Free 1Z0-831 practice questions

Binary numeric promotion turns byte, short and char operands into int before any arithmetic operator runs.

a + b has type int even though both operands are bytes, and b and a are not constants, so the sum needs an explicit cast. A cast binds tighter than +, so it only narrows the whole sum when the sum is in parentheses. Every other form casts an operand, which is then promoted back to int by the addition.

1. Note that b and a are bytes but not final constants.
2. Apply promotion: byte + byte produces an int.
3. Check what each cast actually covers; a cast binds tighter than +.
4. Only a cast around the whole sum turns the int back into a byte.

Exam tip: byte + byte is an int; cast the whole expression, not the operands.

Without braces, an else always pairs with the closest preceding if that does not already have an else (the dangling-else rule). Indentation has no meaning to the compiler.

The code is laid out to suggest the else belongs to if (a > 3). The compiler pairs it with if (b < 5) instead. a > 3 is true, so the inner if runs; b < 5 is false, so its else prints Y. The final print is outside both ifs and prints Z, giving YZ. Reading the else as the outer one's gives Z, which is the trap.

1. Ignore indentation and match each else to the nearest if without an else.
2. Evaluate the outer condition a > 3: true, so the inner if statement runs.
3. Evaluate b < 5: false, so the inner else prints Y.
4. The last print is unconditional and prints Z.

Exam tip: An else belongs to the nearest unmatched if; use braces when you mean otherwise.

The compiler adds a default no-argument constructor only when a class declares no constructors at all.

Box declares Box(int size). Because at least one constructor is written, the compiler does not generate the default no-argument constructor. The call new Box() therefore matches no constructor and javac reports "constructor Box in class Box cannot be applied to given types; required: int". The 0 distractor assumes a default constructor still exists alongside the written one.

1. Count the constructors Box declares: one, taking an int.
2. Recall that a default constructor is generated only when none are declared.
3. Match new Box() against the declared constructors: no match.
4. A failed constructor match is a compile-time error.

Exam tip: Write any constructor and the free no-argument one disappears; add Box() yourself if callers need it.

A checked exception (Exception and its subclasses other than RuntimeException) must be caught or declared wherever it can be thrown.

The compiler checks every throw statement and call for checked exceptions. Exception itself is checked, so the throw inside load() needs either a surrounding catch or a throws Exception clause on load(). Neither exists, so compilation fails, even though the branch would not run for the id 42. The runtime-exception answer assumes Java checks this only when the throw executes; it does not.

1. Classify the thrown type: Exception is checked.
2. Look for a catch in load() that handles it: none.
3. Look for a throws clause on load(): none.
4. Conclude that javac rejects the method, whatever the argument.

Exam tip: Checked exceptions are a compile-time rule: catch them or declare them, even on branches that never run.

Creating an array with new allocates every element and sets it to the type's default value: 0, 0.0, false, '\u0000' or null.

new int[3] creates three int elements set to 0, new String[2] two references set to null, and new boolean[1] one false. Unlike local variables, array elements never need explicit initialisation, so the code compiles. String concatenation turns the null reference into the text null, and length reports the number of elements, 3.

1. Note that each array is created with new and a size.
2. Apply the default value for each element type.
3. Remember that null concatenates as the text null.
4. Read length as the size, not the last index.

Exam tip: Array elements default to 0, false or null; length is the element count.