Java MCQ Questions – Page 145 of 200

Q: What is the output of this code?

A) [A, null, null]:true. `retainAll(Collection c)` retains only the elements in `list` that are also contained in `retainElements`. `list` contains 'A', `null`, 'B', `null`, 'C'. `retainElements` contains 'A', `null`. Both 'A' and both `null`s are present in `retainElements` (as `retainElements.contains(null)` is true). 'B' and 'C' are removed. The list was changed, so `changed` is true.

Q: Given `String text = "JavaProgramming";` what is the correct way to get the substring `"Programming"`?

C) `text.substring(4)`. String indices are 0-based. 'J' is at index 0, 'a' at 1, 'v' at 2, 'a' at 3. 'P' in "Programming" is at index 4. The `substring(beginIndex)` method returns the substring from the specified `beginIndex` to the end of the string.

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

B) StringIndexOutOfBoundsException. The `replace(start, end, str)` method requires the `start` index to be less than or equal to the `end` index. Since 5 is greater than 2, this call throws a `StringIndexOutOfBoundsException`.

Q: Which keyword is used to implement inheritance in Java?

B) extends. In Java, the `extends` keyword is used to specify that a class will inherit from another class, establishing an IS-A relationship.

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

C) java.lang.NullPointerException. The `synchronized` block requires an object reference to acquire a lock. If the reference is `null` when the block is entered, a `NullPointerException` will be thrown at runtime.

Q: What is the compilation error in this code?

A) Incompatible types: `Box` cannot be converted to `int`. An object of a class type (like `Box`) cannot be directly assigned to a primitive variable (like `int`) without explicit type conversion or accessing a field of the object.

Q: What is the output of this Java code?

A) Apple:1 Date:1 Banana:2 Cherry:3. The PriorityQueue sorts by `id` in ascending order. For elements with the same `id` (e.g., '1'), the relative order is not strictly guaranteed by the contract, but in typical implementations, insertion order or internal heap structure dictates which 'equal' element is polled first. Option A represents a common outcome.

Q: Given a 2D array `int[][] matrix = new int[5][8];`, what do `matrix.length` and `matrix[0].length` represent, respectively?

A) Number of rows, Number of columns. `matrix.length` gives the number of rows (the length of the outer array), and `matrix[0].length` gives the number of columns in the first row (the length of the inner array at index 0).

Q: Which compilation error will be thrown by the following code?

B) Invalid '@FunctionalInterface' annotation; ChildProcessor is not a functional interface. `ParentProcessor` has one abstract method (`process`), and `ChildProcessor` adds another (`childProcess`). When `ChildProcessor` extends `ParentProcessor`, it inherits `process()`, resulting in two abstract methods (`process()` and `childProcess()`), violating the single abstract method rule for `@FunctionalInterface`.

Q: What is the most significant difference in program flow control between using a `break` statement and a `return` statement inside a `while` loop when a specific condition is met?

A) `break` terminates the loop and continues execution immediately *after* the loop; `return` terminates the loop and the entire method containing it.. `break` transfers control to the statement immediately following the loop it is within. `return` exits the entire method, regardless of how many loops or conditional blocks are currently active.

Q2881 hard code output

What is the output of this code?

java

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

public class Test {
    public static void main(String[] args) {
        ArrayList<String> list = new ArrayList<>();
        list.add("A");
        list.add(null);
        list.add("B");
        list.add(null);
        list.add("C");

        ArrayList<String> retainElements = new ArrayList<>();
        retainElements.add("A");
        retainElements.add(null); 

        boolean changed = list.retainAll(retainElements);

        System.out.println(list + ":" + changed);
    }
}

Q2882 medium

Given `String text = "JavaProgramming";` what is the correct way to get the substring `"Programming"`?

Q2883 easy code error

What error will occur when this code is executed?

java

public class Main {
    public static void main(String[] args) {
        StringBuffer sb = new StringBuffer("HelloJava");
        sb.replace(5, 2, "World");
        System.out.println(sb);
    }
}

Q2884 easy

Which keyword is used to implement inheritance in Java?

