Keywords in Part 4 of the Object-Oriented Series for Front-end Learning PHP
Nov 16, 2016 am 10:24 AM
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前面的話
php實現(xiàn)面向?qū)ο蟮囊粋€顯著特征是大量使用關(guān)鍵字，本文將詳細介紹關(guān)鍵字
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public
public表示公有，它具有最大的訪問權(quán)限，被定義為公有的類成員可以在任何地方被訪問
如果屬性用 var 定義，則被視為公有，如果方法沒有設(shè)置關(guān)鍵字，則該方法默認為公有
<?<span style="color: #000000;">php
</span><span style="color: #0000ff;">class</span><span style="color: #000000;"> demo{
</span><span style="color: #0000ff;">public</span> <span style="color: #800080;">$public</span> = 1<span style="color: #000000;">;
</span><span style="color: #0000ff;">function</span> test(<span style="color: #800080;">$var</span><span style="color: #000000;">){
</span><span style="color: #0000ff;">echo</span> "{<span style="color: #800080;">$var</span>}000"<span style="color: #000000;">;
}
}
</span><span style="color: #800080;">$d1</span> = <span style="color: #0000ff;">new</span><span style="color: #000000;"> demo;
</span><span style="color: #800080;">$d1</span>->test(<span style="color: #800080;">$d1</span>-><span style="color: #0000ff;">public</span>);<span style="color: #008000;">//</span><span style="color: #008000;">1000</span>
?>
?
protected
protected表示受保護的，被定義為受保護的類成員則可以被其自身以及其子類和父類訪問
<?<span style="color: #000000;">php
</span><span style="color: #0000ff;">class</span><span style="color: #000000;"> demo{
</span><span style="color: #0000ff;">protected</span> <span style="color: #0000ff;">function</span><span style="color: #000000;"> fn(){
</span><span style="color: #0000ff;">echo</span> '111'<span style="color: #000000;">;
}
}
</span><span style="color: #0000ff;">class</span> demo1 <span style="color: #0000ff;">extends</span><span style="color: #000000;"> demo{
</span><span style="color: #0000ff;">function</span><span style="color: #000000;"> test(){
parent</span>::<span style="color: #000000;">fn();
}
}
</span><span style="color: #800080;">$d1</span> = <span style="color: #0000ff;">new</span><span style="color: #000000;"> demo1;
</span><span style="color: #800080;">$d1</span>->test();<span style="color: #008000;">//</span><span style="color: #008000;">111</span>
?>
?
private
private表示私有的，被定義為私有的類成員則只能被其定義所在的類訪問
<?<span style="color: #000000;">php
</span><span style="color: #0000ff;">class</span><span style="color: #000000;"> demo{
</span><span style="color: #0000ff;">private</span> <span style="color: #800080;">$private</span> = 1<span style="color: #000000;">;
</span><span style="color: #0000ff;">function</span><span style="color: #000000;"> test(){
</span><span style="color: #0000ff;">echo</span>(<span style="color: #800080;">$this</span>-><span style="color: #0000ff;">private</span><span style="color: #000000;">);
}
}
</span><span style="color: #800080;">$d1</span> = <span style="color: #0000ff;">new</span><span style="color: #000000;"> demo;
</span><span style="color: #800080;">$d1</span>->test();<span style="color: #008000;">//</span><span style="color: #008000;">1</span>
?>
?
final
PHP5新增了final關(guān)鍵字，它只能用來修飾類和方法，不能使用final這個關(guān)鍵字來修飾成員屬性，因為final是常量的意思，我們在PHP里定義常量使用的是define()函數(shù)和const關(guān)鍵字，所以不能使用final來定義成員屬性
如果父類中的方法被聲明為final，則子類無法覆蓋該方法。如果一個類被聲明為 final，則不能被繼承
<?<span style="color: #000000;">php
</span><span style="color: #0000ff;">class</span><span style="color: #000000;"> BaseClass {
</span><span style="color: #0000ff;">public</span> <span style="color: #0000ff;">function</span><span style="color: #000000;"> test() {
</span><span style="color: #0000ff;">echo</span> "BaseClass::test() called\n"<span style="color: #000000;">;
}
</span><span style="color: #0000ff;">final</span> <span style="color: #0000ff;">public</span> <span style="color: #0000ff;">function</span><span style="color: #000000;"> moreTesting() {
</span><span style="color: #0000ff;">echo</span> "BaseClass::moreTesting() called\n"<span style="color: #000000;">;
}
}
</span><span style="color: #0000ff;">class</span> ChildClass <span style="color: #0000ff;">extends</span><span style="color: #000000;"> BaseClass {
</span><span style="color: #0000ff;">public</span> <span style="color: #0000ff;">function</span><span style="color: #000000;"> moreTesting() {
</span><span style="color: #0000ff;">echo</span> "ChildClass::moreTesting() called\n"<span style="color: #000000;">;
}
}
</span><span style="color: #008000;">//</span><span style="color: #008000;"> Results in Fatal error: Cannot override final method BaseClass::moreTesting()</span>
?>
?
