Which Stream API operation should be used when you have a stream of lists (e.g., `Stream >`) and you want to obtain a single stream of all individual strings (`Stream `)?

C) flatMap(). The `flatMap()` operation is used to transform each element into a stream of zero or more elements, and then flattens these resulting streams into a single stream.

Java MCQ Questions – Page 95 of 200

Q: An `ArrayList` is created using `new ArrayList ()`. If elements are added until its internal capacity needs to grow for the 3rd time, what is the minimum number of elements that must have been added?

B) 23. Default capacity is 10. First growth: 10 -> 15 (on 11th element). Second growth: 15 -> 22 (on 16th element). Third growth: 22 -> 33 (on 23rd element). Therefore, 23 elements must be added to trigger the third capacity growth.

Q: What happens if `next()` is called on a `java.util.Iterator` when `hasNext()` would return `false`?

B) It throws a `NoSuchElementException`.. Calling `next()` when no more elements exist, as indicated by `hasNext()` returning `false`, results in a `NoSuchElementException`.

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

B) Error: Abstract methods do not specify a body. An abstract method is a method declared without an implementation (without a body). Providing a body for an abstract method results in a compilation error.

Q: What is the result of running this Java code?

C) Compilation Error: cannot find symbol method toLowercase(). The correct method name to convert a String to lowercase is toLowerCase(), not toLowercase(). This typo results in a compilation error because the method cannot be found.

Q: What is the output of this code?

A) 100. The `DeepImmutableData` class is designed to be immutable, correctly using defensive copies for the mutable `Date` field in both its constructor and getter. When `new DeepImmutableData("Test", d)` is called, a copy of `d`'s internal state (Jan 1, 2000) is stored. Subsequent modification of the `d` reference in `main` (`d.setYear(101)`) does not affect the `birthDate` field within the `data` object. Therefore, `data.getBirthDate().getYear()` will return 100 (for 2000).

Q: What is the result of compiling and running the following Java code snippet?

C) Throws PatternSyntaxException. The String.split() method takes a regular expression as its argument. The '.' character is a metacharacter in regex that matches any character. When used unescaped as a delimiter, it causes a PatternSyntaxException due to the ambiguity or a zero-length match issue.

Q: What is the output of this code?

A) 2147483647. Before bitwise operations, `byte` operands are promoted to `int`. `b1` as a `byte` is `-1`, which is all ones in two's complement. When promoted to `int`, it becomes `0xFFFFFFFF`. The `>>>` (unsigned right shift) operator shifts bits to the right, filling with zeros from the left. Shifting `0xFFFFFFFF` by 1 position results in `0x7FFFFFFF`, which is `Integer.MAX_VALUE` or `2147483647`.

Q: What is the compilation error in the `main` method?

A) error: RestrictedException(String) has private access in RestrictedException. When a constructor is declared `private`, it can only be accessed from within the class itself. Attempting to call a private constructor from outside its defining class results in a compile-time error, specifically an 'access' error.

Q: What does `synchronized(this)` mean when used in a code block within an instance method?

C) It locks the current object instance.. `synchronized(this)` uses the intrinsic lock of the current object instance to protect the enclosed code block, preventing other threads from executing synchronized methods or blocks on the same instance.

Q1881 hard

An `ArrayList` is created using `new ArrayList<String>()`. If elements are added until its internal capacity needs to grow for the 3rd time, what is the minimum number of elements that must have been added?

Q1882 medium

What happens if `next()` is called on a `java.util.Iterator` when `hasNext()` would return `false`?

Q1883 easy code error

What compilation error will occur in the following Java code?

java

abstract class Processor {
    public abstract void process() {
        System.out.println("Processing data...");
    }
}

Q1884 easy code error

What is the result of running this Java code?

java

public class StringError {
    public static void main(String[] args) {
        String data = "UPPERCASE";
        System.out.println(data.toLowercase());
    }
}

Q1885 hard code output

What is the output of this code?

java

import java.util.Date;

final class DeepImmutableData {
    private final String name;
    private final Date birthDate;

    public DeepImmutableData(String name, Date birthDate) {
        this.name = name;
        this.birthDate = new Date(birthDate.getTime()); // Defensive copy
    }

    public String getName() { return name; }
    public Date getBirthDate() { return new Date(birthDate.getTime()); } // Defensive copy
}

public class Main {
    public static void main(String[] args) {
        Date d = new Date(100, 0, 1); // Jan 1, 2000
        DeepImmutableData data = new DeepImmutableData("Test", d);
        d.setYear(101); // Jan 1, 2001
        System.out.println(data.getBirthDate().getYear());
    }
}

