Java MCQ Questions – Page 26 of 200

Q: After calling the `start()` method on a `Thread` object, what is the immediate state it enters, waiting to be scheduled by the OS?

C) Runnable. When `start()` is called, the thread transitions to the `Runnable` state, meaning it is ready to run and waiting for the scheduler to pick it up. It doesn't immediately enter the `Running` state.

Q: Which method of the `java.io.File` class would you use to confirm if the `File` object represents a regular file?

B) `isFile()`. The `isFile()` method returns true if the `File` object represents an existing regular file, and false if it represents an existing directory or does not exist.

Q: What does this code print?

A) [10, 30]. The `it.remove()` method removes the last element returned by `it.next()`. In this case, 20 and 40 are removed, leaving 10 and 30.

Q: What compilation error will occur in the following Java code?

A) Compile-time error: cannot find symbol method 'setBalance(double)'. The 'Account' class lacks a public 'setBalance' method, so calling it from 'Bank' results in a 'cannot find symbol' compilation error. Direct access 'userAccount.balance' would also be an error due to 'balance' being private, but the method call fails first.

Q: What will be the output when this Java program runs?

A) In try In catch: From try In finally (followed by Error stack trace: From finally). The `try` block throws `RuntimeException`, which is caught. The `finally` block executes next, prints its message, and then throws a new `Error`. When an exception (or `Error`) is thrown from a `finally` block, it supersedes any pending exception from the `try` or `catch` block. The program terminates with the `Error`.

Q: What does this code print?

A) Accumulating: 0 + 1 Accumulating: 1 + 2 Accumulating: 3 + 3 Accumulating: 6 + 4 Result with identity 0: 10 Accumulating: 1 + 2 Accumulating: 3 + 3 Accumulating: 6 + 4 Result without identity: 10 Result with empty list and identity 100: 100 Result with empty list without identity: -1. When `reduce` has an identity, the accumulator is called with the identity and the first element. For an empty stream with an identity, the identity is returned. Without an identity, `reduce` returns an `Optional`, which is empty for an empty stream.

Q: What does this Java code print?

A) 2. The `final` keyword applied to an object reference means the reference cannot be reassigned to point to another object. It does not make the object itself immutable; thus, the contents of the `ArrayList` can still be modified.

Q: What is the output of this code? (Assuming Java 17+)

B) Compilation error: the switch expression does not cover all possible input values. When a `switch` expression operates on a sealed type (like `Shape`), it must be exhaustive. This means all direct permitted subtypes (`Circle` and `Square`) must be covered, or a `default` clause must be provided. Since `Square` is not covered and there's no `default`, it results in a compile-time error.

Q: Given a `HashMap map = new HashMap<>();` and an `Iterator > entryIt = map.entrySet().iterator();`. If `map.put("A", 1); map.put("B", 2);`, and then `entryIt` is obtained and `entryIt.next();` is called, followed by `map.remove("B");` externally, what is the most likely outcome upon a subsequent call to `entryIt.hasNext()` or `entryIt.next()`?

B) A `ConcurrentModificationException` will be thrown, as the underlying map was structurally modified outside the iterator's operations.. Iterators returned by `keySet()`, `entrySet()`, and `values()` views of standard `HashMap` (and most `AbstractMap` implementations) are fail-fast. External structural modifications to the underlying map, such as `map.remove("B")`, will increment the map's `modCount`, causing a subsequent `hasNext()` or `next()` call on the iterator to throw a `ConcurrentModificationException`.

Q: What kind of error will occur when this Java code is executed?

B) Compilation Error: Cannot find symbol method concat(java.lang.String). The `concat()` method belongs to the `String` class, not `StringBuilder`. `StringBuilder` uses `append()` to add characters.

Q501 easy

After calling the `start()` method on a `Thread` object, what is the immediate state it enters, waiting to be scheduled by the OS?

Q502 easy

Which method of the `java.io.File` class would you use to confirm if the `File` object represents a regular file?

Q503 medium code output

What does this code print?

java

import java.util.ArrayList;
import java.util.Iterator;
import java.util.List;

public class Test {
    public static void main(String[] args) {
        List<Integer> numbers = new ArrayList<>(List.of(10, 20, 30, 40));
        Iterator<Integer> it = numbers.iterator();
        while (it.hasNext()) {
            Integer num = it.next();
            if (num == 20 || num == 40) {
                it.remove();
            }
        }
        System.out.println(numbers);
    }
}

Q504 easy code error

What compilation error will occur in the following Java code?

java

class Account {
    private double balance = 100.0;
    // No public setter method for 'balance'
}

public class Bank {
    public static void main(String[] args) {
        Account userAccount = new Account();
        userAccount.setBalance(500.0);
        System.out.println(userAccount.balance);
    }
}

Q505 medium code output

What will be the output when this Java program runs?

java

public class ExceptionInFinally {
    public static void main(String[] args) {
        try {
            System.out.println("In try");
            throw new RuntimeException("From try");
        } catch (RuntimeException e) {
            System.out.println("In catch: " + e.getMessage());
            // No re-throw, but it's caught
        } finally {
            System.out.println("In finally");
            throw new Error("From finally"); // New unchecked error
        }
    }
}

Q506 hard code output

What does this code print?

java

import java.util.Arrays;
import java.util.List;
import java.util.Optional;
import java.util.function.BinaryOperator;

public class ReduceOperation {
    public static void main(String[] args) {
        List<Integer> numbers = Arrays.asList(1, 2, 3, 4);
        BinaryOperator<Integer> sum = (a, b) -> {
            System.out.println("Accumulating: " + a + " + " + b);
            return a + b;
        };

        int result1 = numbers.stream().reduce(0, sum);
        System.out.println("Result with identity 0: " + result1);