static
static關(guān)鍵字表示靜態(tài)的意思，用于修飾類的成員屬性和成員方法（即為靜態(tài)屬性和靜態(tài)方法）
類中的靜態(tài)屬性和靜態(tài)方法不用實例化(new)就可以直接使用類名訪問
[注意]靜態(tài)屬性不能通過一個類已實例化的對象來訪問，但靜態(tài)方法可以
由于靜態(tài)方法不需要通過對象即可調(diào)用，所以偽變量 $this 在靜態(tài)方法中不可用，靜態(tài)屬性不可以由對象通過 -> 操作符來訪問
用靜態(tài)方式調(diào)用一個非靜態(tài)方法會導(dǎo)致一個 E_STRICT 級別的錯誤
就像其它所有的 PHP 靜態(tài)變量一樣，靜態(tài)屬性只能被初始化為文字或常量，不能使用表達式。所以可以把靜態(tài)屬性初始化為整數(shù)或數(shù)組，但不能初始化為另一個變量或函數(shù)返回值，也不能指向一個對象
<?<span style="color: #000000;">php
</span><span style="color: #0000ff;">class</span><span style="color: #000000;"> Foo
{
</span><span style="color: #0000ff;">public</span> <span style="color: #0000ff;">static</span> <span style="color: #800080;">$my_static</span> = 'foo'<span style="color: #000000;">;
</span><span style="color: #0000ff;">public</span> <span style="color: #0000ff;">function</span><span style="color: #000000;"> staticValue() {
</span><span style="color: #0000ff;">return</span> self::<span style="color: #800080;">$my_static</span><span style="color: #000000;">;
}
}
</span><span style="color: #0000ff;">class</span> Bar <span style="color: #0000ff;">extends</span><span style="color: #000000;"> Foo
{
</span><span style="color: #0000ff;">public</span> <span style="color: #0000ff;">function</span><span style="color: #000000;"> fooStatic() {
</span><span style="color: #0000ff;">return</span> parent::<span style="color: #800080;">$my_static</span><span style="color: #000000;">;
}
}
</span><span style="color: #0000ff;">print</span> Foo::<span style="color: #800080;">$my_static</span> . "\n";<span style="color: #008000;">//</span><span style="color: #008000;">'foo'</span>
<span style="color: #800080;">$foo</span> = <span style="color: #0000ff;">new</span><span style="color: #000000;"> Foo();
</span><span style="color: #0000ff;">print</span> <span style="color: #800080;">$foo</span>->staticValue() . "\n";<span style="color: #008000;">//</span><span style="color: #008000;">'foo'</span>
<span style="color: #0000ff;">print</span> <span style="color: #800080;">$foo</span>::<span style="color: #800080;">$my_static</span> . "\n";<span style="color: #008000;">//</span><span style="color: #008000;">'foo'</span>
<span style="color: #0000ff;">print</span> <span style="color: #800080;">$foo</span>->my_static . "\n"; <span style="color: #008000;">//</span><span style="color: #008000;">報錯 </span>
?>
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const
可以把在類中始終保持不變的值定義為常量。在定義和使用常量的時候不需要使用$符號，而是使用const
常量的值必須是一個定值，不能是變量，類屬性，數(shù)學(xué)運算的結(jié)果或函數(shù)調(diào)用
<?<span style="color: #000000;">php
</span><span style="color: #0000ff;">class</span><span style="color: #000000;"> MyClass
{
</span><span style="color: #0000ff;">const</span> <span style="color: #008080;">constant</span> = 'constant value'<span style="color: #000000;">;
</span><span style="color: #0000ff;">function</span><span style="color: #000000;"> showConstant() {
</span><span style="color: #0000ff;">echo</span> self::<span style="color: #008080;">constant</span> . "\n"<span style="color: #000000;">;
}
}
</span><span style="color: #0000ff;">echo</span> MyClass::<span style="color: #008080;">constant</span> . "\n";<span style="color: #008000;">//</span><span style="color: #008000;">'constant value'</span>
<span style="color: #800080;">$classname</span> = "MyClass"<span style="color: #000000;">;
</span><span style="color: #0000ff;">echo</span> <span style="color: #800080;">$classname</span>::<span style="color: #008080;">constant</span> . "\n"; <span style="color: #008000;">//</span><span style="color: #008000;">'constant value'</span>
<span style="color: #800080;">$class</span> = <span style="color: #0000ff;">new</span><span style="color: #000000;"> MyClass();
</span><span style="color: #800080;">$class</span>->showConstant();<span style="color: #008000;">//</span><span style="color: #008000;">'constant value'</span>
<span style="color: #0000ff;">echo</span> <span style="color: #800080;">$class</span>::<span style="color: #008080;">constant</span>."\n";<span style="color: #008000;">//</span><span style="color: #008000;">'constant value'</span>
?>
?
