Detailed explanation of php abstract class feature examples

Characteristics of abstract class

1. Classes defined as abstract cannot be instantiated.
2. If at least one method in it is declared abstract, then this class must be declared abstract.
3. A method defined as abstract only declares its calling method (parameters) and cannot define its specific function implementation.
4. When inheriting an abstract class, the subclass must define all abstract methods in the parent class (the access control of these methods must be the same as that in the parent class (or more relaxed).)

demo

abstract class AbstractClass{
 // 强制要求子类定义这些方法
    abstract protected function getValue();
    abstract protected function prefixValue($prefix);

    // 普通方法（非抽象方法）
    public function printOut() {
        print $this->getValue() . "\n";
    }
class ConcreteClass1 extends AbstractClass{   //父类定义了俩个抽象方法，如子类只实现了其中一个，则报如下错误//Fatal error: Class ConcreteClass1 contains 1 abstract //method and must therefore be declared abstract or //implement the remaining methods //(AbstractClass::prefixValue)
    protected function getValue() {
        return "ConcreteClass1";
    } 
   // 且子类的访问权限不能低于父类，如果将如下子类的访问方式改为private则会报如下错误
    //Fatal error: Access level to //ConcreteClass1::prefixValue() must be protected (as in //class AbstractClass) or weaker
    public function prefixValue($prefix) {
        return "{$prefix}ConcreteClass1";
    }

}

//当实例化一个抽象类时，Fatal error: Cannot instantiate //abstract class AbstractClass
//$obj = new AbstractClass();
// 但是我们可以这么使用##########################################################
abstract class Foo{
     static function bar()
     {
         echo "test\n";
     }
 }

Foo::bar();##########################################################
$obj = new ConcreteClass1();
echo $obj->getValue();

5. The subclass can define an optional parameter (that is, it must have a default value), but it is not included in the declaration of the abstract method of the parent class, then There is no conflict between the two statements.

abstract class AbstractClass{
    // 我们的抽象方法仅需要定义需要的参数
    abstract protected function prefixName($name);
}

class ConcreteClass extends AbstractClass
{

    // 我们的子类可以定义父类签名中不存在的可选参数
    // 如果这里的$separator没有默认值，则会报Fatal error: 、、//Declaration of ConcreteClass::prefixName() must be //compatible with that of AbstractClass::prefixName()
    public function prefixName($name, $separator = ".") {
        if ($name == "Pacman") {            $prefix = "Mr";
        } elseif ($name == "Pacwoman") {            $prefix = "Mrs";
        } else {            $prefix = "";
        }        return "{$prefix}{$separator} {$name}";
    }
}

$class = new ConcreteClass;
echo $class->prefixName("Pacman"), "\n";
echo $class->prefixName("Pacwoman"), "\n";

6. An abstract class does not need an abstract method. On the contrary, a class with an abstract method must be declared as an abstract class.

abstract class A{
     public function show(){
         echo 'A';
     }
 } class B extends A{
     public function hello(){
         echo 'B';         
         parent::show();
     }
 } 
 $obj = new B; 
 $obj->hello(); // BA

7. The order in which abstract classes are defined

// 正确的定义顺序
abstract class horse extends animal {
     public function get_breed() { return "Jersey"; }
 } 
 class cart extends horse {
     public function get_breed() { return "Wood"; }
 }
 // 错误的定义顺序，将会导致horse类找不到
 class cart extends horse {
     public function get_breed() { return "Wood"; }
 } 
 abstract class horse extends animal {
     public function get_breed() { return "Jersey"; }
 }

8. Abstract methods can be staticmethods

abstract class FOO
{
     abstract static function dump();
 } 
 class BAR extends FOO{
     static function dump(){var_dump("BAR");}
 }
BAR::dump(); // string(3) "BAR"

9. When an abstract class inherits another abstract class, the abstract method does not need to be declared as abstract

//Fatal error: Can't inherit abstract function //class1::someFunc() (previously declared abstract in //class2)abstract class class1 { 
   abstract public function someFunc(); 
 } 
 abstract class class2 extends class1 { 
   abstract public function someFunc(); 
 } 

//下面的是正确的
abstract class class1 { 
   abstract public function someFunc(); 
 } 
 abstract class class2 extends class1 { 
 }

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