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javaquerydsl

QueryDSL Code Style: return value from Query methods

I'm learning QueryDSL.

All the code I see looks something like:

JPAQuery<Member> query = queryFactory.selectFrom(member);
query = query.method1().method2().method3();

Do the various methods that let you chain like this actually return a different object? Or could I write it like:

JPAQuery<Member> query = queryFactory.selectFrom(member);
query.method1();
query.method2();
query.method3();

(The basic issue is that some methods are on the superclasses, so they don't return the actual class.)

Clearly, I prefer the first way when it works -- it's clean and clear.

Solution

  • So, in the first example:

    JPAQuery<Member> query = queryFactory.selectFrom(member);
query = query.method1().method2().method3();

    Each method1(), method2(), and method3() call returns a new JPAQuery<Member> object, allowing you to chain these methods together.

    If you have methods from superclasses that do not return the actual class, the chaining still works as long as the methods return an object with the same or compatible interface.

    In your second example:

    JPAQuery<Member> query = queryFactory.selectFrom(member);
query.method1();
query.method2();
query.method3();

    This wouldn't work as intended because each method call operates on the original query object, and you're not capturing the modified instances that the methods return.

    If you ever need to break the chain for some reason (e.g., conditionally apply certain methods), you can assign the intermediate result to a separate variable:

    JPAQuery<Member> query = queryFactory.selectFrom(member);
JPAQuery<Member> intermediateResult = query.method1();
if (someCondition) {
    intermediateResult = intermediateResult.method2();
}
query = intermediateResult.method3();

    This allows you to conditionally apply methods and then continue the chain based on your logic.