        Optional<Integer> result2 = numbers.stream().reduce(sum);
        System.out.println("Result without identity: " + result2.orElse(-1));

        List<Integer> emptyList = Arrays.asList();
        int result3 = emptyList.stream().reduce(100, sum);
        System.out.println("Result with empty list and identity 100: " + result3);

        Optional<Integer> result4 = emptyList.stream().reduce(sum);
        System.out.println("Result with empty list without identity: " + result4.orElse(-1));
    }
}

Q507 easy code output

What does this Java code print?

java

import java.util.ArrayList;
import java.util.List;

public class FinalMutableTest {
    public static void main(String[] args) {
        final List<String> fruits = new ArrayList<>();
        fruits.add("Apple");
        fruits.add("Banana");
        System.out.println(fruits.size());
    }
}

Q508 hard code output

What is the output of this code? (Assuming Java 17+)

java

public class SwitchTest {
    sealed interface Shape permits Circle, Square {}
    record Circle(double radius) implements Shape {}
    record Square(double side) implements Shape {}

    public static void main(String[] args) {
        Shape shape = new Circle(10.0);
        String description = switch (shape) {
            case Circle c -> "Circle with radius " + c.radius();
            // Missing Square case, and no default.
        };
        System.out.println(description);
    }
}

Q509 hard

Given a `HashMap<String, Integer> map = new HashMap<>();` and an `Iterator<Map.Entry<String, Integer>> entryIt = map.entrySet().iterator();`. If `map.put("A", 1); map.put("B", 2);`, and then `entryIt` is obtained and `entryIt.next();` is called, followed by `map.remove("B");` externally, what is the most likely outcome upon a subsequent call to `entryIt.hasNext()` or `entryIt.next()`?

Q510 easy code error

What kind of error will occur when this Java code is executed?

java

public class Test {
    public static void main(String[] args) {
        StringBuilder sb = new StringBuilder("Java");
        sb.concat("World");
        System.out.println(sb);
    }
}

Q511 easy code error

What is the result of running this Java code snippet?

java

import java.util.ArrayList;
import java.util.Arrays;
import java.util.Iterator;

public class IteratorError {
    public static void main(String[] args) {
        ArrayList<String> data = new ArrayList<>(Arrays.asList("One", "Two"));
        Iterator<String> it = data.iterator();
        data.clear(); // Modifying the list after iterator creation
        it.next(); // Attempting to use the invalidated iterator
    }
}

Q512 easy code output

What is the output of this code?

java

public class StringTest {
    public static void main(String[] args) {
        String s1 = "java";
        String s2 = "java";
        System.out.println(s1.equals(s2));
    }
}

Q513 hard

Consider a `try-catch-finally` block. If an exception is thrown in the `try` block, and another unrelated exception is thrown in the `finally` block, which exception will be propagated out of the method?

Q514 easy code error

What is the compilation or runtime error in the following Java code?

java

public class Main {
    public static void main(String[] args) {
        String name = null;
        int len = name.length();
        System.out.println(len);
    }
}

Q515 medium

What is the purpose of the `yield` keyword when used within a `switch` expression in Java?

Q516 hard code error

What kind of error will occur when compiling or running the following Java code?

java

public class SBError {
    public static void main(String[] args) {
        StringBuffer sb = new StringBuffer("Programming"); // Length 11
        char[] destArray = new char[5]; // Can hold 5 chars
        // Attempt to copy 7 chars (from 0 to 7) into destArray starting at index 0
        sb.getChars(0, 7, destArray, 0);
        System.out.println(new String(destArray));
    }
}

Q517 easy code output

What will be printed by this Java program?

java

public class MyClass {
    public static void main(String[] args) {
        int month = 2;
        String season;
        switch (month) {
            case 1:
            case 2:
            case 12: season = "Winter"; break;
            case 3:
            case 4:
            case 5: season = "Spring"; break;
            default: season = "Unknown";
        }
        System.out.print(season);
    }
}

Q518 hard code output

What is the output of this Java code?

java

import java.io.FileWriter;
import java.io.PrintWriter;
import java.io.IOException;
import java.io.FileReader;
import java.io.BufferedReader;
import java.nio.file.Files;
import java.nio.file.Path;

public class PrintWriterAutoFlush {
    public static void main(String[] args) {
        String filename = "autoflush_test.txt";
        try {
            try (PrintWriter pw = new PrintWriter(new FileWriter(filename))) {
                pw.print("First line.");
            }
            try (BufferedReader br = new BufferedReader(new FileReader(filename))) {
                System.out.println("After first PrintWriter: " + br.readLine());
            }

            try (PrintWriter pw = new PrintWriter(new FileWriter(filename, true), true)) {
                pw.println("Second line.");
                pw.print("Third line.");
            }
            try (BufferedReader br = new BufferedReader(new FileReader(filename))) {
                String line; while((line = br.readLine()) != null) { System.out.println("After second PrintWriter: " + line); }
            }
        } catch (IOException e) { System.out.println("Error: " + e.getMessage());
        } finally { try { Files.deleteIfExists(Path.of(filename)); } catch (IOException ignored) {} }
    }
}

Q519 hard code output

What is the output of this code?

java

public class StringIndexOfEmptyTest {
    public static void main(String[] args) {
        String text = "abcde";
        System.out.println(text.indexOf("") + "," + text.indexOf("", 3) + "," + text.lastIndexOf("", 2));
    }
}

Q520 easy

What is the primary characteristic of a `java.util.HashSet` regarding its elements?

☕ Java MCQ Questions – Page 26