What does this print?
String a = "java";
String b = new String("java");
System.out.println((a == b) + " " + a.equals(b));new String creates a distinct heap object, so == (reference comparison) is false, while equals compares characters.
QuizzesModern Java
Core Java & OOP
Strings, equality, inheritance, exceptions, generics and initialization order.
15 questions
Pick an answer to see instantly whether you're right, with an explanation. Questions are shuffled every attempt.
Question 1 of 15Score 0
What does this print?
Integer x = 127, y = 127;
Integer p = 128, q = 128;
System.out.println((x == y) + " " + (p == q));Autoboxing uses Integer.valueOf, which caches -128..127. 128 creates two separate objects, so == is false. Always use equals for boxed values.
Java passes object arguments to methods…
Java is always pass-by-value. For objects the copied value is the reference, so methods can mutate the object but reassigning the parameter has no effect on the caller.
Which statement about method overloading is true?
Overload resolution happens at compile time using declared types. Overriding is the one resolved at runtime.
You override equals but not hashCode. What is the most likely symptom?
Equal objects get different identity hash codes, so hash-based collections look in the wrong bucket.
Which of these exceptions is checked?
IOException extends Exception but not RuntimeException, so it must be caught or declared. The others are unchecked.
What does this method return?
static int f() {
try { return 1; }
finally { return 2; }
}A return in finally overrides the earlier return value (and would also swallow any exception) — which is why you should never return from finally.
In try-with-resources, if both the try body and close() throw, what happens?
The primary exception wins and close failures are available via getSuppressed().
What is printed by new Child()?
class Parent {
Parent() { show(); }
void show() { System.out.println("parent"); }
}
class Child extends Parent {
String name = "child";
@Override void show() { System.out.println(name); }
}The parent constructor runs before the child’s field initialisers, so the overridden show() sees name == null. Never call overridable methods from constructors.
Which declaration accepts a List<Integer>, a List<Double> and a List<Number> for reading numbers?
Generics are invariant, so List<Number> won’t accept List<Integer>. A producer you read from uses ? extends (PECS).
Which is NOT a consequence of type erasure?
Generic methods are perfectly legal. The other three are restrictions caused by erasing type arguments at runtime.
Which keyword prevents a class from being subclassed?
final classes such as String and all records cannot be extended.
A class implements two interfaces that both declare the same default method. What must it do?
Unless a superclass method or a more specific interface resolves it, the compiler forces the class to override and pick an implementation.
Why should you build a long string in a loop with StringBuilder instead of +=?
Strings are immutable, so every concatenation copies all previous characters. StringBuilder appends in place (and is not thread-safe).
Which comparator is safe for any int values?
Subtraction overflows for large or negative values, and option B never returns a negative number. Integer.compare is correct.
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