Quick Answer
The answer is a compilation error. This occurs because the reference type, Object, determines which methods the compiler can access, not the actual type, String, which is what the object was instantiated as. The compiler only sees that obj is declared as Object, and since the Object class does not contain a length() method, the code fails to compile despite the underlying object being a String. On the Oracle Java Foundations 1Z0-811 exam, this concept tests your understanding of compile-time vs runtime behavior, often appearing in trick questions where a reference type hides the methods of the actual type. A common trap is assuming the actual type’s methods are always available, but the compiler strictly enforces the reference type’s contract. Memory tip: “The compiler checks the left side of the assignment, not the right side.”
This 1Z0-811 practice question tests your understanding of object-oriented programming. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. 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.
A developer writes: Object obj = new String("Hello"); System.out.println(obj.length()); What will be the output?
null
Why wrong: Incorrect; this does not produce null output.
Runtime exception
Why wrong: Incorrect; it fails at compile time, not runtime.
5
Why wrong: Incorrect; although the object is a String, the reference type is Object, which does not have length() method.
Compilation error
Correct. The code does not compile because Object class does not have length() method.
0
Why wrong: Incorrect; length() is not called.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
Compilation error
The code fails to compile because the reference variable 'obj' is of type Object, which does not have a length() method. The length() method is defined in the String class, not in Object. Since the compiler checks the declared type (Object) for method availability, it does not find length() and produces a compilation error.
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.
null
Why it's wrong here
Incorrect; this does not produce null output.
Runtime exception
Why it's wrong here
Incorrect; it fails at compile time, not runtime.
5
Why it's wrong here
Incorrect; although the object is a String, the reference type is Object, which does not have length() method.
Compilation error
Why this is correct
Correct. The code does not compile because Object class does not have length() method.
Related concept
Read the scenario before looking for a memorised answer.
0
Why it's wrong here
Incorrect; length() is not called.
Common exam traps
Oracle often tests the distinction between compile-time type and runtime type, trapping candidates who assume that because the object is a String, the length() method is automatically available on any reference to it.
Trap categories for this question
Command / output trap
Incorrect; this does not produce null output.
Detailed technical explanation
In Java, method resolution is based on the compile-time type of the reference variable, not the runtime type. Even though the object is a String, the compiler sees obj as an Object and only allows methods defined in the Object class (e.g., toString(), hashCode()). To call length(), the reference must be explicitly cast to String: ((String) obj).length(). This is a fundamental aspect of Java's static type system and polymorphism.
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 1Z0-811 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
Object-Oriented Programming — This question tests Object-Oriented Programming — Read the scenario before looking for a memorised answer..
The correct answer is: Compilation error — The code fails to compile because the reference variable 'obj' is of type Object, which does not have a length() method. The length() method is defined in the String class, not in Object. Since the compiler checks the declared type (Object) for method availability, it does not find length() and produces a compilation error.
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
This 1Z0-811 practice question is part of Courseiva's free Oracle certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the 1Z0-811 exam.
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