Q1886 hard code error

What is the result of compiling and running the following Java code snippet?

java

public class StringError {
    public static void main(String[] args) {
        String s = "alpha.beta.gamma";
        String[] parts = s.split(".");
        System.out.println(parts.length);
    }
}

Q1887 medium

Which Stream API operation should be used when you have a stream of lists (e.g., `Stream<List<String>>`) and you want to obtain a single stream of all individual strings (`Stream<String>`)?

Q1888 hard code output

What is the output of this code?

java

public class DataTypeChallenge {
    public static void main(String[] args) {
        byte b1 = -1;
        byte b2 = 1;
        int result = b1 >>> b2;
        System.out.println(result);
    }
}

Q1889 hard code error

What is the compilation error in the `main` method?

java

class RestrictedException extends Exception {
    // Private constructor restricts direct external instantiation
    private RestrictedException(String message) {
        super(message);
    }

    // A static factory method might be provided for controlled instantiation
    public static RestrictedException create(String message) {
        return new RestrictedException(message);
    }
}

public class Main {
    public static void main(String[] args) {
        // Attempting to directly call a private constructor
        RestrictedException e = new RestrictedException("Access denied"); // This line causes the compilation error
    }
}

Q1890 easy

What does `synchronized(this)` mean when used in a code block within an instance method?

Q1891 easy

Which of the following describes the key requirement for objects used as keys in a `TreeMap` if no custom `Comparator` is provided?

Q1892 hard code output

What does this code print?

java

public class OperatorChallenge {
    public static void main(String[] args) {
        System.out.println(1 + 2 + "3" + 4 + 5);
    }
}

Q1893 easy code output

What is the output of this Java code snippet?

java

public class Main {
    public static void main(String[] args) {
        int x = 10;
        if (x > 5) {
            System.out.println("X is greater than 5");
        }
    }
}

Q1894 easy code output

What is the output of this code?

java

class BlockCounter {
    private int count = 0;
    public void increment() {
        synchronized(this) {
            count++;
            System.out.print(count);
        }
    }
}

public class MyProgram {
    public static void main(String[] args) throws InterruptedException {
        BlockCounter bc = new BlockCounter();
        Thread t1 = new Thread(() -> bc.increment());
        Thread t2 = new Thread(() -> bc.increment());
        t1.start();
        t2.start();
        t1.join();
        t2.join();
    }
}

Q1895 hard

What is a critical consideration when evaluating a boolean expression in an `if` statement that involves comparison between an `int` primitive and an `Integer` wrapper object using `==`?

Q1896 easy code output

What is the output of this code?

java

class Printer {
    public void print(String s) {
        System.out.print("String: " + s);
    }
    public void print(int i) {
        System.out.print("Int: " + i);
    }
}

public class Main {
    public static void main(String[] args) {
        Printer p = new Printer();
        p.print(123);
    }
}

Q1897 medium code error

What kind of error will occur when executing this Java code?

java

import java.util.Comparator;
import java.util.TreeSet;

public class Main {
    public static void main(String[] args) {
        Comparator<Number> customComparator = (n1, n2) -> {
            Integer i1 = (Integer) n1;
            Integer i2 = (Integer) n2;
            return i1.compareTo(i2);
        };
        TreeSet<Number> set = new TreeSet<>(customComparator);
        set.add(10);
        set.add(5);
        set.add(3.14);
        System.out.println(set.size());
    }
}

Q1898 hard code error

What is the compile-time error in this Java code?

java

abstract class Base {
    public abstract Base();
    private int value;
    Base(int val) {
        this.value = val;
    }
}

Q1899 hard code error

What kind of error will occur when executing the following Java code?

java

import java.util.LinkedList;
import java.util.Queue;

public class QError5 {
    public static void main(String[] args) {
        Queue rawQueue = new LinkedList();
        rawQueue.add("Hello");
        rawQueue.add(123);
        Integer i = (Integer) rawQueue.poll();
        System.out.println(i);
    }
}

Q1900 medium

What distinguishes a daemon thread from a user (non-daemon) thread in Java?

☕ Java MCQ Questions – Page 95