Java MCQ Questions – Page 176 of 200

Q: Which exception is thrown when the `main` method of this Java code is executed?

A) java.lang.NullPointerException. The `String.join()` method's contract specifies that if any element in the iterable is `null`, a `NullPointerException` will be thrown. It does not convert `null` elements to the string "null" automatically.

Q: What will this Java code print to the console?

A) HELLO. The `StaticMethodReference::toUpperCase` is a static method reference that is functionally equivalent to `s -> StaticMethodReference.toUpperCase(s)`. It transforms the input string 'hello' to its uppercase version, 'HELLO'.

Q: What exception is thrown when the `oos.writeObject(inner)` line is executed in the `main` method?

A) java.io.NotSerializableException: OuterClass. Non-static inner classes implicitly hold a reference to their enclosing instance. For the inner class to be serializable, its enclosing `OuterClass` instance must also be serializable. Since `OuterClass` does not implement `Serializable`, a `NotSerializableException` is thrown.

Q: Which exception will be thrown when executing this Java code?

A) java.lang.NullPointerException. Attempting to invoke a method (like length()) on a null reference will result in a NullPointerException at runtime.

Q: What is the output of this Java program?

A) abc. TreeMap iterates over its keys in their natural sorted order. For strings, this is alphabetical order. So, it will iterate 'apple', then 'banana', then 'cherry', resulting in 'abc'.

Q: What is the main conceptual difference between `throw` and `throws` in Java?

C) `throw` is used to explicitly throw an exception object, while `throws` is used in a method signature to declare potential exceptions.. `throw` is an action that initiates an exception, taking an exception object as its argument. `throws` is a declaration that alerts callers that a method might propagate certain exceptions.

Q: What is the output of this code?

A) 5 block 100 ctor. Instance initializers run before any constructor. For `obj1`, the instance initializer sets x=5, s='block', then the no-arg constructor does nothing. For `obj2`, the instance initializer runs first, setting x=5, s='block', then the `ImmutableClass(int val)` constructor runs, reassigning x=100 and s='ctor'.

Q: What does this code print?

A) Violations: 1. The validation cascades through multiple levels of `@Valid` annotations (`Config.main` then `Comp.sub`). `Sub.id` has a `@Size(min=3)` constraint, which is violated by the string "a", leading to one violation.

Q: What will happen when you try to compile this Java code?

A) A compile-time error because 'continue' cannot target a labeled block. The 'continue' statement can only target labeled loops. Attempting to 'continue' a labeled block that is not a loop results in a compile-time error.

Q: What is the output of this code?

A) [10, 20, 30]. A TreeSet stores elements in their natural sorted order. Regardless of the insertion order, the elements will be printed in ascending order.

Q3501 hard code error

Which exception is thrown when the `main` method of this Java code is executed?

java

import java.util.Arrays;
import java.util.List;

public class StringJoinNullElements {
    public static void main(String[] args) {
        List<String> elements = Arrays.asList("a", null, "b");
        try {
            String result = String.join("-", elements);
            System.out.println(result);
        } catch (Exception e) {
            System.out.println(e.getClass().getSimpleName());
        }
    }
}

Q3502 medium code output

What will this Java code print to the console?

java

import java.util.function.Function;

public class StaticMethodReference {
    public static String toUpperCase(String input) {
        return input.toUpperCase();
    }

    public static void main(String[] args) {
        Function<String, String> transformer = StaticMethodReference::toUpperCase;
        System.out.println(transformer.apply("hello"));
    }
}

Q3503 hard code error

What exception is thrown when the `oos.writeObject(inner)` line is executed in the `main` method?

java

import java.io.*;

class OuterClass {
    private String name;
    public OuterClass(String name) { this.name = name; }

    class InnerClass implements Serializable { // Non-static inner class
        private static final long serialVersionUID = 1L;
        private int innerId;
        public InnerClass(int innerId) { this.innerId = innerId; }
        public int getInnerId() { return innerId; }
    }

    public InnerClass createInner(int id) {
        return new InnerClass(id);
    }
}

public class SerializationError4 {
    public static void main(String[] args) throws IOException {
        OuterClass outer = new OuterClass("My Outer");
        OuterClass.InnerClass inner = outer.createInner(100);

