本文簡單探討一下Scala語言Case Class的實現機制
Case Class
Case Class是Scala語言模式匹配功能的基礎。如果定義類的時候加上case關鍵字,那麼它就變成了Case Class,比如下面這個簡單的類CC:
case class CC(x: Int, y: Int)
那麼加上case關鍵字對於一個類來說,到底意味着什麼呢?下文將進行詳細的解釋。單例對象
編譯CC,會產生兩個class:CC.class和CC$.class。這說明,Scala會給case類自動添加一個單例對象。下面是反編譯CC$.class之後的(部分)代碼:
public final class CC$ extends scala.runtime.AbstractFunction2 implements scala.Serializable {
public static final CC$ MODULE$;
static {
new CC$();
}
private CC$() {
MODULE$ = this;
}
public Object readResolve() {
return MODULE$;
}
...
}
apply()和unapply()方法
CC$定義了apply()方法,這樣我們就可以不用敲new關鍵字,而是通過下面這樣的方式來創建CC實例:
val cc = CC(2, 16)
CC$還定義了一個和apply()互補的方法:unapply(),它把CC拆成一個Option<Tuple>。apply()和unapply()方法的代碼如下所示:
public final class CC$ ... {
...
public CC apply(int x, int y) {
return new CC(x, y);
}
public scala.Option<scala.Tuple2<Object, Object>> unapply(CC cc) {
if (cc == null) {
return scala.None$.MODULE$;
}
return new scala.Some(new scala.Tuple2$mcll$sp(cc.x, cc.y))
}
}
Case Class默認是Immutable
Case類的字段,默認會被編譯器加上val關鍵字,也就是說,CC類實際上是下面這樣:
case class CC(val x: Int, val y: Int)
下面是CC.class反編譯之後的相應代碼:
public class CC implements scala.Product, scala.Serializable {
private final int x;
private final int y;
public CC(int x, int y) {
this.x = x;
this.y = y;
scala.Product$class.$init$(this);
}
public int x() {
return x;
}
public int y() {
return y;
}
}
toString()、hashCode()和equals()方法
Scala編譯器負責(根據字段)給Case類重寫toString()、hashCode()和equals()方法。
copy()方法
copy()方法讓Case類實例可以完整的,或者有少量變化的複製自己。如下面的代碼所示:
val cc = CC(1, 2)
val cc1 = cc.copy()
val cc2 = cc.copy(y = 8) // Named arguments
上面的代碼去掉語法糖之後,實際上是下面這樣:
val cc = CC(1, 2)
val cc1 = cc.copy(cc.copy$default$1(), cc.copy$default$2())
val cc2 = cc.copy(cc.copy$default$1(), 8)
下面是反編譯之後copy()和copy$default$n()方法代碼:public class CC ... {
...
public CC copy(int x, int y) {
return new CC(x, y);
}
public int copy$default$1() {
return x();
}
public int copy$default$2() {
return y();
}
...
}
實現Product和Serializable接口
Case類還實現了scala.Product和scala.Serializable接口(Product和Serializable實際上都是Traits)。
完整的反編譯代碼
下面是CC.class和CC$.class的完整代碼,僅供參考:
public class CC implements scala.Product, scala.Serializable {
private final int x;
private final int y;
public CC(int x, int y) {
this.x = x;
this.y = y;
scala.Product$class.$init$(this);
}
public int x() {
return x;
}
public int y() {
return y;
}
public int hashCode() {...}
public boolean equals(Object) {...}
public String toString() {...}
public boolean canEqual(Object obj) {
return obj instanceof CC;
}
public CC copy(int x, int y) {
return new CC(x, y);
}
public int copy$default$1() {
return x();
}
public int copy$default$2() {
return y();
}
// Product
// def productArity: Int
// def productElement(n: Int): Any
// def productIterator: Iterator[Any]
// def productPrefix = ""
public int productArity() {
return 2;
}
public Object productElement(int n) {
switch (n) {
case 0: return x();
case 1: return y();
default: throw new IndexOutOfBoundsException();
}
}
public scala.collection.Iterator productIterator() {
return scala.runtime.ScalaRunTime$.MODULE$.typedProductIterator(this);
}
public String productPrefix() {
return "CC";
}
public static CC apply(int x, int y) {
return CC$.MODULE$.apply(x, y);
}
public static scals.Option<scala.Tuple2<Object, Object>> unapply(CC cc) {
return CC$.MODULE$.unapply(cc);
}
public scala.Function1 static tupled() {
return CC$.MODULE$.tupled();
}
public scala.Function1 static curried() {
return CC$.MODULE$.curried();
}
}
public final class CC$ extends scala.runtime.AbstractFunction2 implements scala.Serializable {
public static final CC$ MODULE$;
static {
new CC$();
}
private CC$() {
MODULE$ = this;
}
public Object readResolve() {
return MODULE$;
}
public String toString() {
return "CC";
}
public CC apply(int x, int y) {
return new CC(x, y);
}
public scala.Option<scala.Tuple2<Object, Object>> unapply(CC cc) {
if (cc == null) {
return scala.None$.MODULE$;
}
return new scala.Some(new scala.Tuple2$mcll$sp(cc.x, cc.y))
}
}
參考資料
<<Programming in Scala>>第二版