
* collection
* (<a href="#optional-restrictions">optional</a>)
* @throws NullPointerException if this collection contains one or more
* null elements and the specified collection does not support
* null elements
* (<a href="#optional-restrictions">optional</a>),
* or if the specified collection is null
* @see #remove(Object)
* @see #contains(Object)
*/
boolean removeAll(Collection
c);
/**
* Removes all of the elements of this collection that satisfy the given
* predicate. Errors or runtime exceptions thrown during iteration or by
* the predicate are relayed to the caller.
*
* @implSpec
* The default implementation traverses all elements of the collection using
* its {@link #iterator}. Each matching element is removed using
* {@link Iterator#remove()}. If the collection's iterator does not
* support removal then an {@code UnsupportedOperationException} will be
* thrown on the first matching element.
*
* @param filter a predicate which returns {@code true} for elements to be
* removed
* @return {@code true} if any elements were removed
* @throws NullPointerException if the specified filter is null
* @throws UnsupportedOperationException if elements cannot be removed
* from this collection. Implementations may throw this exception if a
* matching element cannot be removed or if, in general, removal is not
* supported.
* @since1.8 * /
default boolean removeIf(Predicate<? super E> filter) {
Objects.requireNonNull(filter);
boolean removed = false;
final Iterator<E> each = iterator();
while (each.hasNext()) {
if (filter.test(each.next())) {
each.remove();
removed = true; }}return removed;
}
/**
* Retains only the elements in this collection that are contained in the
* specified collection (optional operation). In other words, removes from
* this collection all of its elements that are not contained in the
* specified collection.
*
* @param c collection containing elements to be retained in this collection
* @return <tt>true</tt> if this collection changed as a result of the call
* @throws UnsupportedOperationException if the <tt>retainAll</tt> operation
* is not supported by this collection
* @throws ClassCastException if the types of one or more elements
* in this collection are incompatible with the specified
* collection
* (<a href="#optional-restrictions">optional</a>)
* @throws NullPointerException if this collection contains one or more
* null elements and the specified collection does not permit null
* elements
* (<a href="#optional-restrictions">optional</a>),
* or if the specified collection is null
* @see #remove(Object)
* @see #contains(Object)
*/
boolean retainAll(Collection
c);
/**
* Removes all of the elements from this collection (optional operation).
* The collection will be empty after this method returns.
*
* @throws UnsupportedOperationException if the <tt>clear</tt> operation
* is not supported by this collection
*/
void clear(a);
// Comparison and hashing
/**
* Compares the specified object with this collection for equality. <p>
*
* While the <tt>Collection</tt> interface adds no stipulations to the
* general contract for the <tt>Object.equals</tt>, programmers who
* implement the <tt>Collection</tt> interface "directly" (in other words,
* create a class that is a <tt>Collection</tt> but is not a <tt>Set</tt>
* or a <tt>List</tt>) must exercise care if they choose to override the
* <tt>Object.equals</tt>. It is not necessary to do so, and the simplest
* course of action is to rely on <tt>Object</tt>'s implementation, but
* the implementor may wish to implement a "value comparison" in place of
* the default "reference comparison." (The <tt>List</tt> and
* <tt>Set</tt> interfaces mandate such value comparisons.)<p>
*
* The general contract for the <tt>Object.equals</tt> method states that
* equals must be symmetric (in other words, <tt>a.equals(b)</tt> if and
* only if <tt>b.equals(a)</tt>). The contracts for <tt>List.equals</tt>
* and <tt>Set.equals</tt> state that lists are only equal to other lists,
* and sets to other sets. Thus, a custom <tt>equals</tt> method for a
* collection class that implements neither the <tt>List</tt> nor
* <tt>Set</tt> interface must return <tt>false</tt> when this collection
* is compared to any list or set. (By the same logic, it is not possible
* to write a class that correctly implements both the <tt>Set</tt> and
* <tt>List</tt> interfaces.)
*
* @param o object to be compared for equality with this collection
* @return <tt>true</tt> if the specified object is equal to this
* collection
*
* @see Object#equals(Object)
* @see Set#equals(Object)
* @see List#equals(Object)
*/
boolean equals(Object o);
/** * Returns the hash code value for this collection. While the * <tt>Collection</tt> interface adds no stipulations to the general * contract for the <tt>Object.hashCode</tt> method, programmers should * take note that any class that overrides the <tt>Object.equals</tt> * method must also override the <tt>Object.hashCode</tt> method in order * to satisfy the general contract for the <tt>Object.hashCode</tt> method. * In particular, <tt>c1.equals(c2)</tt> implies that * <tt>c1.hashCode()==c2.hashCode()</tt>. * *@return the hash code value for this collection
*
* @see Object#hashCode()
* @see Object#equals(Object)
*/
int hashCode(a);
/**
* Creates a {@linkSpliterator} over the elements in this collection. * * Implementations should document characteristic values reported by the * spliterator. Such characteristic values are not required to be reported * if the spliterator reports {@linkSpliterator#SIZED} and this collection * contains no elements. * * <p>The default implementation should be overridden by subclasses that * can return a more efficient spliterator. In order to * preserve expected laziness behavior for the {@link #stream()} and
* {@link #parallelStream()}} methods, spliterators should either have the
* characteristic of {@code IMMUTABLE} or {@code CONCURRENT}, or be
* <em><a href="Spliterator.html#binding">late-binding</a></em>.