Q2885 easy code error

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

java

public class Syncher {
    private Object syncMonitor = null; // Deliberately null

    public void doSynchronizedWork() {
        synchronized (syncMonitor) {
            System.out.println("Work done.");
        }
    }

    public static void main(String[] args) {
        Syncher s = new Syncher();
        s.doSynchronizedWork();
    }
}

Q2886 easy code error

What is the compilation error in this code?

java

class Box {
    int value = 10;
}

public class Main {
    public static void main(String[] args) {
        Box myBox = new Box();
        int x = myBox;
        System.out.println(x);
    }
}

Q2887 hard code output

What is the output of this Java code?

java

import java.util.PriorityQueue;
import java.util.Comparator;

class Item {
    int id;
    String name;
    Item(int id, String name) { this.id = id; this.name = name; }
    @Override public String toString() { return name + ":" + id; }
}

public class QueueTest {
    public static void main(String[] args) {
        PriorityQueue<Item> pq = new PriorityQueue<>(Comparator.comparingInt(item -> item.id));

        pq.offer(new Item(2, "Banana"));
        pq.offer(new Item(1, "Apple"));
        pq.offer(new Item(3, "Cherry"));
        pq.offer(new Item(1, "Date"));

        StringBuilder sb = new StringBuilder();
        while (!pq.isEmpty()) {
            sb.append(pq.poll().toString()).append(" ");
        }
        System.out.println(sb.toString().trim());
    }
}

Q2888 medium

Given a 2D array `int[][] matrix = new int[5][8];`, what do `matrix.length` and `matrix[0].length` represent, respectively?

Q2889 easy code error

Which compilation error will be thrown by the following code?

java

interface ParentProcessor {
    void process();
}

@FunctionalInterface
interface ChildProcessor extends ParentProcessor {
    void childProcess();
}

public class Main {
    public static void main(String[] args) {
        // Attempt to use ChildProcessor
    }
}

Q2890 hard

What is the most significant difference in program flow control between using a `break` statement and a `return` statement inside a `while` loop when a specific condition is met?

Q2891 medium

What is the main purpose of the `newLine()` method in `BufferedWriter`?

Q2892 medium

What is the primary purpose of the `finally` block in Java's exception handling mechanism?

Q2893 medium code output

What is the output of this Java code snippet involving nested switch statements?

java

public class SwitchTest {
    public static void main(String[] args) {
        int x = 1;
        char y = 'B';
        String result = "";
        switch (x) {
            case 1:
                switch (y) {
                    case 'A': result = "1A"; break;
                    case 'B': result = "1B"; break;
                    default: result = "1X";
                }
                break;
            case 2:
                result = "2Y";
                break;
            default:
                result = "ZZ";
        }
        System.out.println(result);
    }
}

Q2894 medium code error

What error will this Java code produce when executed?

java

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

public class Main {
    public static void main(String[] args) {
        List<String> items = new ArrayList<>();
        items.add("Item1");
        Iterator<String> it = items.iterator();
        it.remove(); // Calling remove() before next()
    }
}

Q2895 easy code output

What does this Java code print?

java

public class Main {
    public static void main(String[] args) {
        int a = 20;
        int b = 5;
        if (a > 10) {
            if (b > 10) {
                System.out.println("Inner true");
            } else {
                System.out.println("Inner false");
            }
        } else {
            System.out.println("Outer false");
        }
    }
}

Q2896 hard code output

What is the output of this code?

java

class Base {
    protected Base() { System.out.print("Base ctor "); }
    protected Base(int x) { System.out.print("Base ctor " + x + " "); }
}
class Derived extends Base {
    public Derived() {
        super();
        System.out.print("Derived ctor ");
    }
    public Derived(String s) {
        super(s.length());
        System.out.print("Derived ctor " + s + " ");
    }
}
public class Main {
    public static void main(String[] args) {
        new Derived("Hello");
        new Derived();
    }
}

Q2897 medium code error

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

java

import java.util.TreeSet;
import java.util.SortedSet;

public class Main {
    public static void main(String[] args) {
        TreeSet<Integer> originalSet = new TreeSet<>();
        originalSet.add(10);
        originalSet.add(20);
        originalSet.add(30);
        originalSet.add(40);

        SortedSet<Integer> subSet = originalSet.subSet(15, true, 35, false);
        subSet.add(5);
        System.out.println(subSet.size());
    }
}

Q2898 hard

You insert two distinct objects, `obj1` and `obj2`, into a `TreeMap` as keys. `obj1.equals(obj2)` returns `true`, but `obj1.compareTo(obj2)` returns a non-zero value. What will be the state of the `TreeMap`?

Q2899 easy code error

What compile-time error will occur in the `MyClass()` constructor?

java

import java.io.IOException;

class MyClass {
    MyClass() {
        throw new IOException("Error!");
    }
}
public class Main {
    public static void main(String[] args) {
        MyClass obj = new MyClass();
    }
}

Q2900 medium

Where can the `default` case be placed within a Java `switch` statement or expression?

☕ Java MCQ Questions – Page 145