Java MCQ Questions – Page 173 of 200

Q: Which of the following is a valid way to create a `Runnable` instance using a lambda expression in Java 8 or later?

A) `Runnable r = () -> { System.out.println("Hello"); };`. Since `Runnable` is a functional interface with a single abstract method `run()` (which takes no arguments and returns void), it can be implemented concisely using a lambda expression that matches this signature.

Q: What type of error will occur when compiling this Java code?

C) Compile-time error: incompatible types: boolean cannot be converted to Object. Similar to `int`, primitive types like `boolean` cannot be used directly as the monitor object for a `synchronized` block. The `synchronized` keyword expects an `Object` reference, causing a compile-time error.

Q: What is the output of this code?

A) Welcome!. A `Supplier` lambda provides a value when its `get()` method is called. Here, it supplies the string 'Welcome!' which is then printed.

Q: While `HashSet` makes no guarantees about the iteration order of its elements, under what specific scenario might the elements *appear* to be iterated in insertion order, though it's still not a contractual guarantee?

B) When the `HashSet`'s initial capacity is sufficiently large to prevent any rehashing, and there are no hash collisions.. The iteration order is based on the backing `HashMap`'s bucket iteration order. If there are no hash collisions and no rehashing occurs (due to sufficient initial capacity), elements might appear in a pseudo-insertion order based on their assigned bucket index, but this remains an implementation detail.

Q: What is the output of this Java code?

A) Error: java.io.InvalidClassException: MyExternalizableObject: no such method: readExternal(). Classes implementing `Externalizable` must provide both `writeExternal` and `readExternal` methods. If `readExternal` is missing (or has an incorrect signature), `ObjectInputStream` cannot find the required method for externalization, leading to an `InvalidClassException`.

Q: What is the output of this code?

A) 1. Initially, the map has (1, 'one') and (2, 'two'). `put(1, 'uno')` overwrites the value for key 1, so it becomes (1, 'uno'). Then `remove(2)` removes the entry for key 2. Only one entry (1, 'uno') remains, so `size()` is 1.

Q: In the context of encapsulation, what is a common reason to make a class's constructor `private`?

B) To enforce the Singleton design pattern or control instantiation via factory methods.. A private constructor restricts direct instantiation, enabling patterns like Singleton where only one instance is allowed, or factory methods which control how objects are created, thereby encapsulating object creation.

Q: What is the output of the following Java program?

A) Initial. The `holder::getValue` is an instance method reference on a specific object (`holder`). It refers to the `getValue()` method of that object. When `supplier.get()` is called, it invokes `holder.getValue()`, which returns 'Initial'.

Q: What is the compile-time error when calling `calculate(5, 10)`?

A) Reference to `calculate` is ambiguous, as both overloaded methods match the call.. When `calculate(5, 10)` is called, both arguments are `int`. Java can implicitly widen an `int` to a `long`. Both `calculate(int, long)` and `calculate(long, int)` require one implicit widening conversion, making neither more specific than the other, resulting in an ambiguous method call error on Line 12.

Q: What is the compilation error, if any, for the provided Java code snippet?

B) error: reference to execute is ambiguous. Similar to the `Serializable` example, `null` can be assigned to both `Object` and the `Runnable` interface. Neither method is considered more specific than the other in the context of a `null` argument, causing an ambiguous method call error.

Q3441 medium

Which of the following is a valid way to create a `Runnable` instance using a lambda expression in Java 8 or later?

Q3442 easy code error

What type of error will occur when compiling this Java code?

java

public class PrimitiveBooleanSync {
    private boolean active = false;

    public void toggleActive() {
        synchronized (active) { // Attempting to synchronize on a primitive boolean
            active = !active;
            System.out.println("Active: " + active);
        }
    }

    public static void main(String[] args) {
        new PrimitiveBooleanSync().toggleActive();
    }
}

Q3443 easy code output

What is the output of this code?

java

import java.util.function.Supplier;

public class LambdaTest {
    public static void main(String[] args) {
        Supplier<String> greeter = () -> "Welcome!";
        System.out.println(greeter.get());
    }
}

Q3444 hard

While `HashSet` makes no guarantees about the iteration order of its elements, under what specific scenario might the elements *appear* to be iterated in insertion order, though it's still not a contractual guarantee?

Q3445 medium code error

What is the output of this Java code?

java

import java.io.*;

public class DeserializationError10 {
    static class MyExternalizableObject implements Externalizable {
        String name;

        public MyExternalizableObject() {}
        public MyExternalizableObject(String name) { this.name = name; }

