How to use Java stream map with filters and optional?

Solution 1:

By reducing your problem down I just realized that you always want the value of query.getSystem() to be the context, therefore:

I reduced your code like this:

MockDbTypeEnum newMethod(MockQuery query, MockDbContextHolder dbContextHolder) {
    return Optional
            .ofNullable(query.getSystem())
            .map(MockDbTypeEnum::valueOf)
            .orElse(null);
}

MockDbTypeEnum oldMethod(MockQuery query, MockDbContextHolder dbContextHolder) {

    if (Objects.nonNull(query.getSystem())) {
        if (!query.getSystem().equals(dbContextHolder.getCurrentDb().toString())) {
            dbContextHolder.setCurrentDb(Enum.valueOf(MockDbTypeEnum.class, query.getSystem()));
        }

        return dbContextHolder.getCurrentDb();
    }

    return null;
}

Also here are the mocks and tests I used to prove these methods are functionally the same for your purposes:

@ParameterizedTest
@CsvSource(value = {
        "PSQL, PSQL, PSQL",
        "PSQL, SQL, PSQL",
        "SQL, SQL, SQL",
        "SQL, PSQL, SQL",
        "null, SQL, null",
        "null, PSQL, null"
}, nullValues = {"null"})
void test(String system, MockDbTypeEnum currentDb, MockDbTypeEnum expectedResult) {
    MockQuery query = new MockQuery(system);
    MockDbContextHolder dbContextHolder = new MockDbContextHolder(currentDb);

    MockDbTypeEnum result = oldMethod(query, dbContextHolder);
    assertEquals(expectedResult, result);

    MockDbTypeEnum newResult = newMethod(query, dbContextHolder);
    assertEquals(expectedResult, newResult);
}

enum MockDbTypeEnum {
    PSQL,
    SQL
}

static class MockQuery {

    private final String system;

    public MockQuery(String system) {
        this.system = system;
    }

    public String getSystem() {
        return system;
    }

}

static class MockDbContextHolder {

    private MockDbTypeEnum currentDb;

    public MockDbContextHolder(MockDbTypeEnum currentDb) {
        this.currentDb = currentDb;
    }

    public MockDbTypeEnum getCurrentDb() {
        return currentDb;
    }

    public void setCurrentDb(MockDbTypeEnum currentDb) {
        this.currentDb = currentDb;
    }

}

Here is the result: