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Abstract type (view source)

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===Interface===
{{trans|F#}}
<syntaxhighlight lang="cobol"> INTERFACE-ID.IDENTIFICATION ShapeDIVISION.
INTERFACE-ID. Shape.
PROCEDURE DIVISION.
IDENTIFICATION DIVISION.
METHOD-ID. perimeter.
DATA DIVISION.
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END METHOD perimeter.
IDENTIFICATION DIVISION.
METHOD-ID. shape-area.
DATA DIVISION.
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IDENTIFICATION DIVISION.
CLASS-ID. Rectangle.
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INTERFACE Shape.
IDENTIFICATION DIVISION.
OBJECT IMPLEMENTS Shape.
DATA DIVISION.
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PROCEDURE DIVISION.
IDENTIFICATION DIVISION.
METHOD-ID. perimeter.
DATA DIVISION.
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01 ret USAGE FLOAT-LONG.
PROCEDURE DIVISION RETURNING ret.
COMPUTE ret = width * 2.0 + height * 2.0
GOBACK ret = width * 2.0 + height * 2.0
.END-COMPUTE
GOBACK.
END METHOD perimeter.
IDENTIFICATION DIVISION.
METHOD-ID. shape-area.
DATA DIVISION.
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01 ret USAGE FLOAT-LONG.
PROCEDURE DIVISION RETURNING ret.
COMPUTE ret = width * height
GOBACK ret = width * height
.END-COMPUTE
GOBACK.
END METHOD shape-area.
END OBJECT.
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{{trans|Go}}
<syntaxhighlight lang="emal">
^|EMal doesn'rdoes not support abstract types with partial implementations,
|but we can use interfaces.
|^
type Beast
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writeLine(b.getName() + ", who's a " + b.getKind() + ", cries: " + b.getCry() + ".")
end
^|instantiation works becausbecause a positional variadic constructor
|has been auto generated
|^
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=={{header|Java}}==
Java has an <code>interface</code> and an <code>abstract class</code>. Neither of which can be instantiated, and require some sort of implementation or abstraction.<br />
Methods that don't have an implementation are called abstract methods in Java. A class that contains an abstract method or inherits one but did not override it must be an abstract class; but an abstract class does not need to contain any abstract methods. An abstract class cannot be instantiated. If a method is abstract, it cannot be private.
For an <code>interface</code>, only the <code>private</code> and <code>default</code> access modifiers are allowed, which also implies they require code.<br />
A <code>private</code> method cannot be overridden by a sub-class, and a <code>default</code> method, optionally, can.<br />
A method with no access modifier is inherently <code>public</code>, must not contain code, and requires implementation by its sub-class.<br />
Member fields are allowed, although are effectively <code>public</code>, <code>final</code>, and <code>static</code>, thus requiring a value.<br />
Here is an example of an <code>interface</code>.
<syntaxhighlight lang="java">
interface Example {
String stringA = "rosetta";
String stringB = "code";
 
private String methodA() {
<syntaxhighlight lang="java">public abstract class Abs {
return stringA + " " + stringB;
public abstract int method1(double value);
protected abstract int method2(String name);
int add(int a, int b) {
return a + b;
}
 
}</syntaxhighlight>
default int methodB(int value) {
Before Java 8, interfaces could not implement any methods and all methods were implicitly public and abstract.
return value + 100;
<syntaxhighlight lang="java">public interface Inter {
}
int method1(double value);
 
int method2(String name);
int addmethodC(int avalueA, int bvalueB);
}
}</syntaxhighlight>
</syntaxhighlight>
And here is an example of its implementing class.
<syntaxhighlight lang="java">
class ExampleImpl implements Example {
public int methodB(int value) {
return value + 200;
}
 