* If none of these is practical, the overriding class should describe the
* spliterator's documented policy of binding and structural interference,
* and should override the {@link #stream()} and {@link #parallelStream()}
* methods to create streams using a {@code Supplier} of the spliterator,
* as in:
* <pre>{@code
* Stream<E> s = StreamSupport.stream(() -> spliterator(), spliteratorCharacteristics)
* }</pre>
* <p>These requirements ensure that streams produced by the
* {@link #stream()} and {@link #parallelStream()} methods will reflect the
* contents of the collection as of initiation of the terminal stream
* operation.
*
* @implSpec* The default implementation creates a * <em><a href="Spliterator.html#binding">late-binding</a></em> spliterator * from the collections's {@code Iterator}. The spliterator inherits the
* <em>fail-fast</em> properties of the collection's iterator.
* <p>
* The created {@code Spliterator} reports {@link Spliterator#SIZED}.
*
* @implNote
* The created {@code Spliterator} additionally reports
* {@link Spliterator#SUBSIZED}.
*
* <p>If a spliterator covers no elements then the reporting of additional
* characteristic values, beyond that of {@code SIZED} and {@code SUBSIZED},
* does not aid clients to control, specialize or simplify computation.
* However, this does enable shared use of an immutable and empty
* spliterator instance (see {@link Spliterators#emptySpliterator()}) for
* empty collections, and enables clients to determine if such a spliterator
* covers no elements.
*
* @return a {@code Spliterator} over the elements in this collection
* @since1.8 * /
@Override
default Spliterator<E> spliterator(a) {
return Spliterators.spliterator(this.0);
}
/**
* Returns a sequential {@code Stream} with this collection as its source.
*
* <p>This method should be overridden when the {@link #spliterator()}
* method cannot return a spliterator that is {@code IMMUTABLE},
* {@code CONCURRENT}, or <em>late-binding</em>. (See {@link #spliterator()}
* for details.)
*
* @implSpec
* The default implementation creates a sequential {@code Stream} from the
* collection's {@code Spliterator}.
*
* @return a sequential {@code Stream} over the elements in this collection
* @since1.8 * /
default Stream<E> stream(a) {
return StreamSupport.stream(spliterator(), false);
}
/**
* Returns a possibly parallel {@codeStream} with this collection as its * source. It is allowable for this method to return a sequential stream. * * <p>This method should be overridden when the {@link #spliterator()}
* method cannot return a spliterator that is {@code IMMUTABLE},
* {@code CONCURRENT}, or <em>late-binding</em>. (See {@link #spliterator()}
* for details.)
*
* @implSpec
* The default implementation creates a parallel {@code Stream} from the
* collection's {@code Spliterator}.
*
* @return a possibly parallel {@code Stream} over the elements in this
* collection
* @since1.8 * /
default Stream<E> parallelStream(a) {
return StreamSupport.stream(spliterator(), true); }}Copy the codeWhy use Collection to return
The description of the problem is that the Collection interface is a subtype of Iterable and has a stream method, so it provides iteration and stream access. Therefore, Collection or an appropriate subtype is usually the best return type for a common sequence return method.
Arrays also provide easy iteration and Stream access using the arrays.aslist and stream.of methods. If the returned sequence is small enough to fit easily into memory, it is best to return a standard collection implementation, such as ArrayList or HashSet. But don’t store a large sequence in memory only to return it as a collection.
47. Use stream parallelism sparingly
Even in the best case, parallelizing pipes is unlikely to improve performance if the source comes from the stream. iterate method, or if the intermediate operation limit method is used
48. Check the validity of parameters
Most methods and constructors have restrictions on which values can be passed to the corresponding parameters, such as not being null, having to be greater than 0, etc. For these limitations, a parameter check should be performed at the beginning of the method body to detect errors as soon as possible after they occur
Non-public methods can use assertions to check parameters. An AssertionError is raised if an assertion fails. Note that Java assertions, unlike C, need to display startup when the program is started
// Java-EA class name starts assertion
System.out.println("start");
assert false;
System.out.println("go on");
assert false:"stop";
System.out.println("end");
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