this
當(dāng)一個方法在類定義內(nèi)部被調(diào)用時，有一個可用的偽變量this，特殊對象的引用this就是在對象內(nèi)部的成員方法中，代表本對象的一個引用，但只能在對象的成員方法中使用，不管是在對象內(nèi)部使用$this訪問自己對象內(nèi)部成員。還是在對象外部通過對象的引用名稱訪問對象中的成員，都需要使用特殊的運算符“->”來完成訪問
[注意]this在靜態(tài)方法中不可用
<?<span style="color: #000000;">php
</span><span style="color: #0000ff;">class</span><span style="color: #000000;"> A
{
</span><span style="color: #0000ff;">function</span><span style="color: #000000;"> foo()
{
</span><span style="color: #0000ff;">if</span> (<span style="color: #0000ff;">isset</span>(<span style="color: #800080;">$this</span><span style="color: #000000;">)) {
</span><span style="color: #0000ff;">echo</span> '$this is defined ('<span style="color: #000000;">;
</span><span style="color: #0000ff;">echo</span> <span style="color: #008080;">get_class</span>(<span style="color: #800080;">$this</span><span style="color: #000000;">);
</span><span style="color: #0000ff;">echo</span> ")\n"<span style="color: #000000;">;
} </span><span style="color: #0000ff;">else</span><span style="color: #000000;"> {
</span><span style="color: #0000ff;">echo</span> "\$this is not defined.\n"<span style="color: #000000;">;
}
}
}
</span><span style="color: #0000ff;">class</span><span style="color: #000000;"> B
{
</span><span style="color: #0000ff;">function</span><span style="color: #000000;"> bar()
{
</span><span style="color: #008000;">//</span><span style="color: #008000;"> Note: the next line will issue a warning if E_STRICT is enabled.</span>
A::<span style="color: #000000;">foo();
}
}
</span><span style="color: #800080;">$a</span> = <span style="color: #0000ff;">new</span><span style="color: #000000;"> A();
</span><span style="color: #800080;">$a</span>->foo();<span style="color: #008000;">//</span><span style="color: #008000;">$this is defined (A) </span>
A::foo();<span style="color: #008000;">//</span><span style="color: #008000;">$this is not defined. </span>
<span style="color: #800080;">$b</span> = <span style="color: #0000ff;">new</span><span style="color: #000000;"> B();
</span><span style="color: #800080;">$b</span>->bar();<span style="color: #008000;">//</span><span style="color: #008000;">$this is defined (B) </span>
B::bar();<span style="color: #008000;">//</span><span style="color: #008000;">$this is not defined.</span>
?>
?
self
在類的方法中，不能用this來引用靜態(tài)變量或靜態(tài)方法，而需要用self來引用
<?<span style="color: #000000;">php
</span><span style="color: #0000ff;">class</span><span style="color: #000000;"> MyClass
{
</span><span style="color: #0000ff;">const</span> <span style="color: #008080;">constant</span> = 'constant value'<span style="color: #000000;">;
</span><span style="color: #0000ff;">static</span> <span style="color: #0000ff;">function</span><span style="color: #000000;"> showConstant() {
</span><span style="color: #0000ff;">echo</span> self::<span style="color: #008080;">constant</span> . "\n"<span style="color: #000000;">;
}
}
</span><span style="color: #800080;">$var</span> = <span style="color: #0000ff;">new</span><span style="color: #000000;"> MyClass;
</span><span style="color: #0000ff;">echo</span> <span style="color: #800080;">$var</span>->showConstant();<span style="color: #008000;">//</span><span style="color: #008000;">constant value</span>
?>
?
parent
parent用于調(diào)用父類中定義的成員方法或常量
<?<span style="color: #000000;">php
</span><span style="color: #0000ff;">class</span><span style="color: #000000;"> MyClass{
</span><span style="color: #0000ff;">function</span><span style="color: #000000;"> fn(){
</span><span style="color: #0000ff;">echo</span>('111'<span style="color: #000000;">);
}
</span><span style="color: #0000ff;">const</span> A = 'a'<span style="color: #000000;">;
}
</span><span style="color: #0000ff;">class</span> Class1 <span style="color: #0000ff;">extends</span><span style="color: #000000;"> MyClass{
</span><span style="color: #0000ff;">function</span><span style="color: #000000;"> test(){
</span><span style="color: #0000ff;">echo</span> parent::fn().parent::<span style="color: #000000;">A;
}
}
</span><span style="color: #800080;">$var</span> = <span style="color: #0000ff;">new</span><span style="color: #000000;"> Class1;
</span><span style="color: #800080;">$var</span>->test();<span style="color: #008000;">//</span><span style="color: #008000;">111a</span>
?>
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