public int methodC(int valueA, int valueB) {
return valueA + valueB;
}
}
</syntaxhighlight>
The <code>abstract class</code> is very generalized, and for the most part is just a <code>class</code> that allows for un-implemented methods.<br />
The <code>default</code> access modifier is not used here, as it applies only to an <code>interface</code>.<br />
Additionally, if a method is marked <code>abstract</code>, then the <code>private</code> access modifier is not allowed, as the concept does not apply.<br />
Here is an example of an <code>abstract class</code>.<br />
If the class contains <code>abstract</code> methods then the class definition must also have the <code>abstract</code> keyword.
<syntaxhighlight lang="java">
abstract class Example {
String stringA = "rosetta";
String stringB = "code";
 
private String methodA() {
return stringA + " " + stringB;
}
 
protected int methodB(int value) {
return value + 100;
}
 
public abstract int methodC(int valueA, int valueB);
}
</syntaxhighlight>
Here is an example of a class which <code>extends</code> an <code>abstract class</code>.
<syntaxhighlight lang="java">
class ExampleImpl extends Example {
public int methodC(int valueA, int valueB) {
return valueA + valueB;
}
}
</syntaxhighlight>
 
=={{header|jq}}==
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END TEXTHASHKEY;
</syntaxhighlight>
=={{header|Skew}}==
{{works with|skewc|0.9.19}}
 
In Skew, interfaces must be explicitly implemented with `::`.
 
<syntaxhighlight lang="skew">
@entry
def main {
var rgb = Rgb.new(0, 255, 255)
var hex = Hex.new("00ffff")
 
var color Color
color = hex
color.print
 
color = rgb
color.print
 
(hex as Color).print
}
 
interface Color {
def toString string
def print {
dynamic.console.log(self.toString)
}
}
 
class Rgb :: Color {
var r int, g int, b int
def toString string { return "rgb(\(r), \(g), \(b))" }
}
 
class Hex :: Color {
var code string
def toString string { return "#\(code)" }
}
</syntaxhighlight>
 
 
=={{header|Smalltalk}}==
A class is declared abtract by responding to the query <tt>isAbstract</tt> with true, and defining the required protocol for subclasses to raise an error notification. Optionally, instance creation can be blocked (but seldom done, as you will hit a subclassResponsibility anyway soon). Typically, the IDE provides menu functions to generate these definitions automatically (eg. "Insert Abstract Class" in the refactoring submenu of the class browser):
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The act of giving a signature to a module is called ascription. There are two type of ascription:
Transparent (written <tt>:</tt>) and opaque (written <tt>:></tt>). If a structure is ascribed transparently,
none of the types are abstract. If it is ascribed opaquely, all types are abstract by default, but can be specified
explicitly in the signature, in which case they are not abstract.
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Then say we have a structure ListQueue which implements queues as lists. If we write <tt>ListQueue :> QUEUE</tt>
then the queue type will be abstract, but if we write <tt>ListQueue : QUEUE</tt> or <tt>ListQueue : LIST_QUEUE</tt> it won't.
 
=={{header|Swift}}==
Swift uses Protocols to provide abstract type features. See [https://docs.swift.org/swift-book/documentation/the-swift-programming-language/protocols/ the docs]
 
A trivial example showing required properties and methods, and the means of providing a default implementation.
<syntaxhighlight lang="sml">
protocol Pet {
var name: String { get set }
var favouriteToy: String { get set }
 
func feed() -> Bool
 
func stroke() -> Void
 
}
 
extension Pet {
// Default implementation must be in an extension, not in the declaration above
 
func stroke() {
print("default purr")
}
}
 
struct Dog: Pet {
var name: String
var favouriteToy: String
// Required implementation
func feed() -> Bool {
print("more please")
return false
}
// If this were not implemented, the default from the extension above
// would be called.
func stroke() {
print("roll over")
}
}
 
</syntaxhighlight>
 
=={{header|Tcl}}==
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The Go example, when rewritten in Wren, looks like this.
<syntaxhighlight lang="ecmascriptwren">import "./fmt" for Fmt
 
class Beast{
Anonymous user
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