I have methods toSaveString(StringBuilder) and toSaveString() in several classes and thought of turning those into an interface. The first method would always have to be implemented and the second I could default because it basically only calls the first method every time with a new string builder and returns the resulting string. (Not what default is designed for, but bear with me.)
Now I wouldn't need to implement toSaveString() in the classes implementing the interface, but I would like to change its documentation nonetheless to match the class. Is there a way to achieve this without overriding the toSaveString() method in the implementing class? Because adding three lines to call the default method or five to copy the implementation seems redundant and easy to get errors mixed in.
Also feel free to leave comments about design alternatives here, but the question stays because it is interesting in its own right.
Look at the javadoc of the ArrayList#removeIf method:
/**
* #throws NullPointerException {#inheritDoc}
*/
#Override
public boolean removeIf(Predicate<? super E> filter) {
return removeIf(filter, 0, size);
}
It overrides its superclass Collection#removeIf method:
/**
* 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.
* #since 1.8
*/
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;
}
In your case, you can override only javadoc, and write something like this in the method body:
/**
* custom javadoc
*/
#Override
public boolean customMethod(Object parameter) {
return super.customMethod(parameter);
}
See also: Can I add code to an inherited method without overriding the whole method?
In JDK 8, java.util.Collection starts with
public interface Collection<E> extends Iterable<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();
Interestingly, java.util.Set starts with
public interface Set<E> extends Collection<E> {
// Query Operations
/**
* Returns the number of elements in this set (its cardinality). If this
* set contains more than <tt>Integer.MAX_VALUE</tt> elements, returns
* <tt>Integer.MAX_VALUE</tt>.
*
* #return the number of elements in this set (its cardinality)
*/
int size();
What is the purpose of overriding method declarations in interfaces? Why does not extends suffice?
EDIT java.util.List also shows redundancy, and the javadoc of List.size() is only minimally different that of Collection.size(), and does not introduce any new term:
public interface List<E> extends Collection<E> {
// Query Operations
/**
* Returns the number of elements in this list. If this list contains
* more than <tt>Integer.MAX_VALUE</tt> elements, returns
* <tt>Integer.MAX_VALUE</tt>.
*
* #return the number of elements in this list
*/
int size();
Overriding a method can be used for a lot of reasons besides to change the behavior. It can change the signature of the method (refining the return type with a covariant override), add annotations, broaden the accessibility (turning a protected method into public in a subclass), or refine the specification (expressed as Javadoc). In this case, the override exists so that the Set Javadoc could define the term "cardinality".
I meet a problem when using A.class in program and it return java.lang.NullPointerException at run-time
This is my snipped code:
public synchronized boolean isDeviceEqual(IDevice dev) {
............
if( isDeviceInstanceOf(SimpleDevice.class) ) {
return dev instanceof IDevice
&& XXX();
}
............
}
public boolean isDeviceInstanceOf(Class cls) {
return cls.isAssignableFrom(mDeviceClass);
}
and NPE
java.lang.NullPointerException
at xxx/library/DeviceDescriptor.isDeviceInstanceOf(Ljava/lang/Class;)Z (:0:5)
at xxx/library/DeviceDescriptor.isDeviceEqual(LIDevice;)Z (:0:6)
with above NPE, it means that cls is null in this case but I can't explain why this happens so can anybody help me?
cls.isAssignableFrom(mDeviceClass) will throw a NullPointerException if mDeviceClass, which must be the case here, since you are sure that cls is not null.
/**
* Determines if the class or interface represented by this
* <code>Class</code> object is either the same as, or is a superclass or
* superinterface of, the class or interface represented by the specified
* <code>Class</code> parameter. It returns <code>true</code> if so;
* otherwise it returns <code>false</code>. If this <code>Class</code>
* object represents a primitive type, this method returns
* <code>true</code> if the specified <code>Class</code> parameter is
* exactly this <code>Class</code> object; otherwise it returns
* <code>false</code>.
