Quick Answer
The correct answers are A and C, as both represent valid lambda expression syntax in Java. Option A is valid because it explicitly declares the parameter types and uses parentheses, which is required when types are specified, and the expression body `a + b` implicitly returns the result. Option C is also valid because it uses parentheses with inferred types for two parameters, which is mandatory when there are multiple parameters and types are omitted. On the Oracle Certified Professional Java SE 17 Developer 1Z0-829 exam, this question tests your understanding of lambda syntax rules, specifically the distinction between single-parameter and multi-parameter lambdas, and the requirement for parentheses when types are declared or when there are two or more parameters. A common trap is forgetting that parentheses are required for multiple parameters even when types are inferred, as in option B, or that a block body must include a return statement, as in option D. Remember the memory tip: "One can be bare, but two need a pair"—meaning a single parameter can omit parentheses, but two parameters always require them.
This 1Z0-829 practice question tests your understanding of working with streams and lambda expressions. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
Which TWO are valid lambda expressions? (Choose two.) A. (int a, int b) -> a + b B. a, b -> a + b C. (a, b) -> a + b D. (a, b) -> { a + b; } E. (int a, b) -> a + b
a, b -> a + b
Why wrong: Parentheses required for multiple parameters.
(a, b) -> a + b
Valid lambda with inferred types.
(int a, int b) -> a + b
Valid lambda with explicit types.
(a, b) -> { a + b; }
Why wrong: Block lambda must have return statement if not void.
(int a, b) -> a + b
Why wrong: Cannot mix explicit and implicit types.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
(a, b) -> a + b
Option B is correct because a lambda expression with inferred parameter types can omit the parameter types and parentheses when there is exactly one parameter, but here there are two parameters, so parentheses are required. Option C is correct because it explicitly declares the types of both parameters, which is valid syntax for a lambda expression that returns the sum of the two integers.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
For each option: why learners choose it and why it is or isn't the right answer here.
a, b -> a + b
Why it's wrong here
Parentheses required for multiple parameters.
(a, b) -> a + b
Why this is correct
Valid lambda with inferred types.
Related concept
Read the scenario before looking for a memorised answer.
(int a, int b) -> a + b
Why this is correct
Valid lambda with explicit types.
Related concept
Read the scenario before looking for a memorised answer.
(a, b) -> { a + b; }
Why it's wrong here
Block lambda must have return statement if not void.
(int a, b) -> a + b
Why it's wrong here
Cannot mix explicit and implicit types.
Common exam traps
Oracle often tests the rule that parentheses are mandatory for multiple parameters in a lambda, and that mixing explicit and inferred parameter types is illegal, causing candidates to mistakenly accept options like A or E.
Detailed technical explanation
Under the hood, lambda expressions are compiled into invokedynamic instructions that generate functional interface implementations at runtime. The Java compiler enforces strict syntactic rules: parameter types must be either all specified or all omitted, and parentheses are required for multiple parameters. A common subtlety is that a block body must use 'return' to produce a value, whereas an expression body implicitly returns the result.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
A practitioner preparing for the 1Z0-829 exam encounters this exact type of scenario on the job. The correct answer here is not the most general option — it is the best answer for the specific constraint described. Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option. Real exam questions reward reading the full scenario before eliminating options, because the constraint defines which answer fits.
What to study next
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FAQ
Working with Streams and Lambda Expressions — This question tests Working with Streams and Lambda Expressions — Read the scenario before looking for a memorised answer..
The correct answer is: (a, b) -> a + b — Option B is correct because a lambda expression with inferred parameter types can omit the parameter types and parentheses when there is exactly one parameter, but here there are two parameters, so parentheses are required. Option C is correct because it explicitly declares the types of both parameters, which is valid syntax for a lambda expression that returns the sum of the two integers.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 30, 2026
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