Execution order of of static blocks in an Enum type w.r.t to constructor
This is from Effective Java :
// Implementing a fromString method on an enum type
private static final Map<String, Operation> stringToEnum
= new HashMap<String, Operation>();
static { // Initialize map from constant name to enum constant
for (Operation op : values())
stringToEnum.put(op.toString(), op);
}
// Returns Operation for string, or null if string is invalid
public static Operation fromString(String symbol) {
return stringToEnum.get(symbol);
}
Note that the Operation constants are put into the stringToEnum map from a static block that runs after the constants have been created. Trying to make each constant put itself into the map from its own constructor would cause a compilation error. This is a good thing, because it would cause a NullPointerException if it were legal. Enum constructors aren't permitted to access the enum's static fields, except for compile-time constant fields. This restriction is necessary because these static fields have not yet been initialized when the constructors run.
My question is regarding the line :
"Note that the Operation constants are put into the stringToEnum map from a static block that runs after the constants have been created" .
I thought the static block gets executed before the constructor runs. The are actually executed during class load time.
What am I missing here ?
I understand your question as: why is there a guarantee that the enum constants will be initialised before the static block is run. The answer is given in the JLS, and a specific example is given in #8.9.2.1, with the following explanation:
static initialization occurs top to bottom.
and the enums constants are implicitly final static and are declared before the static initializer block.
EDIT
The behaviour is not different from a normal class. The code below prints:
In constructor: PLUS
PLUS == null MINUS == null
In constructor: MINUS
PLUS != null MINUS == null
In static initialiser
PLUS != null MINUS != null
In constructor: after static
PLUS != null MINUS != null
public class Operation {
private final static Operation PLUS = new Operation("PLUS");
private final static Operation MINUS = new Operation("MINUS");
static {
System.out.println("In static initialiser");
System.out.print("PLUS = " + PLUS);
System.out.println("tMINUS = " + MINUS);
}
public Operation(String s) {
System.out.println("In constructor: " + s);
System.out.print("PLUS = " + PLUS);
System.out.println("tMINUS = " + MINUS);
}
public static void main(String[] args) {
Operation afterStatic = new Operation ("after static");
}
}
Operation常量是静态块中按照出现顺序创建的静态字段。
static {
// instantiate enum instances here
...
// Initialize map from constant name to enum constant
for (Operation op : values())
stringToEnum.put(op.toString(), op);
}
The static blocks execute in order of appearance (you can have multiple static blocks), when the class loader loads the class, eg. it runs before the constructor.
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