1Z0-829 Working with Streams and Lambda Expressions Practice Question
A company processes financial transactions. Each transaction is represented by a Transaction object with fields: amount (double), currency (String), and type (String). The requirement is to compute the total amount of all transactions of type 'SALE' in USD. The transactions are stored in a List<Transaction>. Which code correctly accomplishes this using streams?
⚠ Common exam trap
Oracle often tests the order of stream operations — specifically that filter must come before mapToDouble when the filter condition depends on object fields, otherwise the stream loses access to the original object type.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
transactions.stream().filter(t -> t.type().equals("SALE") && t.currency().equals("USD")).mapToDouble(Transaction::amount).sum()
Ly filters transactions to only those with type 'SALE' and currency 'USD', then maps each to its amount as a double, and sums them using sum(). This satisfies the requirement exactly: total amount of 'SALE' transactions in USD.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
transactions.stream().mapToDouble(Transaction::amount).filter(t -> t.type().equals("SALE")).sum()
Why it's wrong here
mapToDouble returns DoubleStream, which does not have type method; filter cannot be applied after mapToDouble in this way.
- ✗
transactions.stream().filter(t -> t.type().equals("SALE")).filter(t -> t.currency().equals("USD")).mapToDouble(Transaction::amount).sum()
Why it's wrong here
Filters by type then currency, but currency filter is not needed if only SALE transactions are considered; also missing currency check for USD.
- ✗
transactions.stream().map(Transaction::amount).reduce(0.0, (a, b) -> a + b)
Why it's wrong here
Missing filter for type and currency; also reduce works but sum() is more concise.
- ✓
transactions.stream().filter(t -> t.type().equals("SALE") && t.currency().equals("USD")).mapToDouble(Transaction::amount).sum()
Why this is correct
Correctly filters by type and currency, then sums amounts.
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