*
* <p> Specifically, this method tests whether the type represented by the
* specified <code>Class</code> parameter can be converted to the type
* represented by this <code>Class</code> object via an identity conversion
* or via a widening reference conversion. See <em>The Java Language
* Specification</em>, sections 5.1.1 and 5.1.4 , for details.
*
* #param cls the <code>Class</code> object to be checked
* #return the <code>boolean</code> value indicating whether objects of the
* type <code>cls</code> can be assigned to objects of this class
* #exception NullPointerException if the specified Class parameter is
* null.
* #since JDK1.1
*/
public native boolean isAssignableFrom(Class<?> cls);
It seems strange that in isDeviceEqual(IDevice dev) you are not doing anything with dev. I don't know what mDeviceClass is, but perhaps it should be assigned dev.getClass() somewhere.
I have a hashmap called gg. I want to take out its keys using keySet() method and pass them as parameter of another method.
method_A( gg.keySet());
public void method_A(ArrayList<Integer> jj){
....
}
But I got this error:
error: incompatible types: Set<Integer> cannot be converted to ArrayList<Integer>.
Then I saw this:
method_A (new ArrayList<Integer>(gg.keySet()));
What is it doing actually ? It looks like type casting to me. I am puzzled. Could someone explain to me what is going on ?
new ArrayList<Integer>(gg.keySet()) is not type casting. It's more like a copy constructor (though actual copy constructors usually take an argument of the same type that is being instantiated, which is not the case here).
It's creating a new ArrayList<Integer> instance using the constructor that accepts a Collection as an argument. It adds all the elements found in the Collection to the new ArrayList.
Here's the doc of that constructor :
/**
* Constructs a list containing the elements of the specified
* collection, in the order they are returned by the collection's
* iterator.
*
* #param c the collection whose elements are to be placed into this list
* #throws NullPointerException if the specified collection is null
*/
public ArrayList(Collection<? extends E> c)
java.util.Map.Entry as I know is a public static interface in java.util package that
returns collection view of a map but as far now I am confused with the static interface
and as it is Map.Entry is it an inner interface if so how do we have inner static interfaces in java
The definition of Entry happens to live inside the definition of Map (allowed by java). Being static means you don't need an instance of Map to refer to an Entry.
It's easiest to show how to use Map.Entry by an example. Here's how you can iterate over a map
Map<Integer, String> map = new HashMap<Integer, String>();
for (Map.Entry<Integer, String> entry : map.entrySet()) {
Integer key = entry.getKey();
String value = entry.getValue();
// do something with key and/or value etc
// you may also alter the entry's value inside this loop via entry.setValue()
}
There isn't really anything to be confused about.
Yes, Java allows interfaces to be members of classes or other interfaces.
No, that does not mean anything special. It changes absolutely nothing about how you can use such an interface or what you can do with it.
It only changes the name of that interface and creates a strong conceptual link between it and its enclosing type. In this case, a Map.Entry represents an entry of a Map. The designers of the API apparently felt that it made sense to stress this connection by making it a member type.
Example:
public class Outer {
public interface Bar {
Bar get();
}
}
Bar is a nested interface. Nested interfaces are static by default, so you could as well write:
public class Outer {
public static interface Bar {
Bar get();
}
}
Now, what static in this context means is that the interface is a static member, i.e. a member of the class.
You can do this with classes as well:
public class Tree {
private static class Node {
}
}
Here, Node is even private, meaning it's only visible within Tree. So, what's the benefit of this? Why not make Node a public class? Because of better encapsulation. First, the Node is an implementation detail of the Tree, so you don't want it to be visible. Second, if you expose Node via a public API, some client (programmer) could use it in his code. Now, he has a hard dependency on this class. If at some point you want to change the representation of you Tree, and you change/remove the Node class, the client code's may break. And last but not least, your public API becomes smaller, which is also desirable.
So, when to use static member classes/interfaces? Mostly, if you build some sort of Composite object (like a Tree, or a Linked List) or when the class only makes sense in the context of the outer class.
Java allows nested interfaces. You can nest them into classes or interfaces. For instance, Map.Entry is a nested interface defined in the Map interface.
Map implementations (TreeMap, HashMap) provide private implementations of Map.Entry, which are not visible outside the class.
Bohemian's answer addresses how to use Map.Entry.
Yes, it's an inner interface of the Map interface.
/**
* A map entry (key-value pair). The <tt>Map.entrySet</tt> method returns
* a collection-view of the map, whose elements are of this class. The
* <i>only</i> way to obtain a reference to a map entry is from the
* iterator of this collection-view. These <tt>Map.Entry</tt> objects are
* valid <i>only</i> for the duration of the iteration; more formally,
* the behavior of a map entry is undefined if the backing map has been
* modified after the entry was returned by the iterator, except through
* the <tt>setValue</tt> operation on the map entry.
*
* #see Map#entrySet()
* #since 1.2
*/
interface Entry<K,V> {
/**
* Returns the key corresponding to this entry.
*
* #return the key corresponding to this entry
* #throws IllegalStateException implementations may, but are not
* required to, throw this exception if the entry has been
* removed from the backing map.
*/
K getKey();
/**
* Returns the value corresponding to this entry. If the mapping
* has been removed from the backing map (by the iterator's
* <tt>remove</tt> operation), the results of this call are undefined.
*
* #return the value corresponding to this entry
* #throws IllegalStateException implementations may, but are not
* required to, throw this exception if the entry has been
* removed from the backing map.
*/
V getValue();
/**
* Replaces the value corresponding to this entry with the specified
* value (optional operation). (Writes through to the map.) The
* behavior of this call is undefined if the mapping has already been
* removed from the map (by the iterator's <tt>remove</tt> operation).
*
* #param value new value to be stored in this entry
* #return old value corresponding to the entry
* #throws UnsupportedOperationException if the <tt>put</tt> operation
* is not supported by the backing map
* #throws ClassCastException if the class of the specified value
* prevents it from being stored in the backing map
* #throws NullPointerException if the backing map does not permit
* null values, and the specified value is null
* #throws IllegalArgumentException if some property of this value
* prevents it from being stored in the backing map
* #throws IllegalStateException implementations may, but are not
* required to, throw this exception if the entry has been
* removed from the backing map.
*/
V setValue(V value);
/**
* Compares the specified object with this entry for equality.
* Returns <tt>true</tt> if the given object is also a map entry and
* the two entries represent the same mapping. More formally, two
* entries <tt>e1</tt> and <tt>e2</tt> represent the same mapping
* if<pre>
* (e1.getKey()==null ?
* e2.getKey()==null : e1.getKey().equals(e2.getKey())) &&
* (e1.getValue()==null ?
* e2.getValue()==null : e1.getValue().equals(e2.getValue()))
* </pre>
* This ensures that the <tt>equals</tt> method works properly across
* different implementations of the <tt>Map.Entry</tt> interface.
*
* #param o object to be compared for equality with this map entry
* #return <tt>true</tt> if the specified object is equal to this map
* entry
*/
boolean equals(Object o);
/**
* Returns the hash code value for this map entry. The hash code
* of a map entry <tt>e</tt> is defined to be: <pre>
* (e.getKey()==null ? 0 : e.getKey().hashCode()) ^
* (e.getValue()==null ? 0 : e.getValue().hashCode())
* </pre>
* This ensures that <tt>e1.equals(e2)</tt> implies that
* <tt>e1.hashCode()==e2.hashCode()</tt> for any two Entries
* <tt>e1</tt> and <tt>e2</tt>, as required by the general
* contract of <tt>Object.hashCode</tt>.
*
* #return the hash code value for this map entry
* #see Object#hashCode()
* #see Object#equals(Object)
* #see #equals(Object)
*/
int hashCode();
}
For more information about interfaces, see the Interfaces tutorial and this Static Nested Interfaces article.
Inner interfaces are implicitly public and static.
You can have inner interfaces as follows :
1. interface A {
.....
.....
interface B {
....
....
}
}
2. class A {
....
....
interface B {
....
....
}
}
You can access the above inner interface(B) by A.B where A is a class or an interface according to the above two cases.
For example,
class x implements A.B
{
....
....
}