实体类实现Parcelable
接口后枚举字段该怎么写?正好碰到这个问题,记录一下。
枚举类
public enum EnumBean {
STEAM,
EPIC
}
枚举字段序列化,这里直接上写法。实际上用了AS插件Android Parcelable Code Generator
生成,当然还是本着知其所以然的态度,思考一下为啥是这个写法。
public class Test implements Parcelable {
public String name;
public int age;
public EnumBean e;
@Override
public int describeContents() {
return 0;
}
@Override
public void writeToParcel(Parcel dest, int flags) {
dest.writeString(this.name);
dest.writeInt(this.age);
dest.writeInt(this.e == null ? -1 : this.e.ordinal());
}
public void readFromParcel(Parcel source) {
this.name = source.readString();
this.age = source.readInt();
int tmpE = source.readInt();
this.e = tmpE == -1 ? null : EnumBean.values()[tmpE];
}
public Test() {
}
protected Test(Parcel in) {
this.name = in.readString();
this.age = in.readInt();
int tmpE = in.readInt();
this.e = tmpE == -1 ? null : EnumBean.values()[tmpE];
}
public static final Parcelable.Creator<Test> CREATOR = new Parcelable.Creator<Test>() {
@Override
public Test createFromParcel(Parcel source) {
return new Test(source);
}
@Override
public Test[] newArray(int size) {
return new Test[size];
}
};
}
其实从write
和read
字段e就可以看出,这里写入的是枚举类数组下标,读取时根据下标取出枚举类实体。
dest.writeInt(this.e == null ? -1 : this.e.ordinal());
this.e = tmpE == -1 ? null : EnumBean.values()[tmpE];
众所周知枚举中的每个声明都会创建枚举类对象,ordinal()
也就是枚举中声明的顺序,对应到枚举类数组values()
的下标。
看一下编译后的EnumBean.class
文件字节码
//枚举类默认继承Enum
public final enum com/chenxuan/kotlin/EnumBean extends java/lang/Enum
public final static enum Lcom/chenxuan/kotlin/EnumBean; STEAM
public final static enum Lcom/chenxuan/kotlin/EnumBean; EPIC
private final static synthetic [Lcom/chenxuan/kotlin/EnumBean; $VALUES
//构造方法
private <init>(Ljava/lang/String;I)V
L0
LINENUMBER 3 L0
ALOAD 0
ALOAD 1
ILOAD 2
INVOKESPECIAL java/lang/Enum.<init> (Ljava/lang/String;I)V
RETURN
//static块,创建声明的枚举类对象,枚举类数组。
static <clinit>()V
L0
LINENUMBER 4 L0
NEW com/chenxuan/kotlin/EnumBean
DUP
LDC "STEAM"
ICONST_0
INVOKESPECIAL com/chenxuan/kotlin/EnumBean.<init> (Ljava/lang/String;I)V
PUTSTATIC com/chenxuan/kotlin/EnumBean.STEAM : Lcom/chenxuan/kotlin/EnumBean;
L1
LINENUMBER 5 L1
NEW com/chenxuan/kotlin/EnumBean
DUP
LDC "EPIC"
ICONST_1
INVOKESPECIAL com/chenxuan/kotlin/EnumBean.<init> (Ljava/lang/String;I)V
PUTSTATIC com/chenxuan/kotlin/EnumBean.EPIC : Lcom/chenxuan/kotlin/EnumBean;
L2
LINENUMBER 3 L2
ICONST_2
ANEWARRAY com/chenxuan/kotlin/EnumBean
DUP
ICONST_0
GETSTATIC com/chenxuan/kotlin/EnumBean.STEAM : Lcom/chenxuan/kotlin/EnumBean;
AASTORE
DUP
ICONST_1
GETSTATIC com/chenxuan/kotlin/EnumBean.EPIC : Lcom/chenxuan/kotlin/EnumBean;
AASTORE
PUTSTATIC com/chenxuan/kotlin/EnumBean.$VALUES : [Lcom/chenxuan/kotlin/EnumBean;
RETURN
public static values()[Lcom/chenxuan/kotlin/EnumBean;
L0
LINENUMBER 3 L0
GETSTATIC com/chenxuan/kotlin/EnumBean.$VALUES : [Lcom/chenxuan/kotlin/EnumBean;
INVOKEVIRTUAL [Lcom/chenxuan/kotlin/EnumBean;.clone ()Ljava/lang/Object;
CHECKCAST [Lcom/chenxuan/kotlin/EnumBean;
ARETURN