        @Override
        public void writeExternal(ObjectOutput out) throws IOException {
            out.writeObject(name);
        }
        // The readExternal method is intentionally missing
        // @Override
        // public void readExternal(ObjectInput in) throws IOException, ClassNotFoundException {
        //     name = (String) in.readObject();
        // }
    }

    public static void main(String[] args) {
        byte[] data = null;
        try (ByteArrayOutputStream bos = new ByteArrayOutputStream();
             ObjectOutputStream oos = new ObjectOutputStream(bos)) {
            oos.writeObject(new MyExternalizableObject("Test"));
            data = bos.toByteArray();
        } catch (IOException e) { e.printStackTrace(); }

        try (ByteArrayInputStream bis = new ByteArrayInputStream(data);
             ObjectInputStream ois = new ObjectInputStream(bis)) {
            MyExternalizableObject obj = (MyExternalizableObject) ois.readObject();
            System.out.println("Deserialized: " + obj.name);
        } catch (Exception e) {
            System.out.println("Error: " + e.getClass().getName() + ": " + e.getMessage().split(";")[0]);
        }
    }
}

Q3446 medium code output

What is the output of this code?

java

import java.util.HashMap;

public class Test {
    public static void main(String[] args) {
        HashMap<Integer, String> map = new HashMap<>();
        map.put(1, "one");
        map.put(2, "two");
        map.put(1, "uno"); // Overwrites value for key 1
        map.remove(2);
        System.out.println(map.size());
    }
}

Q3447 medium

In the context of encapsulation, what is a common reason to make a class's constructor `private`?

Q3448 medium code output

What is the output of the following Java program?

java

import java.util.function.Supplier;

class DataHolder {
    private String value = "Initial";
    public String getValue() { return value; }
}

public class InstanceMethodReference {
    public static void main(String[] args) {
        DataHolder holder = new DataHolder();
        Supplier<String> supplier = holder::getValue;
        System.out.println(supplier.get());
    }
}

Q3449 hard code error

What is the compile-time error when calling `calculate(5, 10)`?

java

class Calculator {
    public void calculate(int a, long b) {
        System.out.println("int, long");
    }
    public void calculate(long a, int b) {
        System.out.println("long, int");
    }
}
public class Tester {
    public static void main(String[] args) {
        Calculator calc = new Calculator();
        calc.calculate(5, 10); // Line 12
    }
}

Q3450 hard code error

What is the compilation error, if any, for the provided Java code snippet?

java

class Overloader {
    public void execute(Object o) {}
    public void execute(Runnable r) {}

    public static void main(String[] args) {
        Overloader o = new Overloader();
        o.execute(null); // Line X
    }
}

Q3451 medium

Which of the following variable names is considered invalid in Java?

Q3452 easy code error

What error will this Java code produce during compilation?

java

public class ArrayError {
    public static void main(String[] args) {
        int[][][] cube = new int[2][3];
        System.out.println(cube[0][0][0]);
    }
}

Q3453 hard code error

What runtime error will this Java code snippet throw?

java

public class Test {
    public static void main(String[] args) {
        int[][] grid = new int[2][2];
        grid[0][0] = 1;
        grid[1] = null;
        System.out.println(grid[1][0]);
    }
}

Q3454 medium code output

What is the output of this Java code?

java

import java.util.function.IntBinaryOperator;

class Counter {
    private int value;
    public Counter(int initial) { this.value = initial; }
    public int add(int x, int y) {
        value += (x + y);
        return value;
    }
}

public class MethodRefTest {
    public static void main(String[] args) {
        Counter counter = new Counter(10);
        IntBinaryOperator adder = counter::add;
        int res1 = adder.applyAsInt(2, 3);
        int res2 = adder.applyAsInt(1, 1);
        System.out.println(res1 + " " + res2);
    }
}

Q3455 medium

Consider a scenario where an `Object` reference `obj` holds an instance of `String`. What happens if you try to cast `obj` to a `StringBuilder` without checking its actual type?

Q3456 medium

Is `HashSet` inherently thread-safe?

Q3457 easy

Which statement is true regarding an interface that *can* be used as a functional interface even without the `@FunctionalInterface` annotation?

Q3458 medium code output

What is the output of this Java program?

java

import java.util.HashSet;
import java.util.Set;

public class Main {
    public static void main(String[] args) {
        Set<String> fruits = new HashSet<>();
        fruits.add("apple");
        fruits.add("mango");
        fruits.add("orange");
        System.out.println(fruits.contains("grape"));
    }
}

Q3459 hard code error

What is the error in the execution of this Java code snippet?

java

public class BuilderError {
    public static void main(String[] args) {
        StringBuilder sb = new StringBuilder();
        sb.appendCodePoint(-1);
        System.out.println(sb);
    }
}

Q3460 hard code output

What is the output of this Java code?

java

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

public class FileWriterNullCharWrite {
    public static void main(String[] args) {
        String filename = "null_char_test.txt";
        String s = "Hello\u0000World"; // Null character
        try (FileWriter fw = new FileWriter(filename)) {
            fw.write(s);
        } catch (IOException e) { System.out.println("Error: " + e.getMessage()); }

        try (BufferedReader br = new BufferedReader(new FileReader(filename))) {
            String line = br.readLine();
            System.out.println("\'" + (line != null ? line.replace("\u0000", "[NULL]") : "[Empty]") + "\'");
        } catch (IOException e) { System.out.println("Error reading: " + e.getMessage());
        } finally { try { Files.deleteIfExists(Path.of(filename)); } catch (IOException ignored) {} }
    }
}

☕ Java MCQ Questions – Page 173