        ByteArrayOutputStream bos = new ByteArrayOutputStream();
        ObjectOutputStream oos = new ObjectOutputStream(bos);
        oos.writeObject(inner); // Error occurs here
        oos.close();
    }
}

Q3504 medium code error

Which exception will be thrown when executing this Java code?

java

public class StringError1 {
    public static void main(String[] args) {
        String s = null;
        int len = s.length();
        System.out.println(len);
    }
}

Q3505 medium code output

What is the output of this Java program?

java

import java.util.TreeMap;
import java.util.Map;

public class TreeMapIteration {
    public static void main(String[] args) {
        TreeMap<String, Integer> map = new TreeMap<>();
        map.put("banana", 2);
        map.put("apple", 1);
        map.put("cherry", 3);
        StringBuilder sb = new StringBuilder();
        for (Map.Entry<String, Integer> entry : map.entrySet()) {
            sb.append(entry.getKey().charAt(0));
        }
        System.out.println(sb.toString());
    }
}

Q3506 easy

What is the main conceptual difference between `throw` and `throws` in Java?

Q3507 hard code output

What is the output of this code?

java

class ImmutableClass {
    final int x;
    final String s;
    { x = 5; s = "block"; } // Instance initializer block
    ImmutableClass() {}
    ImmutableClass(int val) {
        this.x = val; // Re-initializes x after instance block
        this.s = "ctor";
    }
    public String toString() { return x + " " + s; }
}
public class Main {
    public static void main(String[] args) {
        ImmutableClass obj1 = new ImmutableClass();
        ImmutableClass obj2 = new ImmutableClass(100);
        System.out.println(obj1 + " " + obj2);
    }
}

Q3508 medium code output

What does this code print?

java

import jakarta.validation.*;
import jakarta.validation.constraints.Size;
import java.util.Set;

public class Test {
    static class Sub { @Size(min=3) public String id; public Sub(String id) { this.id = id; } }
    static class Comp { @Valid public Sub sub; public Comp(Sub s) { this.sub = s; } }
    static class Config {
        @Valid public Comp main;
        public Config(Comp c) { this.main = c; }
    }
    public static void main(String[] args) {
        Validator validator = Validation.buildDefaultValidatorFactory().getValidator();
        Config cfg = new Config(new Comp(new Sub("a")));
        Set<ConstraintViolation<Config>> violations = validator.validate(cfg);
        System.out.println("Violations: " + violations.size());
    }
}

Q3509 easy code error

What will happen when you try to compile this Java code?

java

public class Main {
    public static void main(String[] args) {
        myBlock: { // A labeled block, not a loop or switch
            for (int i = 0; i < 3; i++) {
                if (i == 1) continue myBlock;
            }
            System.out.println("Block finished");
        }
        System.out.println("Main finished");
    }
}

Q3510 easy code output

What is the output of this code?

java

import java.util.TreeSet;

public class Main {
    public static void main(String[] args) {
        TreeSet<Integer> ts = new TreeSet<>();
        ts.add(30);
        ts.add(10);
        ts.add(20);
        System.out.println(ts);
    }
}

Q3511 hard code error

What is the compile-time error in the `Sub` class?

java

class Super {
    public static void display() {
        System.out.println("Super static");
    }
}
class Sub extends Super {
    @Override
    public void display() {
        System.out.println("Sub instance");
    }
}
public class Main {
    public static void main(String[] args) {
        Sub.display();
    }
}

Q3512 hard

Considering memory footprint on a 64-bit JVM with compressed ordinary object pointers (OOPs), how does an `long[]` array compare to an `Object[]` array of the same length, holding `Long` objects, assuming both arrays store equivalent numerical data?

Q3513 medium code output

What is the output of this Java code?

java

class NameThread extends Thread {
    public void run() {
        System.out.println("Child: " + Thread.currentThread().getName());
    }
}
public class Main {
    public static void main(String[] args) throws InterruptedException {
        NameThread t = new NameThread();
        t.start();
        t.join(); 
        System.out.println("Main: " + Thread.currentThread().getName());
    }
}

Q3514 medium code output

What does this code print?

java

import java.io.File;
import java.io.IOException;
import java.util.Arrays;

public class DirectoryListing {
    public static void main(String[] args) throws IOException {
        File dir = new File("my_test_dir");
        dir.mkdir();
        new File(dir, "file_b.txt").createNewFile();
        new File(dir, "subdir_a").mkdir();
        new File(dir, "file_c.txt").createNewFile();
        