public static valueOf(Ljava/lang/String;)Lcom/chenxuan/kotlin/EnumBean;
L0
LINENUMBER 3 L0
LDC Lcom/chenxuan/kotlin/EnumBean;.class
ALOAD 0
INVOKESTATIC java/lang/Enum.valueOf (Ljava/lang/Class;Ljava/lang/String;)Ljava/lang/Enum;
CHECKCAST com/chenxuan/kotlin/EnumBean
ARETURN
很好理解,声明了两个EnumBean类型的变量STEAM和EPIC并在static块中初始化;同时声明了EnumBean数组$VALUES,按照枚举声明顺序添加到数组。
需注意枚举变量初始化时调用的构造方法传入了两个参数:声明的枚举变量对应的字符串STEAM、下标0。
LDC "STEAM"
ICONST_0
INVOKESPECIAL com/chenxuan/kotlin/EnumBean.<init> (Ljava/lang/String;I)V
EnumBean构造方法调用父类Enum的构造方法
private <init>(Ljava/lang/String;I)V
L0
LINENUMBER 3 L0
ALOAD 0
ALOAD 1
ILOAD 2
INVOKESPECIAL java/lang/Enum.<init> (Ljava/lang/String;I)V
RETURN
继续看Enum的构造方法
private final String name;
private final int ordinal;
protected Enum(String name, int ordinal) {
this.name = name;
this.ordinal = ordinal;
}
public final int ordinal() {
return ordinal;
}
如此一来,ordinal
就和枚举声明顺序或者说枚举数组下标联系起来了。
回看readFromParcel
中EnumBean.values()[tmpE]
,由上述字节码可知values()
方法调用$VALUES.clone()
也就是枚举类数组,然后根据数组下标取出枚举类实体。
还有我们常用的valueOf()
方法,调用父类Enum.valueOf()
传入了EnumBean.class
。
public static valueOf(Ljava/lang/String;)Lcom/chenxuan/kotlin/EnumBean;
L0
LINENUMBER 3 L0
LDC Lcom/chenxuan/kotlin/EnumBean;.class
ALOAD 0
INVOKESTATIC java/lang/Enum.valueOf (Ljava/lang/Class;Ljava/lang/String;)Ljava/lang/Enum;
CHECKCAST com/chenxuan/kotlin/EnumBean
ARETURN
Enum.valueOf()
public static <T extends Enum<T>> T valueOf(Class<T> enumType,
String name) {
Objects.requireNonNull(enumType, "enumType == null");
Objects.requireNonNull(enumType, "name == null");
T[] values = getSharedConstants(enumType);
if (values == null) {
throw new IllegalArgumentException(enumType.toString() + " is not an enum type.");
}
for (int i = values.length - 1; i >= 0; --i) {
T value = values[i];
if (name.equals(value.name())) {
return value;
}
}
throw new IllegalArgumentException(
"No enum constant " + enumType.getCanonicalName() + "." + name);
}
Enum.getSharedConstants()
根据传入的class获取枚举类数组values
public static <T extends Enum<T>> T[] getSharedConstants(Class<T> enumType) {
return (T[]) sharedConstantsCache.get(enumType);
}
private static final BasicLruCache<Class<? extends Enum>, Object[]> sharedConstantsCache
= new BasicLruCache<Class<? extends Enum>, Object[]>(64) {
@Override protected Object[] create(Class<? extends Enum> enumType) {
return enumValues(enumType);
}
};
private static Object[] enumValues(Class<? extends Enum> clazz) {
if (!clazz.isEnum()) {
return null;
}
try {
Method valueMethod = clazz.getDeclaredMethod("values");
return (Object[]) valueMethod.invoke(null);
} catch (NoSuchMethodException | IllegalAccessException | InvocationTargetException e) {
throw new RuntimeException(e);
}
}
这里通过反射获取到枚举类数组,然后迭代数组找到name相同的枚举类。而name其实就是声明的枚举变量对应的字符串,在static块中初始化时调用到父类Enum构造方法赋值。
NEW com/chenxuan/kotlin/EnumBean
DUP
LDC "STEAM"
ICONST_0
INVOKESPECIAL com/chenxuan/kotlin/EnumBean.<init> (Ljava/lang/String;I)V
PUTSTATIC com/chenxuan/kotlin/EnumBean.STEAM : Lcom/chenxuan/kotlin/EnumBean;