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Applied to the list structure, the visitor pattern takes the following coding pattern. The clarity following code is simplified as the Functional Java language level would accept. The Full Java language level code can be downloaded here.
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/**
* Abstract list structure.
*/
abstract class IntList {
abstract Object accept(IntListVisitor v);
ConsIntList cons(int n) {
return new ConsIntList(n, this);
}
}
/**
* Concrete empty list structure containing nothing.
*/
class EmptyIntList extends IntList {
Object accept(IntListVisitor v) {
return v.forEmptyIntList(this);
}
}
/**
* Concrete non-empty list structure containing an int, called first,
* and a rest, which is a list structure.
*/
class ConsIntList extends IntList {
int first;
IntList rest;
Object accept(IntListVisitor v) {
return v.forConsIntList(this);
}
}
/**
* Abstract operation on IntList.
*/
interface IntListVisitor {
Object forEmptyIntList(EmptyIntList host);
Object forConsIntList(ConsIntList host);
}
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