        String[] list = dir.list();
        Arrays.sort(list); // Ensures consistent order
        
        String output = String.join(" ", list);
        
        // Cleanup
        new File(dir, "file_b.txt").delete();
        new File(dir, "file_c.txt").delete();
        new File(dir, "subdir_a").delete();
        dir.delete();
        
        System.out.println(output);
    }
}

Q3515 easy code output

What is the output of this code?

java

import java.io.BufferedWriter;
import java.io.StringWriter;
import java.io.IOException;

public class Main {
    public static void main(String[] args) {
        StringWriter stringWriter = new StringWriter();
        try (BufferedWriter writer = new BufferedWriter(stringWriter)) {
            writer.write("Line1");
            writer.newLine();
            writer.write("Line2");
            writer.flush();
            System.out.print(stringWriter.toString().replace("\n", "\\n"));
        } catch (IOException e) {
            System.out.print("Error");
        }
    }
}

Q3516 hard code error

What kind of error will this Java code snippet produce when executed?

java

import java.util.Arrays;
import java.util.List;

public class FixedSizeListModification {
    public static void main(String[] args) {
        String[] namesArray = {"Alice", "Bob"};
        List<String> fixedSizeList = Arrays.asList(namesArray);
        
        fixedSizeList.add("Charlie"); // Attempt to add to a fixed-size list
    }
}

Q3517 hard code error

What exception is thrown when the `ois.readObject()` line is executed during deserialization?

java

import java.io.*;
import java.io.ObjectInputFilter.Config;
import java.io.ObjectInputFilter.FilterInfo;
import java.io.ObjectInputFilter.Status;

class FilteredClass implements Serializable {
    private static final long serialVersionUID = 1L;
    String data;
    public FilteredClass(String data) { this.data = data; }
    public String toString() { return data; }
}

public class SerializationError9 {
    public static void main(String[] args) throws Exception {
        // Serialize an object
        FilteredClass obj = new FilteredClass("Sensitive Data");
        ByteArrayOutputStream bos = new ByteArrayOutputStream();
        ObjectOutputStream oos = new ObjectOutputStream(bos);
        oos.writeObject(obj);
        byte[] serializedData = bos.toByteArray();
        oos.close();

        // Deserialization with a filter that rejects FilteredClass
        ByteArrayInputStream bis = new ByteArrayInputStream(serializedData);
        ObjectInputStream ois = new ObjectInputStream(bis);

        // Set a filter that rejects FilteredClass
        ois.setObjectInputFilter(info -> {
            if (info.serialClass() != null && info.serialClass().getName().equals("FilteredClass")) {
                return Status.REJECTED; // Explicitly reject objects of type FilteredClass
            }
            return Status.UNDECIDED;
        });

        // Error occurs here due to the filter
        FilteredClass deserialized = (FilteredClass) ois.readObject();
        ois.close();
        System.out.println(deserialized);
    }
}

Q3518 medium code error

What is the outcome when this Java code is executed?

java

import java.util.LinkedList;
import java.util.List;

public class Test {
    public static void main(String[] args) {
        LinkedList<Double> values = new LinkedList<>();
        values.add(1.1);
        List<Double> myList = values;
        myList.addFirst(0.0);
        System.out.println(myList);
    }
}

Q3519 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<Integer> data = new ArrayList<>();
        data.add(1);
        data.add(2);
        Iterator<Integer> it1 = data.iterator();
        Iterator<Integer> it2 = data.iterator();
        it1.next(); // Retrieves 1 for it1
        it2.remove(); // Calling remove() for it2 without its own next() call
    }
}

Q3520 easy code output

What is the output of this code?

java

import java.io.BufferedWriter;
import java.io.StringWriter;
import java.io.IOException;

public class Main {
    public static void main(String[] args) {
        StringWriter stringWriter = new StringWriter();
        try (BufferedWriter writer = new BufferedWriter(stringWriter)) {
            char[] chars = {'J', 'A', 'V', 'A'};;
            writer.write(chars, 1, 2);
            writer.flush();
            System.out.print(stringWriter.toString());
        } catch (IOException e) {
            System.out.print("Error");
        }
    }
}

☕ Java MCQ Questions – Page 176